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 also be
+ | +---+.....|.EGP |RTA|-----|.....+---+ +---+ |-----|RTX| | +---+ +---+ | |RTB|-----| +---+ | | +---+ | |RTC|-----| +---+ | | + Figure 16: Forwarding address example 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 link advertisements, 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 LS 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 link 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 link advertisements are not flooded into/throughout stub areas (see Section 3.6). (4) Else if the advertisement's LS 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 the receiving router itself (i.e., is considered a self-originated advertisement), the router must take special action, either updating the advertisement or in some cases flushing it from the routing domain. For a description of how self-originated advertisements are detected and subsequently handled, 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 Exchange process. In this case, restart the Database Exchange 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 Link State Update 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 its 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 has its LS age field set to MaxAge, the instance of age MaxAge is considered to be more recent. o Else, if the LS age fields of the two instances differ by more than MaxAgeDiff, the instance having the smaller (younger) LS 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 OSPF 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 new advertisement's LS type field: Router links and network links advertisements 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 advertisements The best route to the destination described by the summary link advertisement must be recalculated (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 advertisements The best route to the destination described by the AS external link advertisement must be recalculated (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 link advertisements (LS Type = 5) AS external link advertisements 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 LS 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 examine the next neighbor. o If the two copies are the same instance, then delete the advertisement from the Link state request list, and examine 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, examine the next neighbor. (d) At this point we are not positive that the 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 out the interface 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 that 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 Link State Update 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 for a router to receive self- originated link state advertisements via the flooding procedure. A self-originated advertisement is detected when either 1) the advertisement's Advertising Router is equal to the router's own Router ID or 2) the advertisement is a network links advertisement and its Link State ID is equal to one of the router's own IP interface addresses. However, if the received self-originated advertisement is newer 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 most cases, the router must then advance the advertisement's LS sequence number one past the received LS sequence number, and originate a new instance of the advertisement. It may be the case the router no longer wishes to originate the
received advertisement. Possible examples include: 1) the advertisement is a summary link or AS external link and the router no longer has an (advertisable) route to the destination, 2) the advertisement is a network links advertisement but the router is no longer Designated Router for the network or 3) the advertisement is a network links advertisement whose Link State ID is one of the router's own IP interface addresses but whose Advertising Router is not equal to the router's own Router ID (this latter case should be rare, and it indicates that the router's Router ID has changed since originating the advertisement). In all these cases, instead of updating the advertisement, the advertisement should be flushed from the routing domain by incrementing the received 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 Link State Acknowledgment packet; it enables a single Link State Acknowledgment packet to indicate acknowledgments to several neighbors at once (through multicasting); and it randomizes the Link State 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. 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 Link State Acknowledgment packets must be unicast separately over each adjacency (i.e., 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 Designated Router, and RT3 as Backup Designated Router 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
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 acknowledg- Delayed ack- more recent than ment sent if adver- nowledgment sent. database copy, but tisement received was not flooded from Designated back out receiving Router, otherwise interface do nothing _______________________________________________________________ Advertisement is a Delayed acknowledg- No acknowledgment duplicate, and was ment sent if adver- sent. treated as an im- tisement received plied acknowledg- from Designated ment (see Section Router, otherwise 13, step 7a). do nothing _______________________________________________________________ Advertisement is a Direct acknowledg- Direct acknowledg- duplicate, and was ment sent. ment sent. not treated as an implied ack- nowledgment. _______________________________________________________________ Advertisement's LS Direct acknowledg- Direct acknowledg- age is equal to ment sent. ment sent. MaxAge, 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.
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 Link State Update 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 Link State Acknowledgment packet is discarded. Otherwise, for each acknowledgment in the Link State Acknowledgment 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 LS age field. The LS age is expressed in seconds. An advertisement's LS 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 LS age is incremented by InfTransDelay. An advertisement's LS 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 LS 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 creating a Database summary list for a newly forming adjacency, 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 must be removed immediately from the router's link state database as soon as both 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 LS age hits a multiple of CheckAge, its LS checksum should be verified. If the LS 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 LS age to MaxAge and reflooding the advertisement. This procedure follows the same course as flushing an advertisement whose LS 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 LS 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. Premature aging is also be used when unexpectedly receiving self-originated advertisements during the flooding procedure (see Section 13.4). A router may only prematurely age its own self-originated link state advertisements. The router may not prematurely age advertisements that have been originated by other routers. An advertisement is considered self-originated when either 1) the advertisement's Advertising Router is equal to the router's own Router ID or 2) the advertisement is a network links advertisement and its Link State ID is equal to one of the router's own IP interface addresses. 15. Virtual Links The single backbone area (Area ID = 0.0.0.0) 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 physically 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 in this document 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 InterfaceUp event occurs for a virtual link when its corresponding TOS 0 routing table entry becomes reachable. Conversely, the InterfaceDown 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. Note that a virtual link whose underlying path has cost greater than hexadecimal 0xffff (the maximum size of an interface cost in a router links advertisement) should be considered inoperational (i.e., treated the same as if the path did not exist). 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 OSPF protocol packets over the virtual link. Note that when one (or both) of the virtual link endpoints connect to the transit area via an unnumbered point-to-point link, it may be impossible to calculate either the virtual interface's IP address and/or the virtual neighbor's IP address, thereby causing the virtual link to fail.
