12.4. Originating LSAs Into any given OSPF area, a router will originate several LSAs. Each router originates a router-LSA. If the router is also the Designated Router for any of the area's networks, it will originate network-LSAs for those networks. Area border routers originate a single summary-LSA for each known inter-area destination. AS boundary routers originate a single AS- external-LSA for each known AS external destination. Destinations are advertised one at a time so that the change in any single route can be flooded without reflooding the entire collection of routes. During the flooding procedure, many LSAs can be carried by a single Link State Update packet. As an example, consider Router RT4 in Figure 6. It is an area border router, having a connection to Area 1 and the backbone. Router RT4 originates 5 distinct LSAs into the backbone (one router-LSA, and one summary-LSA for each of the networks N1-N4). Router RT4 will also originate 8 distinct LSAs into Area 1 (one router-LSA and seven summary-LSAs as pictured in Figure 7). If RT4 has been selected as Designated Router for Network N3, it will also originate a network- LSA for N3 into Area 1. In this same figure, Router RT5 will be originating 3 distinct AS- external-LSAs (one for each of the networks N12-N14). These will be flooded throughout the entire AS, assuming that none of the areas
have been configured as stubs. However, if area 3 has been configured as a stub area, the AS-external-LSAs for networks N12-N14 will not be flooded into area 3 (see Section 3.6). Instead, Router RT11 would originate a default summary- LSA that would be flooded throughout area 3 (see Section 12.4.3). This instructs all of area 3's internal routers to send their AS external traffic to RT11. Whenever a new instance of an LSA is originated, its LS sequence number is incremented, its LS age is set to 0, its LS checksum is calculated, and the LSA is added to the link state database and flooded out the appropriate interfaces. See Section 13.2 for details concerning the installation of the LSA into the link state database. See Section 13.3 for details concerning the flooding of newly originated LSAs. The ten events that can cause a new instance of an LSA to be originated are: (1) The LS age field of one of the router's self-originated LSAs reaches the value LSRefreshTime. In this case, a new instance of the LSA is originated, even though the contents of the LSA (apart from the LSA header) will be the same. This guarantees periodic originations of all LSAs. This periodic updating of LSAs adds robustness to the link state algorithm. LSAs that solely describe unreachable destinations should not be refreshed, but should instead be flushed from the routing domain (see Section 14.1). When whatever is being described by an LSA changes, a new LSA is originated. However, two instances of the same LSA may not be originated within the time period MinLSInterval. This may require that the generation of the next instance be delayed by up to MinLSInterval. The following events may cause the contents of an LSA to change. These events should cause new originations if and only if the contents of the new LSA would be different: (2) An interface's state changes (see Section 9.1). This may mean that it is necessary to produce a new instance of the router-LSA. (3) An attached network's Designated Router changes. A new router-LSA should be originated. Also, if the router itself is now the Designated Router, a new network-LSA should be produced. If the router itself is no longer the Designated Router, any network-LSA that it might have originated for the network should be flushed from the routing domain (see Section 14.1).
(4) One of the neighboring routers changes to/from the FULL state. This may mean that it is necessary to produce a new instance of the router-LSA. Also, if the router is itself the Designated Router for the attached network, a new network-LSA should be produced. The next four events concern area border routers only: (5) An intra-area route has been added/deleted/modified in the routing table. This may cause a new instance of a summary- LSA (for this route) to be originated in each attached area (possibly including the backbone). (6) An inter-area route has been added/deleted/modified in the routing table. This may cause a new instance of a summary- LSA (for this route) to be originated in each attached area (but NEVER for the backbone). (7) The router becomes newly attached to an area. The router must then originate summary-LSAs into the newly attached area for all pertinent intra-area and inter-area routes in the router's routing table. See Section 12.4.3 for more details. (8) When the state of one of the router's configured virtual links changes, it may be necessary to originate a new router-LSA into the virtual link's Transit area (see the discussion of the router-LSA's bit V in Section 12.4.1), as well as originating a new router-LSA into the backbone. The last two events concern AS boundary routers (and former AS boundary routers) only: (9) An external route gained through direct experience with an external routing protocol (like BGP) changes. This will cause an AS boundary router to originate a new instance of an AS-external-LSA. (10) A router ceases to be an AS boundary router, perhaps after restarting. In this situation the router should flush all AS-external-LSAs that it had previously originated. These LSAs can be flushed via the premature aging procedure specified in Section 14.1.
