13. The Flooding Procedure Link State Update packets provide the mechanism for flooding link state advertisements. A Link State Update packet may contain several distinct advertisements, and floods each advertisement one hop further from its point of origination. To make the flooding procedure reliable, each advertisement must be acknowledged separately. Acknowledgments are transmitted in Link State Acknowledgment packets. Many separate acknowledgments can be grouped together into a single packet. The flooding procedure starts when a Link State Update packet has been received. Many consistency checks have been made on the received packet before being handed to the flooding procedure (see Section 8.2). In particular, the Link State Update packet has been associated with a particular neighbor, and a particular area. If the neighbor is in a lesser state than Exchange, the packet should be dropped without further processing. All types of link state advertisements, other than AS external links, are associated with a specific area. However, link state advertisements do not contain an area field. A link state advertisement's area must be deduced from the Link State Update packet header. For each link state advertisement contained in the packet, the following steps are taken: (1) Validate the advertisement's link state checksum. If the checksum turns out to be invalid, discard the advertisement and get the next one from the Link State Update packet. (2) Examine the link state advertisement's LS type. If the LS type is unknown, discard the advertisement and get the next one from the Link State Update Packet. This specification defines LS Types 1-5 (see Section 4.3). (3) Else if this is a AS external advertisement (LS type = 5), and the area has been configured as a stub area, discard the advertisement and get the next one from the Link State Update Packet. AS external advertisements are not flooded into/throughout stub areas (see Section 3.6). (4) Else if the advertisement's age is equal to MaxAge, and there is currently no instance of the advertisement in the router's link state database, then take the following actions:
(a) Acknowledge the receipt of the advertisement by sending a Link State Acknowledgment packet back to the sending neighbor (see Section 13.5). (b) Purge all outstanding requests for equal or previous instances of the advertisement from the sending neighbor's Link State Request list (see Section 10). (c) If the sending neighbor is in state Exchange or in state Loading, then install the MaxAge advertisement in the link state database. Otherwise, simply discard the advertisement. In either case, examine the next advertisement (if any) listed in the Link State Update packet. (5) Otherwise, find the instance of this advertisement that is currently contained in the router's link state database. If there is no database copy, or the received advertisement is more recent than the database copy (see Section 13.1 below for the determination of which advertisement is more recent) the following steps must be performed: (a) If there is already a database copy, and if the database copy was installed less than MinLSInterval seconds ago, discard the new advertisement (without acknowledging it) and examine the next advertisement (if any) listed in the Link State Update packet. (b) Otherwise immediately flood the new advertisement out some subset of the router's interfaces (see Section 13.3). In some cases (e.g., the state of the receiving interface is DR and the advertisement was received from a router other than the Backup DR) the advertisement will be flooded back out the receiving interface. This occurrence should be noted for later use by the acknowledgment process (Section 13.5). (c) Remove the current database copy from all neighbors' Link state retransmission lists. (d) Install the new advertisement in the link state database (replacing the current database copy). This may cause the routing table calculation to be scheduled. In addition, timestamp the new advertisement with the current time (i.e., the time it was received). The flooding procedure cannot overwrite the newly installed advertisement until MinLSInterval seconds have elapsed. The advertisement installation process is discussed further in Section 13.2. (e) Possibly acknowledge the receipt of the advertisement by sending a Link State Acknowledgment packet back out the receiving
interface. This is explained below in Section 13.5. (f) If this new link state advertisement indicates that it was originated by this router itself, the router must advance the advertisement's link state sequence number, and issue a new instance of the advertisement (see Section 13.4). (6) Else, if there is an instance of the advertisement on the sending neighbor's Link state request list, an error has occurred in the Database Description process. In this case, restart the Database Description process by generating the neighbor event BadLSReq for the sending neighbor and stop processing the Link State Update packet. (7) Else, if the received advertisement is the same instance as the database copy (i.e., neither one is more recent) the following two steps should be performed: (a) If the advertisement is listed in the Link state retransmission list for the receiving adjacency, the router itself is expecting an acknowledgment for this advertisement. The router should treat the received advertisement as an acknowledgment, by removing the advertisement from the Link state retransmission list. This is termed an "implied acknowledgment". Its occurrence should be noted for later use by the acknowledgment process (Section 13.5). (b) Possibly acknowledge the receipt of the advertisement by sending a Link State Acknowledgment packet back out the receiving interface. This is explained below in Section 13.5. (8) Else, the database copy is more recent. Note an unusual event to network management, discard the advertisement and process the next link state advertisement contained in the packet. 13.1 Determining which link state is newer When a router encounters two instances of a link state advertisement, it must determine which is more recent. This occurred above when comparing a received advertisement to the database copy. This comparison must also be done during the database exchange procedure which occurs during adjacency bring-up. A link state advertisement is identified by its LS type, Link State ID and Advertising Router. For two instances of the same advertisement, the LS sequence number, LS age, and LS checksum fields are used to determine which instance is more recent:
o The advertisement having the newer LS sequence number is more recent. See Section 12.1.6 for an explanation of the LS sequence number space. If both instances have the same LS sequence number, then: o If the two instances have different LS checksums, then the instance having the larger LS checksum (when considered as a 16-bit unsigned integer) is considered more recent. o Else, if only one of the instances is of age MaxAge, the instance of age MaxAge is considered to be more recent. o Else, if the ages of the two instances differ by more than MaxAgeDiff, the instance having the smaller (younger) age is considered to be more recent. o Else, the two instances are considered to be identical. 13.2 Installing link state advertisements in the database Installing a new link state advertisement in the database, either as the result of flooding or a newly self originated advertisement, may cause the routing table structure to be recalculated. The contents of the new advertisement should be compared to the old instance, if present. If there is no difference, there is no need to recalculate the routing table. (Note that even if the contents are the same, the LS checksum will probably be different, since the checksum covers the LS sequence number.) If the contents are different, the following pieces of the routing table must be recalculated, depending on the LS type field: Router links, network links The entire routing table must be recalculated, starting with the shortest path calculations for each area (not just the area whose topological database has changed). The reason that the shortest path calculation cannot be restricted to the single changed area has to do with the fact that AS boundary routers may belong to multiple areas. A change in the area currently providing the best route may force the router to use an intra-area route provided by a different area. Summary link The best route to the destination described by the summary link advertisement must be re-examined (see Section 16.5). If this destination is an AS boundary router, it may also be necessary to
re-examine all the AS external link advertisements. AS external link The best route to the destination described by the AS external link advertisement must be re-examined (see Section 16.6). Also, any old instance of the advertisement must be removed from the database when the new advertisement is installed. This old instance must also be removed from all neighbors' Link state retransmission lists (see Section 10). 13.3 Next step in the flooding procedure When a new (and more recent) advertisement has been received, it must be flooded out some set of the router's interfaces. This section describes the second part of flooding procedure (the first part being the processing that occurred in Section 13), namely, selecting the outgoing interfaces and adding the advertisement to the appropriate neighbors' Link state retransmission lists. Also included in this part of the flooding procedure is the maintenance of the neighbors' Link state request lists. This section is equally applicable to the flooding of an advertisement that the router itself has just originated (see Section 12.4). For these advertisements, this section provides the entirety of the flooding procedure (i.e., the processing of Section 13 is not performed, since, for example, the advertisement has not been received from a neighbor and therefore does not need to be acknowledged). Depending upon the advertisement's LS type, the advertisement can be flooded out only certain interfaces. These interfaces, defined by the following, are called the eligible interfaces: AS external links (LS Type = 5) AS external links are flooded throughout the entire AS, with the exception of stub areas (see Section 3.6). The eligible interfaces are all the router's interfaces, excluding virtual links and those interfaces attaching to stub areas. All other types All other types are specific to a single area (Area A). The eligible interfaces are all those interfaces attaching to the Area A. If Area A is the backbone, this includes all the virtual links.