o In each endpoint's router links advertisement for the backbone, the virtual link is represented as a Type 4 link whose Link ID is set to the virtual neighbor's OSPF Router ID and whose Link Data is set to the virtual interface's IP address. See Section 12.4.1 for more information. Note that it may be the case that there is a TOS 0 path, but no non-zero TOS paths, between the two endpoint routers. In this case, both routers must revert to being non-TOS-capable, clearing the T-bit in the Options field of their backbone router links advertisements. 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 into the transit areas 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. During the area's shortest-path tree calculation, the area's TransitCapability is also calculated for later use in Step 4. (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) In area border routers connecting to one or more transit areas (i.e, non-backbone areas whose TransitCapability is found to be TRUE), the transit areas' summary link advertisements are examined to see whether better paths exist using the transit areas than were found in Steps 2-3 above. (5) Routes to external destinations are calculated, through examination of AS external link advertisements. The locations of the AS boundary routers (which originate the AS external link advertisements) have 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 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 router's 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 set of 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 set of shortest paths 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 (i.e., the Dijkstra algorithm) can now be summarized as follows. At each iteration of the algorithm, there is a list of candidate vertices. Paths from the root to these vertices have been found, but not necessarily the shortest ones. However, the paths to the candidate vertex that is closest to the root are guaranteed to be shortest; this vertex 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 algorithm 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). Set Area A's TransitCapability to FALSE. (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 A's link state database based on the Vertex ID. If this is a router links advertisement, and bit V of the router links advertisement (see Section A.4.2) is set, set Area A's TransitCapability to TRUE. In any case, 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 LS age is equal to MaxAge, or it does not have a link back to vertex V, examine the next link in V's advertisement. (c) If vertex W is already on the shortest-path tree, examine the next link in the advertisement.
(d) 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 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 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 procedure 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). Note that when there is a choice of vertices closest to the root, network vertices must be chosen before router vertices in order to necessarily find all equal-cost paths. This is consistent with the tie-breakers that were introduced in the modified Dijkstra algorithm used by OSPF's Multicast routing extensions (MOSPF). (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 (call it ABR), 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 in addition ABR is the endpoint of one of the calculating router's configured virtual links that uses Area A as its Transit area: the virtual link is declared up, the IP address of the virtual interface is set to the IP address of the outgoing interface calculated above for ABR, and the virtual neighbor's IP address is set to the ABR interface address (contained in ABR's router links advertisement) that points back to the root of the shortest-path tree; equivalently, this is the interface that points back to ABR's parent vertex on the shortest-path tree (similar to the calculation in Section 16.1.1). 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: The area providing the shortest path is always chosen; if more than one area provides paths with the same minimum cost, the area with the largest OSPF Area ID (when considered as an unsigned 32-bit integer) is chosen. 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 body of 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) 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 stub 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. (2) 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 more efficient algorithms exist for calculating the tree; for example, the incremental SPF algorithm described in [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 2d of Stage 1). In stage 2 a shorter path is discovered each time the destination's routing table entry is modified (Step 2 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. If the connecting OSPF interface in this case is a virtual link, the setting of the next hop should be deferred until the calculation in Section 16.3. In the second case, the parent vertex is a network that directly connects the calculating router to the destination router. 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, or if the advertisement's LS age is equal to MaxAge, then examine the 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 advertisement 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 advertisement 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 (for Type 3 summary links, N's address is obtained by masking the advertisement's Link State ID with the network/subnet mask contained in the body of the advertisement), and the area border originating the advertisement BR. Look up the routing table entry for BR having Area 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 "type 1 external" or "type 2 external", then install the inter-area path to N, with associated area 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.