The construction of each type of LSA is explained in detail below. In general, these sections describe the contents of the LSA body (i.e., the part coming after the 20-byte LSA header). For information concerning the building of the LSA header, see Section 12.1. 12.4.1. Router-LSAs A router originates a router-LSA for each area that it belongs to. Such an LSA describes the collected states of the router's links to the area. The LSA is flooded throughout the particular area, and no further. The format of a router-LSA is shown in Appendix A (Section A.4.2). The first 20 bytes of the LSA consist of the generic LSA header that was discussed in Section 12.1. router-LSAs have LS type = 1. A router also indicates whether it is an area border router, or an AS boundary router, by setting the appropriate bits .................................... . 192.1.2 Area 1 . . + . . | . . | 3+---+1 . . N1 |--|RT1|-----+ . . | +---+ \ . . | \ _______N3 . . + \/ \ . 1+---+ . * 192.1.1 *------|RT4| . + /\_______/ . +---+ . | / | . . | 3+---+1 / | . . N2 |--|RT2|-----+ 1| . . | +---+ +---+8 . 6+---+ . | |RT3|----------------|RT6| . + +---+ . +---+ . 192.1.3 |2 . 184.108.40.206|7 . | . | . +------------+ . . 192.1.4 (N4) . .................................... Figure 15: Area 1 with IP addresses shown (bit B and bit E, respectively) in its router-LSAs. This enables paths to those types of routers to be saved in the routing table, for later processing of summary-LSAs and AS-external-LSAs. Bit B should be set whenever the router is actively attached to two or more areas, even if the router is not currently attached to the OSPF backbone
area. Bit E should never be set in a router-LSA for a stub area (stub areas cannot contain AS boundary routers). In addition, the router sets bit V in its router-LSA for Area A if and only if the router is the endpoint of one or more fully adjacent virtual links having Area A as their Transit area. The setting of bit V enables other routers in Area A to discover whether the area supports transit traffic (see TransitCapability in Section 6). The router-LSA then describes the router's working connections (i.e., interfaces or links) to the area. Each link is typed according to the kind of attached network. Each link is also labelled with its Link ID. This Link ID gives a name to the entity that is on the other end of the link. Table 18 summarizes the values used for the Type and Link ID fields. Link type Description Link ID __________________________________________________ 1 Point-to-point Neighbor Router ID link 2 Link to transit Interface address of network Designated Router 3 Link to stub IP network number network 4 Virtual link Neighbor Router ID Table 18: Link descriptions in the router-LSA. In addition, the Link Data field is specified for each link. This field gives 32 bits of extra information for the link. For links to transit networks, numbered point-to-point links and virtual links, this field specifies the IP interface address of the associated router interface (this is needed by the routing table calculation, see Section 16.1.1). For links to stub networks, this field specifies the stub network's IP address mask. For unnumbered point- to-point links, the Link Data field should be set to the unnumbered interface's MIB-II [Ref8] ifIndex value. Finally, the cost of using the link for output is specified. The output cost of a link is configurable. With the exception of links to stub networks, the output cost must always be non-zero. To further describe the process of building the list of link descriptions, suppose a router wishes to build a router-LSA for Area A. The router examines its collection of interface data structures. For each interface, the following steps are taken:
o If the attached network does not belong to Area A, no links are added to the LSA, and the next interface should be examined. o If the state of the interface is Down, no links are added. o If the state of the interface is Loopback, add a Type 3 link (stub network) as long as this is not an interface to an unnumbered point-to-point network. The Link ID should be set to the IP interface address, the Link Data set to the mask 0xffffffff (indicating a host route), and the cost set to 0. o Otherwise, the link descriptions added to the router-LSA depend on the OSPF interface type. Link descriptions used for point-to-point interfaces are specified in Section 220.127.116.11, for virtual links in Section 18.104.22.168, for broadcast and NBMA interfaces in 22.214.171.124, and for Point-to-MultiPoint interfaces in 126.96.36.199. After consideration of all the router interfaces, host links are added to the router-LSA by examining the list of attached hosts belonging to Area A. A host route is represented as a Type 3 link (stub network) whose Link ID is the host's IP address, Link Data is the mask of all ones (0xffffffff), and cost the host's configured cost (see Section C.7). 188.8.131.52. Describing point-to-point interfaces For point-to-point interfaces, one or more link descriptions are added to the router-LSA as follows: o If the neighboring router is fully adjacent, add a Type 1 link (point-to-point). The Link ID should be set to the Router ID of the neighboring router. For numbered point-to-point networks, the Link Data should specify the IP interface address. For unnumbered point-to-point networks, the Link Data field should specify the interface's MIB-II [Ref8] ifIndex value. The cost should be set to the output cost of the point-to-point interface. o In addition, as long as the state of the interface is "Point-to-Point" (and regardless of the neighboring router state), a Type 3 link (stub network) should be added. There are two forms that this stub link can take:
Option 1 Assuming that the neighboring router's IP address is known, set the Link ID of the Type 3 link to the neighbor's IP address, the Link Data to the mask 0xffffffff (indicating a host route), and the cost to the interface's configured output cost. Option 2 If a subnet has been assigned to the point-to-point link, set the Link ID of the Type 3 link to the subnet's IP address, the Link Data to the subnet's mask, and the cost to the interface's configured output cost. 184.108.40.206. Describing broadcast and NBMA interfaces For operational broadcast and NBMA interfaces, a single link description is added to the router-LSA as follows: o If the state of the interface is Waiting, add a Type 3 link (stub network) with Link ID set to the IP network number of the attached network, Link Data set to the attached network's address mask, and cost equal to the interface's configured output cost. o Else, there has been a Designated Router elected for the attached network. If the router is fully adjacent to the Designated Router, or if the router itself is Designated Router and is fully adjacent to at least one other router, add a single Type 2 link (transit network) with Link ID set to the IP interface address of the attached network's Designated Router (which may be the router itself), Link Data set to the router's own IP interface address, and cost equal to the interface's configured output cost. Otherwise, add a link as if the interface state were Waiting (see above). 220.127.116.11. Describing virtual links For virtual links, a link description is added to the router-LSA only when the virtual neighbor is fully adjacent. In this case, add a Type 4 link (virtual link) with Link ID set to the Router ID of the virtual neighbor, Link Data set to the IP interface address associated with the virtual link and cost set to the cost calculated for the virtual link during the routing table calculation (see Section 15).