Link state databases must remain synchronized over all adjacencies associated with the above eligible interfaces. This is accomplished by executing the following steps on each eligible interface. It should be noted that this procedure may decide not to flood a link state advertisement out a particular interface, if there is a high probability that the attached neighbors have already received the advertisement. However, in these cases the flooding procedure must be absolutely sure that the neighbors eventually do receive the advertisement, so the advertisement is still added to each adjacency's Link state retransmission list. For each eligible interface: (1) Each of the neighbors attached to this interface are examined, to determine whether they must receive the new advertisement. The following steps are executed for each neighbor: (a) If the neighbor is in a lesser state than Exchange, it does not participate in flooding, and the next neighbor should be examined. (b) Else, if the adjacency is not yet full (neighbor state is Exchange or Loading), examine the Link state request list associated with this adjacency. If there is an instance of the new advertisement on the list, it indicates that the neighboring router has an instance of the advertisement already. Compare the new advertisement to the neighbor's copy: o If the new advertisement is less recent, then try the next neighbor. o If the two copies are the same instance, then delete the advertisement from the Link state request list, and try the next neighbor. o Else, the new advertisement is more recent. Delete the advertisement from the Link state request list. (c) If the new advertisement was received from this neighbor, try the next neighbor. (d) At this point we are not positive that the new neighbor has an up-to-date instance of this new advertisement. Add the new advertisement to the Link state retransmission list for the adjacency. This ensures that the flooding procedure is reliable; the advertisement will be retransmitted at intervals until an acknowledgment is seen from the neighbor.
(2) The router must now decide whether to flood the new link state advertisement out this interface. If in the previous step, the link state advertisement was NOT added to any of the Link state retransmission lists, there is no need to flood the advertisement and the next interface should be examined. (3) If the new advertisement was received on this interface, and it was received from either the Designated Router or the Backup Designated Router, chances are all the neighbors have received the advertisement already. Therefore, examine the next interface. (4) If the new advertisement was received on this interface, and the interface state is Backup (i.e., the router itself is the Backup Designated Router), examine the next interface. The Designated Router will do the flooding on this interface. If the Designated Router fails, this router will end up retransmitting the updates. (5) If this step is reached, the advertisement must be flooded out the interface. Send a Link State Update packet (with the new advertisement as contents) out the interface. The advertisement's LS age must be incremented by InfTransDelay (which must be > 0) when copied into the outgoing packet (until the LS age field reaches its maximum value of MaxAge). On broadcast networks, the Link State Update packets are multicast. The destination IP address specified for the Link State Update Packet depends on the state of the interface. If the interface state is DR or Backup, the address AllSPFRouters should be used. Otherwise, the address AllDRouters should be used. On non-broadcast, multi-access networks, separate Link State Update packets must be sent, as unicasts, to each adjacent neighbor (i.e., those in state Exchange or greater). The destination IP addresses for these packets are the neighbors' IP addresses. 13.4 Receiving self-originated link state It is a common occurrence to receive a self-originated link state advertisement via the flooding procedure. If the advertisement received is a newer instance than the last instance that the router actually originated, the router must take special action. The reception of such an advertisement indicates that there are link state advertisements in the routing domain that were originated before the last time the router was restarted. In this case, the router must advance the sequence number for the advertisement one past the received sequence number, and originate a new instance of the advertisement.
Note also that if the type of the advertisement is Summary link or AS external link, the router may no longer have an (advertisable) route to the destination. In this case, the advertisement should be flushed from the routing domain by incrementing the advertisement's LS age to MaxAge and reflooding (see Section 14.1). 13.5 Sending Link State Acknowledgment packets Each newly received link state advertisement must be acknowledged. This is usually done by sending Link State Acknowledgment packets. However, acknowledgments can also be accomplished implicitly by sending Link State Update packets (see step 7a of Section 13). Many acknowledgments may be grouped together into a single Link State Acknowledgment packet. Such a packet is sent back out the interface that has received the advertisements. The packet can be sent in one of two ways: delayed and sent on an interval timer, or sent directly (as a unicast) to a particular neighbor. The particular acknowledgment strategy used depends on the circumstances surrounding the receipt of the advertisement. Sending delayed acknowledgments accomplishes several things: it facilitates the packaging of multiple acknowledgments in a single packet; it enables a single packet to indicate acknowledgments to several neighbors at once (through multicasting); and it randomizes the acknowledgment packets sent by the various routers attached to a multi- access network. The fixed interval between a router's delayed transmissions must be short (less than RxmtInterval) or needless retransmissions will ensue. Direct acknowledgments are sent to a particular neighbor in response to the receipt of duplicate link state advertisements. These acknowledgments are sent as unicasts, and are sent immediately when the duplicate is received. The precise procedure for sending Link State Acknowledgment packets is described in Table 19. The circumstances surrounding the receipt of the advertisement are listed in the left column. The acknowledgment action then taken is listed in one of the two right columns. This action depends on the state of the concerned interface; interfaces in state Backup behave differently from interfaces in all other states. Action taken in state Circumstances Backup All other states ______________________________________________________________
