12.4.3. Summary links

Each summary link advertisement describes a route to a single destination. Summary link advertisements are flooded throughout a single area only. The destination described is one that is external to the area, yet still belonging to the Autonomous System.

Summary link advertisements 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:

• Only Destination Types of network and AS boundary router are advertised in summary link advertisements. If the routing table entry's Destination Type is area border router, examine the next routing table entry.

• AS external routes are never advertised in summary link advertisements. If the routing table entry has Path- type of type 1 external or type 2 external, examine the next routing table entry.

• Else, if the area associated with this set of paths is the Area A itself, do not generate a summary link advertisement for the route.[14]

• Else, if the next hops associated with this set of paths belong to Area A itself, do not generate a summary link advertisement for the route.[15] This is the logical equivalent of a Distance Vector protocol's split horizon logic.

• Else, if the routing table cost equals or exceeds the value LSInfinity, a summary link advertisement cannot be generated for this route.

• Else, if the destination of this route is an AS boundary router, generate a Type 4 link state advertisement 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. These advertisements should not be generated if Area A has been configured as a stub area.

• Else, the Destination type is network. If this is an inter-area route, generate a Type 3 advertisement 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 F for details) and metric equal to the routing table cost.

• 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 link advertisements. Remember that an area has been defined as a 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 advertisement is made for each range. When the range's status indicates Advertise, a Type 3 advertisement 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 F for details) and cost equal to the smallest cost of any of the component networks. When the range's status indicates DoNotAdvertise, the Type 3 advertisement 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 advertisement 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 F for details) and metric equal to the network's routing table cost.

  If virtual links are being used to provide/increase connectivity of the backbone, routing information concerning the backbone networks should not be condensed before being summarized into the virtual links' Transit areas. 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 links into Transit areas. The existence of virtual links is determined during the shortest path calculation for the Transit areas (see Section 16.1).

If a router advertises a summary advertisement for a destination which then becomes unreachable, the router must then flush the advertisement 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 advertisement should also be flushed from the routing domain.

For an example of summary link advertisements, 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 link advertisements. Consider in particular Router RT4. Its routing table was calculated as the example in Section 11.3. RT4 originates summary link advertisements into both the backbone and Area 1. Into the backbone, Router RT4 originates separate advertisements for each of the networks N1-N4. Into Area 1, Router RT4 originates separate advertisements for networks N6-N8 and the AS boundary routers RT5,RT7. It also condenses host routes Ia and Ib into a single summary link advertisement. Finally, the routes to networks N9,N10,N11 and Host H1 are advertised by a single summary link advertisement. This condensation was originally performed by the router RT11.

These advertisements are illustrated graphically in Figures 7 and 8. Two of the summary link advertisements originated by Router RT4 follow. The actual IP addresses for the networks and routers in question have been assigned in Figure 15.

              ; summary link advertisement for Network N1,
              ; originated by Router RT4 into the backbone

              LS age = 0                  ;always true on origination
              Options = (T-bit|E-bit)     ;TOS-capable
              LS type = 3                 ;summary link to IP net
              Link State ID = 192.1.2.0   ;N1's IP network number
              Advertising Router = 192.1.1.4       ;RT4's ID
                     TOS = 0
                     metric = 4

              ; summary link advertisement for AS boundary router RT7
              ; originated by Router RT4 into Area 1

              LS age = 0                  ;always true on origination
              Options = (T-bit|E-bit)     ;TOS-capable
              LS type = 4                 ;summary link to ASBR
              Link State ID = Router RT7's ID
              Advertising Router = 192.1.1.4       ;RT4's ID
                     TOS = 0
                     metric = 14

Summary link advertisements pertain to a single destination (IP network or AS boundary router). However, for a single destination there may be separate sets of paths, and therefore separate routing table entries, for each Type of Service. All these entries must be considered when building the summary link advertisement for the destination; a single advertisement must specify the separate costs (if they exist) for each TOS. The encoding of TOS in OSPF link state advertisements is described in Section 12.3.

Clearing the T-bit in the Options field of a summary link advertisement indicates that there is a TOS 0 path to the destination, but no paths for non-zero TOS. This can happen when non-TOS-capable routers exist in the routing domain (see Section 2.4).