18.104.22.168. Describing Point-to-MultiPoint interfaces For operational Point-to-MultiPoint interfaces, one or more link descriptions are added to the router-LSA as follows: o A single Type 3 link (stub network) is added with Link ID set to the router's own IP interface address, Link Data set to the mask 0xffffffff (indicating a host route), and cost set to 0. o For each fully adjacent neighbor associated with the interface, add an additional Type 1 link (point-to-point) with Link ID set to the Router ID of the neighboring router, Link Data set to the IP interface address and cost equal to the interface's configured output cost. 22.214.171.124. Examples of router-LSAs Consider the router-LSAs generated by Router RT3, as pictured in Figure 6. The area containing Router RT3 (Area 1) has been redrawn, with actual network addresses, in Figure 15. Assume that the last byte of all of RT3's interface addresses is 3, giving it the interface addresses 126.96.36.199 and 188.8.131.52, and that the other routers have similar addressing schemes. In addition, assume that all links are functional, and that Router IDs are assigned as the smallest IP interface address. RT3 originates two router-LSAs, one for Area 1 and one for the backbone. Assume that Router RT4 has been selected as the Designated router for network 184.108.40.206. RT3's router-LSA for Area 1 is then shown below. It indicates that RT3 has two connections to Area 1, the first a link to the transit network 220.127.116.11 and the second a link to the stub network 18.104.22.168. Note that the transit network is identified by the IP interface of its Designated Router (i.e., the Link ID = 22.214.171.124 which is the Designated Router RT4's IP interface to 126.96.36.199). Note also that RT3 has indicated that it is an area border router.
; RT3's router-LSA for Area 1 LS age = 0 ;always true on origination Options = (E-bit) ; LS type = 1 ;indicates router-LSA Link State ID = 188.8.131.52 ;RT3's Router ID Advertising Router = 184.108.40.206 ;RT3's Router ID bit E = 0 ;not an AS boundary router bit B = 1 ;area border router #links = 2 Link ID = 220.127.116.11 ;IP address of Desig. Rtr. Link Data = 18.104.22.168 ;RT3's IP interface to net Type = 2 ;connects to transit network # TOS metrics = 0 metric = 1 Link ID = 22.214.171.124 ;IP Network number Link Data = 0xffffff00 ;Network mask Type = 3 ;connects to stub network # TOS metrics = 0 metric = 2 Next RT3's router-LSA for the backbone is shown. It indicates that RT3 has a single attachment to the backbone. This attachment is via an unnumbered point-to-point link to Router RT6. RT3 has again indicated that it is an area border router. ; RT3's router-LSA for the backbone LS age = 0 ;always true on origination Options = (E-bit) ; LS type = 1 ;indicates router-LSA Link State ID = 126.96.36.199 ;RT3's router ID Advertising Router = 188.8.131.52 ;RT3's router ID bit E = 0 ;not an AS boundary router bit B = 1 ;area border router #links = 1 Link ID = 184.108.40.206 ;Neighbor's Router ID Link Data = 0.0.0.3 ;MIB-II ifIndex of P-P link Type = 1 ;connects to router # TOS metrics = 0 metric = 8
12.4.2. Network-LSAs A network-LSA is generated for every transit broadcast or NBMA network. (A transit network is a network having two or more attached routers). The network-LSA describes all the routers that are attached to the network. The Designated Router for the network originates the LSA. The Designated Router originates the LSA only if it is fully adjacent to at least one other router on the network. The network-LSA is flooded throughout the area that contains the transit network, and no further. The network-LSA lists those routers that are fully adjacent to the Designated Router; each fully adjacent router is identified by its OSPF Router ID. The Designated Router includes itself in this list. The Link State ID for a network-LSA is the IP interface address of the Designated Router. This value, masked by the network's address mask (which is also contained in the network-LSA) yields the network's IP address. A router that has formerly been the Designated Router for a network, but is no longer, should flush the network-LSA that it had previously originated. This LSA is no longer used in the routing table calculation. It is flushed by prematurely incrementing the LSA's age to MaxAge and reflooding (see Section 14.1). In addition, in those rare cases where a router's Router ID has changed, any network-LSAs that were originated with the router's previous Router ID must be flushed. Since the router may have no idea what it's previous Router ID might have been, these network-LSAs are indicated by having their Link State ID equal to one of the router's IP interface addresses and their Advertising Router equal to some value other than the router's current Router ID (see Section 13.4 for more details). 220.127.116.11. Examples of network-LSAs Again consider the area configuration in Figure 6. Network-LSAs are originated for Network N3 in Area 1, Networks N6 and N8 in Area 2, and Network N9 in Area 3. Assuming that Router RT4 has been selected as the Designated Router for Network N3, the following network-LSA is generated by RT4 on behalf of Network N3 (see Figure 15 for the address assignments):