Action taken in state Circumstances Backup All other states ______________________________________________________________ Advertisement has No acknowledgment No acknowledgment been flooded back sent. sent. out receiving in- terface (see Sec- tion 13, step 5b). ______________________________________________________________ Advertisement is Delayed ack- Delayed ack- more recent than nowledgment sent nowledgment sent. database copy, but if advertisement was not flooded received from DR, back out receiving otherwise do noth- interface ing ______________________________________________________________ Advertisement is a Delayed ack- No acknowledgment duplicate, and was nowledgment sent sent. treated as an im- if advertisement plied acknowledg- received from DR, ment (see Section otherwise do noth- 13, step 7a). ing ______________________________________________________________ Advertisement is a Direct acknowledg- Direct acknowledg- duplicate, and was ment sent. ment sent. not treated as an implied ack- nowledgment. ______________________________________________________________ Advertisement's age Direct acknowledg- Direct acknowledg- is equal to MaxAge, ment sent. ment sent. and there is no current instance of the advertisement in the link state database (see Section 13, step 4). Table 19: Sending link state acknowledgements. Delayed acknowledgments must be delivered to all adjacent routers associated with the interface. On broadcast networks, this is accomplished by sending the delayed Link State Acknowledgment packets as multicasts. The Destination IP address used depends on the state of the interface. If the state is DR or Backup, the destination AllSPFRouters is used. In other states, the destination AllDRouters is used. On non-broadcast networks, delayed acks must be unicast separately over
each adjacency (neighbor whose state is >= Exchange). The reasoning behind sending the above packets as multicasts is best explained by an example. Consider the network configuration depicted in Figure 15. Suppose RT4 has been elected as DR, and RT3 as Backup for the network N3. When router RT4 floods a new advertisement to network N3, it is received by routers RT1, RT2, and RT3. These routers will not flood the advertisement back onto net N3, but they still must ensure that their topological databases remain synchronized with their adjacent neighbors. So RT1, RT2, and RT4 are waiting to see an acknowledgment from RT3. Likewise, RT4 and RT3 are both waiting to see acknowledgments from RT1 and RT2. This is best achieved by sending the acknowledgments as multicasts. The reason that the acknowledgment logic for Backup DRs is slightly different is because they perform differently during the flooding of link state advertisements (see Section 13.3, step 4). 13.6 Retransmitting link state advertisements Advertisements flooded out an adjacency are placed on the adjacency's Link state retransmission list. In order to ensure that flooding is reliable, these advertisements are retransmitted until they are acknowledged. The length of time between retransmissions is a configurable per-interface value, RxmtInterval. If this is set too low for an interface, needless retransmissions will ensue. If the value is set too high, the speed of the flooding, in the face of lost packets, may be affected. Several retransmitted advertisements may fit into a single Link State Update packet. When advertisements are to be retransmitted, only the number fitting in a single Link State Update packet should be transmitted. Another packet of retransmissions can be sent when some of the advertisements are acknowledged, or on the next firing of the retransmission timer. Link State Update Packets carrying retransmissions are always sent as unicasts (directly to the physical address of the neighbor). They are never sent as multicasts. Each advertisement's LS age must be incremented by InfTransDelay (which must be > 0) when copied into the outgoing packet (until the LS age field reaches its maximum value of MaxAge). If the adjacent router goes down, retransmissions may occur until the adjacency is destroyed by OSPF's Hello Protocol. When the adjacency is destroyed, the Link state retransmission list is cleared.
13.7 Receiving link state acknowledgments Many consistency checks have been made on a received Link State Acknowledgment packet before it is handed to the flooding procedure. In particular, it has been associated with a particular neighbor. If this neighbor is in a lesser state than Exchange, the packet is discarded. Otherwise, for each acknowledgment in the packet, the following steps are performed: o Does the advertisement acknowledged have an instance on the Link state retransmission list for the neighbor? If not, examine the next acknowledgment. Otherwise: o If the acknowledgment is for the same instance that is contained on the list, remove the item from the list and examine the next acknowledgment. Otherwise: o Log the questionable acknowledgment, and examine the next one. 14. Aging The Link State Database Each link state advertisement has an age field. The age is expressed in seconds. An advertisement's age field is incremented while it is contained in a router's database. Also, when copied into a Link State Update Packet for flooding out a particular interface, the advertisement's age is incremented by InfTransDelay. An advertisement's age is never incremented past the value MaxAge. Advertisements having age MaxAge are not used in the routing table calculation. As a router ages its link state database, an advertisement's age may reach MaxAge. At this time, the router must attempt to flush the advertisement from the routing domain. This is done simply by reflooding the MaxAge advertisement just as if it was a newly originated advertisement (see Section 13.3). When a Database summary list for a newly adjacent neighbor is formed, any MaxAge advertisements present in the link state database are added to the neighbor's Link state retransmission list instead of the neighbor's Database summary list. See Section 10.3 for more details. A MaxAge advertisement is removed entirely from the router's link state database when a) it is no longer contained on any neighbor Link state retransmission lists and b) none of the router's neighbors are in states Exchange or Loading.
When, in the process of aging the link state database, an advertisement's age hits a multiple of CheckAge, its checksum should be verified. If the checksum is incorrect, a program or memory error has been detected, and at the very least the router itself should be restarted. 14.1 Premature aging of advertisements A link state advertisement can be flushed from the routing domain by setting its age to MaxAge and reflooding the advertisement. This procedure follows the same course as flushing an advertisement whose age has naturally reached the value MaxAge (see Section 14). In particular, the MaxAge advertisement is removed from the router's link state database as soon as a) it is no longer contained on any neighbor Link state retransmission lists and b) none of the router's neighbors are in states Exchange or Loading. We call the setting of an advertisement's age to MaxAge premature aging. Premature aging is used when it is time for a self-originated advertisement's sequence number field to wrap. At this point, the current advertisement instance (having LS sequence number of 0x7fffffff) must be prematurely aged and flushed from the routing domain before a new instance with sequence number 0x80000001 can be originated. See Section 12.1.6 for more information. Premature aging can also be used when, for example, one of the router's previously advertised external routes is no longer reachable. In this circumstance, the router can flush its external advertisement from the routing domain via premature aging. This procedure is preferable to the alternative, which is to originate a new advertisement for the destination specifying a metric of LSInfinity. A router may only prematurely age its own (self-originated) link state advertisements. These are the link state advertisements having the router's own OSPF Router ID in the Advertising Router field. 15. Virtual Links The single backbone area (Area ID = 0) cannot be disconnected, or some areas of the Autonomous System will become unreachable. To establish/maintain connectivity of the backbone, virtual links can be configured through non-backbone areas. Virtual links serve to connect separate components of the backbone. The two endpoints of a virtual link are area border routers. The virtual link must be configured in both routers. The configuration information in each router consists of the other virtual endpoint (the other area border router), and the non-
backbone area the two routers have in common (called the transit area). Virtual links cannot be configured through stub areas (see Section 3.6). The virtual link is treated as if it were an unnumbered point-to-point network (belonging to the backbone) joining the two area border routers. An attempt is made to establish an adjacency over the virtual link. When this adjacency is established, the virtual link will be included in backbone router links advertisements, and OSPF packets pertaining to the backbone area will flow over the adjacency. Such an adjacency has been referred to as a "virtual adjacency". In each endpoint router, the cost and viability of the virtual link is discovered by examining the routing table entry for the other endpoint router. (The entry's associated area must be the configured transit area). Actually, there may be a separate routing table entry for each Type of Service. These are called the virtual link's corresponding routing table entries. The Interface Up event occurs for a virtual link when its corresponding TOS 0 routing table entry becomes reachable. Conversely, the Interface Down event occurs when its TOS 0 routing table entry becomes unreachable. In other words, the virtual link's viability is determined by the existence of an intra-area path, through the transit area, between the two endpoints. The other details concerning virtual links are as follows: o AS external links are NEVER flooded over virtual adjacencies. This would be duplication of effort, since the same AS external links are already flooded throughout the virtual link's transit area. For this same reason, AS external link advertisements are not summarized over virtual adjacencies during the database exchange process. o The cost of a virtual link is NOT configured. It is defined to be the cost of the intra-area path between the two defining area border routers. This cost appears in the virtual link's corresponding routing table entry. When the cost of a virtual link changes, a new router links advertisement should be originated for the backbone area. o Just as the virtual link's cost and viability are determined by the routing table build process (through construction of the routing table entry for the other endpoint), so are the IP interface address for the virtual interface and the virtual neighbor's IP address. These are used when sending protocol packets over the virtual link. o In each endpoint's router links advertisement for the backbone, the virtual link is represented as a link having link type 4, Link ID set to the virtual neighbor's OSPF Router ID and Link Data set to the virtual interface's IP address. See Section 12.4.1 for more information. Also, it may be the case that there is a TOS 0 path,
but no non-zero TOS paths to the other endpoint router. In this case, non-zero TOS costs must be set to LSInfinity in the router links advertisement. o When virtual links are configured for the backbone, information concerning backbone networks should not be condensed before being summarized for the transit areas. In other words, each backbone network should be advertised in a separate summary link advertisement, regardless of the backbone's configured area address ranges. See Section 12.4.3 for more information. o The time between link state retransmissions, RxmtInterval, is configured for a virtual link. This should be well over the expected round-trip delay between the two routers. This may be hard to estimate for a virtual link. It is better to err on the side of making it too large. 16. Calculation Of The Routing Table This section details the OSPF routing table calculation. Using its attached areas' link state databases as input, a router runs the following algorithm, building its routing table step by step. At each step, the router must access individual pieces of the link state databases (e.g., a router links advertisement originated by a certain router). This access is performed by the lookup function discussed in Section 12.2. The lookup process may return a link state advertisement whose LS age is equal to MaxAge. Such an advertisement should not be used in the routing table calculation, and is treated just as if the lookup process had failed. The OSPF routing table's organization is explained in Section 11. Two examples of the routing table build process are presented in Sections 11.2 and 11.3. This process can be broken into the following steps: (1) The present routing table is invalidated. The routing table is built again from scratch. The old routing table is saved so that changes in routing table entries can be identified. (2) The intra-area routes are calculated by building the shortest path tree for each attached area. In particular, all routing table entries whose Destination type is "area border router" are calculated in this step. This step is described in two parts. At first the tree is constructed by only considering those links between routers and transit networks. Then the stub networks are incorporated into the tree.
(3) The inter-area routes are calculated, through examination of summary link advertisements. If the router is attached to multiple areas (i.e., it is an area border router), only backbone summary link advertisements are examined. (4) For those routing entries whose next hop is over a virtual link, a real (physical) next hop is calculated. The real next hop will be on one of the router's directly attached networks. This step only concerns routers having configured virtual links. (5) Routes to external destinations are calculated, through examination of AS external link advertisements. The location of the AS boundary routers (which originate the AS external link advertisements) has been determined in steps 2-4. Steps 2-5 are explained in further detail below. The explanations describe the calculations for TOS 0 only. It may also be necessary to perform each step (separately) for each of the non-zero TOS values. For more information concerning the building of non-zero TOS routes see Section 16.9. Changes made to routing table entries as a result of these calculations can cause the OSPF protocol to take further actions. For example, a change to an intra-area route will cause an area border router to originate new summary link advertisements (see Section 12.4). See Section 16.7 for a complete list of the OSPF protocol actions resulting from routing table table changes. 16.1 Calculating the shortest-path tree for an area This calculation yields the set of intra-area routes associated with an area (called hereafter Area A). A router calculates the shortest-path tree using itself as the root. The formation of the shortest path tree is done here in two stages. In the first stage, only links between routers and transit networks are considered. Using the Dijkstra algorithm, a tree is formed from this subset of the link state database. In the second stage, leaves are added to the tree by considering the links to stub networks. The procedure will be explained using the graph terminology that was introduced in Section 2. The area's link state database is represented as a directed graph. The graph's vertices are routers, transit networks and stub networks. The first stage of the procedure concerns only the transit vertices (routers and transit networks) and their connecting links. Throughout the shortest path calculation, the following data is also associated with each transit vertex:
Vertex (node) ID A 32-bit number uniquely identifying the vertex. For router vertices this is the OSPF Router ID. For network vertices, this is the IP address of the network's Designated Router. A link state advertisement Each transit vertex has an associated link state advertisement. For router vertices, this is a router links advertisement. For transit networks, this is a network links advertisement (which is actually originated by the network's Designated Router). In any case, the advertisement's Link State ID is always equal to the above Vertex ID. List of next hops The list of next hops for the current shortest paths from the root to this vertex. There can be multiple shortest paths due to the equal-cost multipath capability. Each next hop indicates the outgoing router interface to use when forwarding traffic to the destination. On multi-access networks, the next hop also includes the IP address of the next router (if any) in the path towards the destination. Distance from root The link state cost of the current shortest path(s) from the root to the vertex. The link state cost of a path is calculated as the sum of the costs of the path's constituent links (as advertised in router links and network links advertisements). One path is said to be "shorter" than another if it has a smaller link state cost. The first stage of the procedure can now be summarized as follows. At each iteration of the algorithm, there is a list of candidate vertices. The shortest paths from the root to these vertices have not (necessarily) been found. The candidate vertex closest to the root is added to the shortest-path tree, removed from the candidate list, and its adjacent vertices are examined for possible addition to/modification of the candidate list. The algorithm then iterates again. It terminates when the candidate list becomes empty. The following steps describe the first stage in detail. Remember that we are computing the shortest path tree for Area A. All references to link state database lookup below are from Area A's database. (1) Initialize the algorithm's data structures. Clear the list of candidate vertices. Initialize the shortest-path tree to only the root (which is the router doing the calculation).