; Network-LSA for Network N3 LS age = 0 ;always true on origination Options = (E-bit) ; LS type = 2 ;indicates network-LSA Link State ID = 18.104.22.168 ;IP address of Desig. Rtr. Advertising Router = 22.214.171.124 ;RT4's Router ID Network Mask = 0xffffff00 Attached Router = 126.96.36.199 ;Router ID Attached Router = 188.8.131.52 ;Router ID Attached Router = 184.108.40.206 ;Router ID Attached Router = 220.127.116.11 ;Router ID 12.4.3. Summary-LSAs The destination described by a summary-LSA is either an IP network, an AS boundary router or a range of IP addresses. Summary-LSAs are flooded throughout a single area only. The destination described is one that is external to the area, yet still belongs to the Autonomous System. Summary-LSAs are originated by area border routers. The precise summary routes to advertise into an area are determined by examining the routing table structure (see Section 11) in accordance with the algorithm described below. Note that only intra-area routes are advertised into the backbone, while both intra-area and inter-area routes are advertised into the other areas. To determine which routes to advertise into an attached Area A, each routing table entry is processed as follows. Remember that each routing table entry describes a set of equal-cost best paths to a particular destination: o Only Destination Types of network and AS boundary router are advertised in summary-LSAs. If the routing table entry's Destination Type is area border router, examine the next routing table entry. o AS external routes are never advertised in summary-LSAs. If the routing table entry has Path-type of type 1 external or type 2 external, examine the next routing table entry. o Else, if the area associated with this set of paths is the Area A itself, do not generate a summary-LSA for the route.
o Else, if the next hops associated with this set of paths belong to Area A itself, do not generate a summary-LSA for the route. This is the logical equivalent of a Distance Vector protocol's split horizon logic. o Else, if the routing table cost equals or exceeds the value LSInfinity, a summary-LSA cannot be generated for this route. o Else, if the destination of this route is an AS boundary router, a summary-LSA should be originated if and only if the routing table entry describes the preferred path to the AS boundary router (see Step 3 of Section 16.4). If so, a Type 4 summary-LSA is originated for the destination, with Link State ID equal to the AS boundary router's Router ID and metric equal to the routing table entry's cost. Note: these LSAs should not be generated if Area A has been configured as a stub area. o Else, the Destination type is network. If this is an inter-area route, generate a Type 3 summary-LSA for the destination, with Link State ID equal to the network's address (if necessary, the Link State ID can also have one or more of the network's host bits set; see Appendix E for details) and metric equal to the routing table cost. o The one remaining case is an intra-area route to a network. This means that the network is contained in one of the router's directly attached areas. In general, this information must be condensed before appearing in summary-LSAs. Remember that an area has a configured list of address ranges, each range consisting of an [address,mask] pair and a status indication of either Advertise or DoNotAdvertise. At most a single Type 3 summary-LSA is originated for each range. When the range's status indicates Advertise, a Type 3 summary-LSA is generated with Link State ID equal to the range's address (if necessary, the Link State ID can also have one or more of the range's "host" bits set; see Appendix E for details) and cost equal to the largest cost of any of the component networks. When the range's status indicates DoNotAdvertise, the Type 3 summary-LSA is suppressed and the component networks remain hidden from other areas. By default, if a network is not contained in any explicitly configured address range, a Type 3 summary-LSA is generated with Link State ID equal to the network's address (if necessary, the Link State ID can also have one or more of the network's "host" bits set; see Appendix E for details) and metric equal to the network's routing table cost.