(2) Call the vertex just added to the tree vertex V. Examine the link state advertisement associated with vertex V. This is a lookup in the area link state database based on the Vertex ID. Each link described by the advertisement gives the cost to an adjacent vertex. For each described link, (say it joins vertex V to vertex W): (a) If this is a link to a stub network, examine the next link in V's advertisement. Links to stub networks will be considered in the second stage of the shortest path calculation. (b) Otherwise, W is a transit vertex (router or transit network). Look up the vertex W's link state advertisement (router links or network links) in Area A's link state database. If the advertisement does not exist, or its age is equal to MaxAge, or it does not have a link back to vertex V, examine the next link in V's advertisement. Both ends of a link must advertise the link before it will be used for data traffic. (c) If vertex W is already on the shortest-path tree, examine the next link in the advertisement. (d) If the cost of the link (from V to W) is LSInfinity, the link should not be used for data traffic. In this case, examine the next link in the advertisement. (e) Calculate the link state cost D of the resulting path from the root to vertex W. D is equal to the sum of the link state cost of the (already calculated) shortest path to vertex V and the advertised cost of the link between vertices V and W. If D is: o Greater than the value that already appears for vertex W on the candidate list, then examine the next link. o Equal to the value that appears for vertex W on the the candidate list, calculate the set of next hops that result from using the advertised link. Input to this calculation is the destination (W), and its parent (V). This calculation is shown in Section 16.1.1. This set of hops should be added to the next hop values that appear for W on the candidate list. o Less than the value that appears for vertex W on the the candidate list, or if W does not yet appear on the candidate list, then set the entry for W on the candidate list to indicate a distance of D from the root. Also calculate the list of next hops that result from using the advertised link, setting the next hop values for W accordingly. The next hop calculation is described in Section 16.1.1; it
takes as input the destination (W) and its parent (V). (3) If at this step the candidate list is empty, the shortest-path tree (of transit vertices) has been completely built and this stage of the algorithm terminates. Otherwise, choose the vertex belonging to the candidate list that is closest to the root, and add it to the shortest-path tree (removing it from the candidate list in the process). (4) Possibly modify the routing table. For those routing table entries modified, the associated area will be set to Area A, the path type will be set to intra-area, and the cost will be set to the newly discovered shortest path's calculated distance. If the newly added vertex is an area border router, a routing table entry is added whose destination type is "area border router". The Options field found in the associated router links advertisement is copied into the routing table entry's Optional capabilities field. If the newly added vertex is an AS boundary router, the routing table entry of type "AS boundary router" for the destination is located. Since routers can belong to more than one area, it is possible that several sets of intra-area paths exist to the AS boundary router, each set using a different area. However, the AS boundary router's routing table entry must indicate a set of paths which utilize a single area. The area leading to the routing table entry is selected as follows: A set of intra-area paths having no virtual next hops is always preferred over a set of intra-area paths in which some virtual next hops appear ; all other things being equal the set of paths having lower cost is preferred. Note that whenever an AS boundary router's routing table entry is added/modified, the Options found in the associated router links advertisement is copied into the routing table entry's Optional capabilities field. If the newly added vertex is a transit network, the routing table entry for the network is located. The entry's destination ID is the IP network number, which can be obtained by masking the Vertex ID (Link State ID) with its associated subnet mask (found in the associated network links advertisement). If the routing table entry already exists (i.e., there is already an intra-area route to the destination installed in the routing table), multiple vertices have mapped to the same IP network. For example, this can occur when a new Designated Router is being established. In this case, the current routing table entry should be overwritten if and only if the newly found path is just as short and the current routing table entry's Link State Origin has a smaller Link State ID than the newly added vertex' link state advertisement.
If there is no routing table entry for the network (the usual case), a routing table entry for the IP network should be added. The routing table entry's Link State origin should be set to the newly added vertex' link state advertisement. (5) Iterate the algorithm by returning to Step 2. The stub networks are added to the tree in the procedure's second stage. In this stage, all router vertices are again examined. Those that have been determined to be unreachable in the above first phase are discarded. For each reachable router vertex (call it V), the associated router links advertisement is found in the link state database. Each stub network link appearing in the advertisement is then examined, and the following steps are executed: (1) If the cost of the stub network link is LSInfinity, the link should not be used for data traffic. In this case, go on to examine the next stub network link in the advertisement. (2) Otherwise, Calculate the distance D of stub network from the root. D is equal to the distance from the root to the router vertex (calculated in stage 1), plus the stub network link's advertised cost. Compare this distance to the current best cost to the stub network. This is done by looking up the network's current routing table entry. If the calculated distance D is larger, go on to examine the next stub network link in the advertisement. (3) If this step is reached, the stub network's routing table entry must be updated. Calculate the set of next hops that would result from using the stub network link. This calculation is shown in Section 16.1.1; input to this calculation is the destination (the stub network) and the parent vertex (the router vertex). If the distance D is the same as the current routing table cost, simply add this set of next hops to the routing table entry's list of next hops. In this case, the routing table already has a Link State origin. If this Link State origin is a router links advertisement whose Link State ID is smaller than V's Router ID, reset the Link State origin to V's router links advertisement. Otherwise D is smaller than the routing table cost. Overwrite the current routing table entry by setting the routing table entry's cost to D, and by setting the entry's list of next hops to the newly calculated set. Set the routing table entry's Link State origin to V's router links advertisement. Then go on to examine the next stub network link.
For all routing table entries added/modified in the second stage, the associated area will be set to Area A and the path type will be set to intra-area. When the list of reachable router links is exhausted, the second stage is completed. At this time, all intra-area routes associated with Area A have been determined. The specification does not require that the above two stage method be used to calculate the shortest path tree. However, if another algorithm is used, an identical tree must be produced. For this reason, it is important to note that links between transit vertices must be bidirectional in ordered to be included in the above tree. It should also be mentioned that algorithms exist for incrementally updating the shortest-path tree (see [BBN]). 16.1.1 The next hop calculation This section explains how to calculate the current set of next hops to use for a destination. Each next hop consists of the outgoing interface to use in forwarding packets to the destination together with the next hop router (if any). The next hop calculation is invoked each time a shorter path to the destination is discovered. This can happen in either stage of the shortest-path tree calculation (see Section 16.1). In stage 1 of the shortest-path tree calculation a shorter path is found as the destination is added to the candidate list, or when the destination's entry on the candidate list is modified (Step 2e of Stage 1). In stage 2 a shorter path is discovered each time the destination's routing table entry is modified (Step 3 of Stage 2). The set of next hops to use for the destination may be recalculated several times during the shortest-path tree calculation, as shorter and shorter paths are discovered. In the end, the destination's routing table entry will always reflect the next hops resulting from the absolute shortest path(s). Input to the next hop calculation is a) the destination and b) its parent in the current shortest path between the root (the calculating router) and the destination. The parent is always a transit vertex (i.e., always a router or a transit network). If there is at least one intervening router in the current shortest path between the destination and the root, the destination simply inherits the set of next hops from the parent. Otherwise, there are two cases. In the first case, the parent vertex is the root (the calculating router itself). This means that the destination is either a directly connected network or directly connected router. The next hop in this case is simply the OSPF interface connecting to the network/router; no next hop router is required.