If an area is capable of carrying transit traffic (i.e., its TransitCapability is set to TRUE), routing information concerning backbone networks should not be condensed before being summarized into the area. Nor should the advertisement of backbone networks into transit areas be suppressed. In other words, the backbone's configured ranges should be ignored when originating summary-LSAs into transit areas. If a router advertises a summary-LSA for a destination which then becomes unreachable, the router must then flush the LSA from the routing domain by setting its age to MaxAge and reflooding (see Section 14.1). Also, if the destination is still reachable, yet can no longer be advertised according to the above procedure (e.g., it is now an inter-area route, when it used to be an intra-area route associated with some non-backbone area; it would thus no longer be advertisable to the backbone), the LSA should also be flushed from the routing domain. 18.104.22.168. Originating summary-LSAs into stub areas The algorithm in Section 12.4.3 is optional when Area A is an OSPF stub area. Area border routers connecting to a stub area can originate summary-LSAs into the area according to the Section 12.4.3's algorithm, or can choose to originate only a subset of the summary-LSAs, possibly under configuration control. The fewer LSAs originated, the smaller the stub area's link state database, further reducing the demands on its routers' resources. However, omitting LSAs may also lead to sub-optimal inter-area routing, although routing will continue to function. As specified in Section 12.4.3, Type 4 summary-LSAs (ASBR-summary- LSAs) are never originated into stub areas. In a stub area, instead of importing external routes each area border router originates a "default summary-LSA" into the area. The Link State ID for the default summary-LSA is set to DefaultDestination, and the metric set to the (per-area) configurable parameter StubDefaultCost. Note that StubDefaultCost need not be configured identically in all of the stub area's area border routers. 22.214.171.124. Examples of summary-LSAs Consider again the area configuration in Figure 6. Routers RT3, RT4, RT7, RT10 and RT11 are all area border routers, and therefore are originating summary-LSAs. Consider in particular Router RT4. Its routing table was calculated as the example in Section 11.3. RT4 originates summary-LSAs into both the backbone and Area 1. Into the backbone, Router RT4 originates separate LSAs for each of the
networks N1-N4. Into Area 1, Router RT4 originates separate LSAs for networks N6-N8 and the AS boundary routers RT5,RT7. It also condenses host routes Ia and Ib into a single summary-LSA. Finally, the routes to networks N9,N10,N11 and Host H1 are advertised by a single summary-LSA. This condensation was originally performed by the router RT11. These LSAs are illustrated graphically in Figures 7 and 8. Two of the summary-LSAs originated by Router RT4 follow. The actual IP addresses for the networks and routers in question have been assigned in Figure 15. ; Summary-LSA for Network N1, ; originated by Router RT4 into the backbone LS age = 0 ;always true on origination Options = (E-bit) ; LS type = 3 ;Type 3 summary-LSA Link State ID = 126.96.36.199 ;N1's IP network number Advertising Router = 188.8.131.52 ;RT4's ID metric = 4 ; Summary-LSA for AS boundary router RT7 ; originated by Router RT4 into Area 1 LS age = 0 ;always true on origination Options = (E-bit) ; LS type = 4 ;Type 4 summary-LSA Link State ID = Router RT7's ID Advertising Router = 184.108.40.206 ;RT4's ID metric = 14 12.4.4. AS-external-LSAs AS-external-LSAs describe routes to destinations external to the Autonomous System. Most AS-external-LSAs describe routes to specific external destinations; in these cases the LSA's Link State ID is set to the destination network's IP address (if necessary, the Link State ID can also have one or more of the network's "host" bits set; see Appendix E for details). However, a default route for the Autonomous System can be described in an AS-external-LSA by setting the LSA's Link State ID to DefaultDestination (0.0.0.0). AS-external-LSAs are originated by AS boundary routers. An AS boundary router originates a single AS-external-LSA for each external route that it has learned, either through another routing protocol (such as BGP), or through configuration information.
AS-external-LSAs are the only type of LSAs that are flooded throughout the entire Autonomous System; all other types of LSAs are specific to a single area. However, AS-external-LSAs are not flooded into/throughout stub areas (see Section 3.6). This enables a reduction in link state database size for routers internal to stub areas. The metric that is advertised for an external route can be one of two types. Type 1 metrics are comparable to the link state metric. Type 2 metrics are assumed to be larger than the cost of any intra-AS path. If a router advertises an AS-external-LSA for a destination which then becomes unreachable, the router must then flush the LSA from the routing domain by setting its age to MaxAge and reflooding (see Section 14.1). 220.127.116.11. Examples of AS-external-LSAs Consider once again the AS pictured in Figure 6. There are two AS boundary routers: RT5 and RT7. Router RT5 originates three AS- external-LSAs, for networks N12-N14. Router RT7 originates two AS- external-LSAs, for networks N12 and N15. Assume that RT7 has learned its route to N12 via BGP, and that it wishes to advertise a Type 2 metric to the AS. RT7 would then originate the following LSA for N12: ; AS-external-LSA for Network N12, ; originated by Router RT7 LS age = 0 ;always true on origination Options = (E-bit) ; LS type = 5 ;AS-external-LSA Link State ID = N12's IP network number Advertising Router = Router RT7's ID bit E = 1 ;Type 2 metric metric = 2 Forwarding address = 0.0.0.0 In the above example, the forwarding address field has been set to 0.0.0.0, indicating that packets for the external destination should be forwarded to the advertising OSPF router (RT7). This is not always desirable. Consider the example pictured in Figure 16. There are three OSPF routers (RTA, RTB and RTC) connected to a common network. Only one of these routers, RTA, is exchanging BGP information with the non-OSPF router RTX. RTA must then originate AS- external-LSAs for those destinations it has learned from RTX. By using the AS- external-LSA's forwarding address field, RTA can specify that packets
for these destinations be forwarded directly to RTX. Without this feature, Routers RTB and RTC would take an extra hop to get to these destinations. Note that when the forwarding address field is non-zero, it should point to a router belonging to another Autonomous System. A forwarding address can also be specified for the default route. For example, in figure 16 RTA may want to specify that all externally- destined packets should by default be forwarded to its BGP peer RTX. The resulting AS-external-LSA is pictured below. Note that the Link State ID is set to DefaultDestination. ; Default route, originated by Router RTA ; Packets forwarded through RTX LS age = 0 ;always true on origination Options = (E-bit) ; LS type = 5 ;AS-external-LSA Link State ID = DefaultDestination ; default route Advertising Router = Router RTA's ID bit E = 1 ;Type 2 metric metric = 1 Forwarding address = RTX's IP address In figure 16, suppose instead that both RTA and RTB exchange BGP information with RTX. In this case, RTA and RTB would originate the same set of AS-external-LSAs. These LSAs, if they specify the same metric, would be functionally equivalent since they would specify the same destination and forwarding address (RTX). This leads to a clear duplication of effort. If only one of RTA or RTB originated the set of AS-external-LSAs, the routing would remain the same, and the size of the link state database would decrease. However, it must be unambiguously defined as to which router originates the LSAs (otherwise neither may, or the identity of the originator may oscillate). The following rule is thereby established: if two routers, both reachable from one another, originate functionally equivalent AS-external-LSAs (i.e., same destination, cost and non- zero forwarding address), then the LSA originated by the router having the highest OSPF Router ID is used. The router having the lower OSPF Router ID can then flush its LSA. Flushing an LSA is discussed in Section 14.1. 13. The Flooding Procedure Link State Update packets provide the mechanism for flooding LSAs. A Link State Update packet may contain several distinct LSAs, and floods each LSA one hop further from its point of origination. To
make the flooding procedure reliable, each LSA must be acknowledged separately. Acknowledgments are transmitted in Link State Acknowledgment packets. Many separate acknowledgments can also 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. + | +---+.....|.BGP |RTA|-----|.....+---+ +---+ |-----|RTX| | +---+ +---+ | |RTB|-----| +---+ | | +---+ | |RTC|-----| +---+ | | + Figure 16: Forwarding address example All types of LSAs, other than AS-external-LSAs, are associated with a specific area. However, LSAs do not contain an area field. An LSA's area must be deduced from the Link State Update packet header. For each LSA contained in a Link State Update packet, the following steps are taken: (1) Validate the LSA's LS checksum. If the checksum turns out to be invalid, discard the LSA and get the next one from the Link State Update packet. (2) Examine the LSA's LS type. If the LS type is unknown, discard the LSA 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 an AS-external-LSA (LS type = 5), and the area
has been configured as a stub area, discard the LSA and get the next one from the Link State Update Packet. AS-external-LSAs are not flooded into/throughout stub areas (see Section 3.6). (4) Else if the LSA's LS age is equal to MaxAge, and there is currently no instance of the LSA in the router's link state database, then take the following actions: (a) Acknowledge the receipt of the LSA 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 LSA 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 LSA in the link state database. Otherwise, simply discard the LSA. In either case, examine the next LSA (if any) listed in the Link State Update packet. (5) Otherwise, find the instance of this LSA that is currently contained in the router's link state database. If there is no database copy, or the received LSA is more recent than the database copy (see Section 13.1 below for the determination of which LSA 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 MinLSArrival seconds ago, discard the new LSA (without acknowledging it) and examine the next LSA (if any) listed in the Link State Update packet. (b) Otherwise