In the second case, the destination is a router, and its parent vertex is a network. The list of next hops is then determined by examining the destination's router links advertisement. For each link in the advertisement that points back to the parent network, the link's Link Data field provides the IP address of a next hop router. The outgoing interface to use can then be derived from the next hop IP address (or it can be inherited from the parent network). 16.2 Calculating the inter-area routes The inter-area routes are calculated by examining summary link advertisements. If the router has active attachments to multiple areas, only backbone summary link advertisements are examined. Routers attached to a single area examine that area's summary links. In either case, the summary links examined below are all part of a single area's link state database (call it Area A). Summary link advertisements are originated by the area border routers. Each summary link advertisement in Area A is considered in turn. Remember that the destination described by a summary link advertisement is either a network (type 3 summary link advertisements) or an AS boundary router (type 4 summary link advertisements). For each summary link advertisement: (1) If the cost specified by the advertisement is LSInfinity, then examine the next advertisement. (2) If the advertisement was originated by the calculating router itself, examine the next advertisement. (3) If the collection of destinations described by the summary link falls into one of the router's configured area address ranges (see Section 3.5) and the particular area address range is active, the summary link should be ignored. Active means that there are one or more reachable (by intra-area paths) networks contained in the area range. In this case, all addresses in the area range are assumed to be either reachable via intra-area paths, or else to be unreachable by any other means. (4) Else, call the destination described by the advertisement N, and the area border originating the advertisement BR. Look up the routing table entry for BR having A as its associated area. If no such entry exists for router BR (i.e., BR is unreachable in Area A), do nothing with this advertisement and consider the next in the list. Else, this advertisement describes an inter-area path to destination N, whose cost is the distance to BR plus the cost specified in the
advertisement. Call the cost of this inter-area path IAC. (5) Next, look up the routing table entry for the destination N. (The entry's Destination type is either Network or AS boundary router.) If no entry exists for N or if the entry's path type is "AS external", install the inter-area path to N, with associated area A, cost IAC, next hop equal to the list of next hops to router BR, and advertising router equal to BR. (6) Else, if the paths present in the table are intra-area paths, do nothing with the advertisement (intra-area paths are always preferred). (7) Else, the paths present in the routing table are also inter-area paths. Install the new path through BR if it is cheaper, overriding the paths in the routing table. Otherwise, if the new path is the same cost, add it to the list of paths that appear in the routing table entry. 16.3 Resolving virtual next hops This step is only necessary in area-border routers having configured virtual links. In these routers, some of the routing table entries may have virtual next hops. That is, one or more of the next hops installed in Sections 16.1 and 16.2 may be over a virtual link. However, when forwarding data traffic to a destination, the next hops must always be on a directly attached network. In this section, each virtual next hop is replaced by a real next hop. In the process a new routing table distance is calculated that may be smaller than the previously calculated distance. In this case, the list of next hops is pruned so that only those giving rise to the new shortest distance are included, and the routing table entry's distance is updated accordingly. ______________________________________ (Figure not included in text version.) Figure 17: Resolving virtual next hops ______________________________________
This resolution of virtual next hops is done only for Destination types Network or AS Boundary router. Suppose that one of a routing table entry's next hops is a virtual link. This is determined by the following combination: the routing table entry's path type is either intra-area or inter-area, the area associated with the routing table entry must be the backbone, yet the next hop belongs to a different area (the virtual link's transit area). Let N be the above entry's destination, and A the virtual link's transit area. The real next hop (and new distance) is calculated as follows. Let D be a distance counter, and set the real next hop NH to null. Then, look up all the summary link advertisements for N in area A's database, performing the following steps for each advertisement: (1) Call the border router that originated the advertisement BR. If there is no routing table entry for BR having A as associated area (i.e., BR is unreachable through Area A), examine the next advertisement. (2) Else, let X be the distance to BR via Area A. If the cost advertised by BR (call it Y) to the destination is LSInfinity, examine the next summary link advertisement. Else, the cost to destination N through area border router BR is X+Y. (3) If next hop NH is null or X+Y is smaller is smaller than D, set D to X+Y and set the next hop NH to the next hop specified in router BR's routing table entry. At this point, the real next hop NH should be set, and the distance D calculated should be less than or equal to the cost originally specified in destination N's routing table entry. This same calculation should be done for all of N's virtual next hops, and then N's new cost set to the minimum calculated distance, with the its new set of next hops that combination of non-virtual and recalculated next hops that correspond to this (possibly same as original) distance. The resolving of virtual next hops may produce unexpected results. After the virtual next hops are resolved, traffic that was originally scheduled to go over the virtual link may instead take a different path through the virtual link's transit area. In other words, virtual links allow transit traffic to be forwarded through an area, but do not dictate the precise path that the traffic will take. As an example, consider the Autonomous System pictured in Figure 17. There is a single non-backbone area (Area 1) that physically divides the backbone into two separate pieces. To maintain connectivity of the
backbone, a virtual link has been configured between routers RT1 and RT4. On the right side of the figure, network N1 belongs to the backbone. The dotted lines indicate that there is a much shorter intra-area backbone path between router RT5 and network N1 (cost 20) than there is between router RT4 and network N1 (cost 100). Both router RT4 and router RT5 will inject summary link advertisements for network N1 into Area 1. After the shortest-path tree has been calculated for the backbone, router RT1 (one end of the virtual link) will have selected router RT4 as the virtual next hop for all data traffic destined for network N1. However, since router RT5 is so much closer to network N1, all routers internal to Area 1 (e.g., routers RT2 and RT3) will forward their network N1 traffic towards router RT5, instead of RT4. And indeed, after resolving the virtual next hop by the above calculation, router RT1 will also forward network N1 traffic towards RT5. So, in this example the virtual link enables network N1 traffic to be forwarded through the transit Area 1, but the actual path the data traffic takes does not follow the virtual link. 16.4 Calculating AS external routes AS external routes are calculated by examining AS external link advertisements. Each of the AS external link advertisements is considered in turn. Most AS external advertisements describe routes to specific IP destinations. An AS external advertisement can also describe a default route for the Autonomous System (destination = DefaultDestination). For each AS external link advertisement: (1) If the cost specified by the advertisement is LSInfinity, then examine the next advertisement. (2) If the advertisement was originated by the calculating router itself, examine the next advertisement. (3) Call the destination described by the advertisement N. Look up the routing table entry for the AS boundary router (ASBR) that originated the advertisement. If no entry exists for router ASBR (i.e., ASBR is unreachable), do nothing with this advertisement and consider the next in the list. Else, this advertisement describes an AS external path to destination N. Examine the forwarding address specified in the external advertisement. This indicates the IP address to which packets for the destination should be forwarded. If forwarding address is set to 0.0.0.0, packets should be sent to the ASBR
itself. Otherwise, look up the forwarding address in the routing table. An intra-area or inter-area path must exist to the forwarding address. If no such path exists, do nothing with the advertisement and consider the next in the list. Call the routing table distance to the forwarding address X (when the forwarding address is set to 0.0.0.0, this is the distance to the ASBR itself), and the cost specified in the advertisement Y. X is in terms of the link state metric, and Y is a Type 1 or 2 external metric. (4) Next, look up the routing table entry for the destination N. If no entry exists for N, install the AS external path to N, with next hop equal to the list of next hops to the forwarding address, and advertising router equal to ASBR. If the external metric type is 1, then the path-type is set to type 1 external and the cost is equal to X+Y. If the external metric type is 2, the the path-type is set to type 2 external, the link state component of the route's cost is X, and the Type 2 cost is Y. (5) Else, if the paths present in the table are not type 1 or type 2 external paths, do nothing (AS external paths have the lowest priority). (6) Otherwise, compare the cost of this new AS external path to the ones present in the table. Type 1 external paths are always shorter than Type 2 external paths. Type 1 external paths are compared by looking at the sum of the distance to the forwarding address and the advertised Type 1 metric (X+Y). Type 2 external paths are compared by looking at the advertised Type 2 metrics, and then if necessary, the distance to the forwarding addresses. If the new path is shorter, it replaces the present paths in the routing table entry. If the new path is the same cost, it is added to the routing table entry's list of paths. 16.5 Incremental updates --- summary links When a new summary link advertisement is received, it is not necessary to recalculate the entire routing table. Call the destination described by the summary link advertisement N, and let A be the area to which the advertisement belongs. Look up the routing table entry for N. If the next hop to N is a virtual link through Area A (this means that the entry's associated area is the backbone, and the listed next hop does not belong to the backbone, but instead belongs to Area A), the real next hop must again
be resolved. This means running the algorithm in Section 16.3 for destination N only. Else, if there is an intra-area route to destination N nothing need be done (intra-area routes always take precedence). Otherwise, if Area A is the router's sole attached area, or Area A is the backbone, the procedure in Section 16.2 will have to be performed, but only for those summary link advertisements whose destination is N. Before this procedure is performed, the present routing table entry for N should be invalidated (but kept for comparison purposes). If this procedure leads to a virtual next hop, the algorithm in Section 16.3 will again have to be performed in order to calculate the real next hop. If N's routing table entry changes, and N is an AS boundary router, the AS external links will have to be reexamined (Section 16.4). 16.6 Incremental updates --- AS external links When a new AS external link advertisement is received, it is not necessary to recalculate the entire routing table. Call the destination described by the AS external link advertisement N. If there is already an intra-area or inter-area route to the destination, no recalculation is necessary (these routes take precedence). Otherwise, the procedure in Section 16.4 will have to be performed, but only for those AS external link advertisements whose destination is N. Before this procedure is performed, the present routing table entry for N should be invalidated. 16.7 Events generated as a result of routing table changes Changes to routing table entries sometimes cause the OSPF area border routers to take additional actions. These routers need to act on the following routing table changes: o The cost or path type of a routing table entry has changed. If the destination described by this entry is a Network or AS boundary router, and this is not simply a change of AS external routes, new summary link advertisements may have to be generated (potentially one for each attached area, including the backbone). See Section 12.4.3 for more information. If a previously advertised entry has been deleted, or is no longer advertisable to a particular area, the advertisement must be flushed from the routing domain by setting its age to MaxAge and reflooding (see Section 14.1).
o A routing table entry associated with a configured virtual link has changed. The destination of such a routing table entry is an area border router. The change indicates a modification to the virtual link's cost or viability. If the entry indicates that the area border router is newly reachable (via TOS 0), the corresponding virtual link is now operational. An Interface Up event should be generated for the virtual link, which will cause a virtual adjacency to begin to form (see Section 10.3). At this time the virtual interface's IP address and the virtual neighbor's IP address are also calculated. If the entry indicates that the area border router is no longer reachable (via TOS 0), the virtual link and its associated adjacency should be destroyed. This means an Interface Down event should be generated for the associated virtual link. If the cost of the entry has changed, and there is a fully established virtual adjacency, a new router links advertisement for the backbone must be originated. This in turn may cause further routing table changes. 16.8 Equal-cost multipath The OSPF protocol maintains multiple equal-cost routes to all destinations. This can be seen in the steps used above to calculate the routing table, and in the definition of the routing table structure. Each one of the multiple routes will be of the same type (intra-area, inter-area, type 1 external or type 2 external), cost, and will have the same associated area. However, each route specifies a separate next hop and advertising router. There is no requirement that a router running OSPF keep track of all possible equal-cost routes to a destination. An implementation may choose to keep only a fixed number of routes to any given destination. This does not affect any of the algorithms presented in this specification. 16.9 Building the non-zero-TOS portion of the routing table The OSPF protocol can calculate a different set of routes for each IP TOS (see Section 2.4). Support for TOS-based routing is optional. TOS-capable and non-TOS-capable routers can be mixed in an OSPF routing domain. Routers not supporting TOS calculate only the TOS 0 route to each destination. These routes are then used to forward all data