immediately flood the new LSA 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 LSA was received from a router other than the Backup DR) the LSA 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 LSA 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 LSA with the current time (i.e., the time it was received). The flooding procedure cannot overwrite the newly installed LSA until MinLSArrival seconds have elapsed. The LSA installation process is discussed further in Section 13.2. (e) Possibly acknowledge the receipt of the LSA by sending a Link State Acknowledgment packet back out the receiving interface. This is explained below in Section 13.5. (f) If this new LSA indicates that it was originated by the receiving router itself (i.e., is considered a self- originated LSA), the router must take special action, either updating the LSA or in some cases flushing it from the routing domain. For a description of how self-originated LSAs are detected and subsequently handled, see Section 13.4. (6) Else, if there is an instance of the LSA 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 LSA 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 LSA is listed in the Link state retransmission list for the receiving adjacency, the router itself is expecting an acknowledgment for this LSA. The router should treat the received LSA as an acknowledgment by removing the LSA 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 LSA 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. If the database copy has LS age equal to MaxAge and LS sequence number equal to MaxSequenceNumber, simply discard the received LSA without acknowledging it. (In this case, the LSA's LS sequence number is wrapping, and the MaxSequenceNumber LSA must be completely flushed before any new LSA instance can be introduced). Otherwise, send the database copy back to the sending neighbor,
encapsulated within a Link State Update Packet. The Link State Update Packet should be unicast to the neighbor. In so doing, do not put the database copy of the LSA on the neighbor's link state retransmission list, and do not acknowledge the received (less recent) LSA instance. 13.1. Determining which LSA is newer When a router encounters two instances of an LSA, it must determine which is more recent. This occurred above when comparing a received LSA to its database copy. This comparison must also be done during the Database Exchange procedure which occurs during adjacency bring- up. An LSA is identified by its LS type, Link State ID and Advertising Router. For two instances of the same LSA, the LS sequence number, LS age, and LS checksum fields are used to determine which instance is more recent: o The LSA 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 LSAs in the database Installing a new LSA in the database, either as the result of flooding or a newly self-originated LSA, may cause the OSPF routing table structure to be recalculated. The contents of the new LSA should be compared to the old instance, if present. If there is no difference, there is no need to recalculate the routing table. When comparing an LSA to its previous instance, the following are all considered to be differences in contents: o The LSA's Options field has changed. o One of the LSA instances has LS age set to MaxAge, and the other does not. o The length field in the LSA header has changed. o The body of the LSA (i.e., anything outside the 20-byte LSA header) has changed. Note that this excludes changes in LS Sequence Number and LS Checksum. If the contents are different, the following pieces of the routing table must be recalculated, depending on the new LSA's LS type field: Router-LSAs and network-LSAs The entire routing table must be recalculated, starting with the shortest path calculations for each area (not just the area whose link-state 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-LSAs The best route to the destination described by the summary-LSA 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-LSAs. AS-external-LSAs The best route to the destination described by the AS-external-LSA must be recalculated (see Section 16.6). Also, any old instance of the LSA must be removed from the database when the new LSA 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) LSA 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 LSA 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 LSA that the router itself has just originated (see Section 12.4). For these LSAs, this section provides the entirety of the flooding procedure (i.e., the processing of Section 13 is not performed, since, for example, the LSA has not been received from a neighbor and therefore does not need to be acknowledged). Depending upon the LSA's LS type, the LSA can be flooded out only certain interfaces. These interfaces, defined by the following, are called the eligible interfaces: AS-external-LSAs (LS Type = 5) AS-external-LSAs 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 an LSA out a particular interface, if there is a high probability that the attached neighbors have already received the LSA. However, in these cases the flooding procedure must be absolutely sure that the neighbors eventually do receive the LSA, so the LSA 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 LSA. 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 LSA on the list, it indicates that the neighboring router has an instance of the LSA already. Compare the new LSA to the neighbor's copy: o If the new LSA is less recent, then examine the next neighbor. o If the two copies are the same instance, then delete the LSA from the Link state request list, and examine the next neighbor. o Else, the new LSA is more recent. Delete the LSA from the Link state request list. (c) If the new LSA 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 LSA. Add the new LSA to the Link state retransmission list for the adjacency. This ensures that the flooding procedure is reliable; the LSA will be retransmitted at intervals until an acknowledgment is seen from the neighbor. (2) The router must now decide whether to flood the new LSA out this interface. If in the previous step, the LSA was NOT added to any of the Link state retransmission lists, there is no need to flood the LSA out the interface and the next interface should be examined. (3) If the new LSA 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 LSA already. Therefore, examine the next interface.