traffic, regardless of the TOS indications in the data packet's IP header. A router that does not support TOS indicates this fact to the other OSPF routers by clearing the T-bit in the Options field of its router links advertisement. The above sections detailing the routing table calculations handle the TOS 0 case only. In general, for routers supporting TOS-based routing, each piece of the routing table calculation must be rerun separately for the non-zero TOS values. When calculating routes for TOS X, only TOS X metrics can be used. Any link state advertisement may specify a separate cost for each TOS (a cost for TOS 0 must always be specified). The encoding of TOS in OSPF link state advertisements is described in Section 12.3. An advertisement can specify that it is restricted to TOS 0 (i.e., non- zero TOS is not handled) by clearing the T-bit in the link state advertisement's Option field. Such advertisements are not used when calculating routes for non-zero TOS. For this reason, it is possible that a destination is unreachable for some non-zero TOS. In this case, the TOS 0 path is used when forwarding packets (see Section 11.1). The following lists the modifications needed when running the routing table calculation for a non-zero TOS value (called TOS X). In general, routers and advertisements that do not support TOS are omitted from the calculation. Calculating the shortest-path tree (Section 16.1). Routers that do not support TOS-based routing should be omitted from the shortest-path tree calculation. These routers are identified as those having the T-bit reset in their router links advertisements. Such routers should never be added to the Dijktra algorithm's candidate list, nor should their router links advertisements be examined when adding the stub networks to the tree. Calculating the inter-area routes (Section 16.2). Inter-area paths are the concatenation of a path to an area border router with a summary link. When calculating TOS X routes, both path components must also specify TOS X. In other words, only TOS X paths to the area border router are examined, and the area border router must be advertising a TOS X route to the destination. Note that this means that summary link advertisements having the T-bit reset in their Options field are not considered. Resolving virtual next hops (Section 16.3). This calculation again considers the concatenation of a path to an area border router with a summary link. As with inter-area routes, only TOS X paths to the area border router are examined, and the
area border router must be advertising a TOS X route to the destination. Calculating AS external routes (Section 16.4). This calculation considers the concatenation of a path to a forwarding address with an AS external link. Only TOS X paths to the forwarding address are examined, and the AS boundary router must be advertising a TOS X route to the destination. Note that this means that AS external link advertisements having the T-bit reset in their Options field are not considered. In addition, the advertising AS boundary router must also be reachable for its advertisements to be considered (see Section 16.4). However, if the advertising router and the forwarding address are not one in the same, the advertising router need only be reachable via TOS 0.
The graph's vertices represent either routers, transit networks, or stub networks. Since routers may belong to multiple areas, it is not possible to color the graph's vertices. It is possible for all of a router's interfaces to be unnumbered point-to-point links. In this case, an IP address must be assigned to the router. This address will then be advertised in the router's router links advertisement as a host route. Note that in these cases both interfaces, the non-virtual and the virtual, would have the same IP address. Note that no host route is generated for, and no IP packets can be addressed to, interfaces to unnumbered point-to-point networks. This is regardless of such an interface's state. It is instructive to see what happens when the Designated Router for the network crashes. Call the Designated Router for the network RT1, and the the Backup Designated Router RT2. If router RT1 crashes (or maybe its interface to the network dies), the other routers on the network will detect RT1's absence within RouterDeadInterval seconds. All routers may not detect this at precisely the same time; the routers that detect RT1's absence before RT2 does will, for a time, select RT2 to be both Designated Router and Backup Designated Router. When RT2 detects that RT1 is gone it will move itself to Designated Router. At this time, the remaining router having highest Router Priority will be selected as Backup Designated Router. On point-to-point networks, the lower level protocols indicate whether the neighbor is up and running. Likewise, existence of the neighbor on virtual links is indicated by the routing table calculation. However, in both these cases, the Hello Protocol is still used. This ensures that communication between the neighbors is bidirectional, and that each of the neighbors has a functioning routing protocol layer. When the identity of the Designated Router is changing, it may be quite common for a neighbor in this state to send the router a Database Description packet; this means that there is some momentary disagreement on the Designated Router's identity. Note that it is possible for a router to resynchronize any of its fully established adjacencies by setting the adjacency's state back to ExStart. This will cause the other end of the adjacency to process a Seq Number Mismatch event, and therefore to also go back to ExStart state.
The address space of IP networks and the address space of OSPF Router IDs may overlap. That is, a network may have an IP address which is identical (when considered as a 32-bit number) to some router's Router ID. It is assumed that, for two different address ranges matching the destination, one range is more specific than the other. Non- contiguous subnet masks can be configured to violate this assumption. Such subnet mask configurations cannot be handled by the OSPF protocol. MaxAgeDiff is an architectural constant. It indicates the maximum dispersion of ages, in seconds, that can occur for a single link state instance as it is flooded throughout the routing domain. If two advertisements differ by more than this, they are assumed to be different instances of the same advertisement. This can occur when a router restarts and loses track of its previous sequence number. See Section 13.4 for more details. When two advertisements have different checksums, they are assumed to be separate instances. This can occur when a router restarts, and loses track of its previous sequence number. In this case, since the two advertisements have the same sequence number, it is not possible to determine which link state is actually newer. If the wrong advertisement is accepted as newer, the originating router will originate another instance. See Section 13.4 for further details. There is one instance where a lookup must be done based on partial information. This is during the routing table calculation, when a network links advertisement must be found based solely on its Link State ID. The lookup in this case is still well defined, since no two network advertisements can have the same Link State ID. This clause covers the case: Inter-area routes are not summarized to the backbone. This is because inter-area routes are always associated with the backbone area. By keeping more information in the routing table, it is possible for an implementation to recalculate the shortest path tree only for a single area. In fact, there are incremental algorithms that allow an implementation to recalculate only a portion of the shortest path tree [BBN]. These algorithms are beyond the scope of this specification. This is how the Link state request list is emptied, which eventually causes the neighbor state to transition to Full. See Section 10.9 for more details.
It should be a relatively rare occurrence for an advertisement's age to reach MaxAge. Usually, the advertisement will be replaced by a more recent instance before it ages out. Only the TOS 0 routes are important here. This is because all routing protocol packets are sent with TOS= 0. See Appendix A. It may be the case that paths to certain destinations do not vary based on TOS. For these destinations, the routing calculation need not be repeated for each TOS value. In addition, there need only be a single routing table entry for these destinations (instead of a separate entry for each TOS value). Strictly speaking, because of equal-cost multipath, the algorithm does not create a tree. We continue to use the "tree" terminology because that is what occurs most often in the existing literature. This means that before data traffic will flow between a pair of neighboring routers, their link state databases must be synchronized. Before synchronization (neighbor state < Full), a router will not include the connection to its neighbor in its link state advertisements. As a result of this clause, when a virtual link exists between the calculating router and an AS boundary router, the intra-area path through the virtual link's transit area is always preferred over the virtual link itself. Note the similarity between this procedure and the calculation of inter-area routes by a router internal to Area A. When the forwarding address is non-zero, it should point to a router belonging to another Autonomous System. See Section 12.4.4 for more details.