(4) If the new LSA 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. However, if the Designated Router fails the router (i.e., the Backup Designated Router) will end up retransmitting the updates. (5) If this step is reached, the LSA must be flooded out the interface. Send a Link State Update packet (including the new LSA as contents) out the interface. The LSA's LS age must be incremented by InfTransDelay (which must be > 0) when it is copied into the outgoing Link State Update packet (until the LS age field reaches the 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 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 LSAs It is a common occurrence for a router to receive self-originated LSAs via the flooding procedure. A self-originated LSA is detected when either 1) the LSA's Advertising Router is equal to the router's own Router ID or 2) the LSA is a network-LSA and its Link State ID is equal to one of the router's own IP interface addresses. However, if the received self-originated LSA is newer than the last instance that the router actually originated, the router must take special action. The reception of such an LSA indicates that there are LSAs in the routing domain that were originated by the router before the last time it was restarted. In most cases, the router must then advance the LSA's LS sequence number one past the received LS sequence number, and originate a new instance of the LSA. It may be the case the router no longer wishes to originate the received LSA. Possible examples include: 1) the LSA is a summary-LSA or AS-external-LSA and the router no longer has an (advertisable)
route to the destination, 2) the LSA is a network-LSA but the router is no longer Designated Router for the network or 3) the LSA is a network-LSA 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 LSA). In all these cases, instead of updating the LSA, the LSA should be flushed from the routing domain by incrementing the received LSA's LS age to MaxAge and reflooding (see Section 14.1). 13.5. Sending Link State Acknowledgment packets Each newly received LSA 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 which received the LSAs. 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 LSA. Sending delayed acknowledgments accomplishes several things: 1) it facilitates the packaging of multiple acknowledgments in a single Link State Acknowledgment packet, 2) it enables a single Link State Acknowledgment packet to indicate acknowledgments to several neighbors at once (through multicasting) and 3) it randomizes the Link State Acknowledgment packets sent by the various routers attached to a common 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 LSAs. 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 LSA 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 interface state is DR or Backup, the destination AllSPFRouters is used. In all 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). Action taken in state Circumstances Backup All other states _______________________________________________________________ LSA has No acknowledgment No acknowledgment been flooded back sent. sent. out receiving in- terface (see Sec- tion 13, step 5b). _______________________________________________________________ LSA 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 _______________________________________________________________ LSA 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 _______________________________________________________________ LSA is a Direct acknowledg- Direct acknowledg- duplicate, and was ment sent. ment sent. not treated as an implied ack- nowledgment. _______________________________________________________________ LSA's LS Direct acknowledg- Direct acknowledg- age is equal to ment sent. ment sent. MaxAge, and there is no current instance of the LSA in the link state database (see Section 13, step 4). Table 19: Sending link state acknowledgments.
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 LSA to Network N3, it is received by routers RT1, RT2, and RT3. These routers will not flood the LSA back onto net N3, but they still must ensure that their link-state 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 LSAs (see Section 13.3, step 4). 13.6. Retransmitting LSAs LSAs flooded out an adjacency are placed on the adjacency's Link state retransmission list. In order to ensure that flooding is reliable, these LSAs 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 LSAs may fit into a single Link State Update packet. When LSAs are to be retransmitted, only the number fitting in a single Link State Update packet should be sent. Another packet of retransmissions can be sent whenever some of the LSAs 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 LSA's LS age must be incremented by InfTransDelay (which must be > 0) when it is copied into the outgoing Link State Update packet (until the LS age field reaches the maximum value of MaxAge). If an 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 LSA 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 LSA has an LS age field. The LS age is expressed in seconds. An LSA'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 LSA's LS age is incremented by InfTransDelay. An LSA's LS age is never incremented past the value MaxAge. LSAs having age MaxAge are not used in the routing table calculation. As a router ages its link state database, an LSA's LS age may reach MaxAge. At this time, the router must attempt to flush the LSA from the routing domain. This is done simply by reflooding the MaxAge LSA just as if it was a newly originated LSA (see Section 13.3). When creating a Database summary list for a newly forming adjacency, any MaxAge LSAs 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 LSA 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 LSA'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 LSAs An LSA can be flushed from the routing domain by setting its LS age to MaxAge and reflooding the LSA. This procedure follows the same course as flushing an LSA whose LS age has naturally reached the value MaxAge (see Section 14). In particular, the MaxAge LSA 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 LSA's LS age to MaxAge "premature aging". Premature aging is used when it is time for a self-originated LSA's sequence number field to wrap. At this point, the current LSA instance (having LS sequence number MaxSequenceNumber) must be prematurely aged and flushed from the routing domain before a new instance with sequence number equal to InitialSequenceNumber 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 AS- external-LSA from the routing domain via premature aging. This procedure is preferable to the alternative, which is to originate a new LSA for the destination specifying a metric of LSInfinity. Premature aging is also be used when unexpectedly receiving self- originated LSAs during the flooding procedure (see Section 13.4). A router may only prematurely age its own self-originated LSAs. The router may not prematurely age LSAs that have been originated by other routers. An LSA is considered self- originated when either 1) the LSA's Advertising Router is equal to the router's own Router ID or 2) the LSA is a network-LSA 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 and 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-LSAs, 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). This is called the virtual link's corresponding routing table entry. The InterfaceUp event occurs for a virtual link when its corresponding routing table entry becomes reachable. Conversely, the InterfaceDown event occurs when its 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-LSA) 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-LSAs are NEVER flooded over virtual adjacencies. This would be duplication of effort, since the same AS-external-LSAs are already flooded throughout the virtual link's Transit area. For this same reason, AS-external-LSAs 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-LSA 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-LSA 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. o A non-backbone area can carry transit data traffic (i.e., is considered a "transit area") if and only if it serves as the Transit area for one or more fully adjacent virtual links (see TransitCapability in Sections 6 and 16.1). Such an area requires special treatment when summarizing backbone networks into it (see Section 12.4.3), and during the routing calculation (see Section 16.3). 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.