12.4.5. AS external links

AS external link advertisements describe routes to destinations external to the Autonomous System. Most AS external link advertisements describe routes to specific external destinations; in these cases the advertisement'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 F for details). However, a default route for the Autonomous System can be described in an AS external link advertisement by setting the advertisement's Link State ID to DefaultDestination (0.0.0.0). AS external link advertisements are originated by AS boundary routers. An AS boundary router originates a single AS external link advertisement for each external route that it has learned, either through another routing protocol (such as EGP), or through configuration information.

In general, AS external link advertisements are the only type of link state advertisements that are flooded throughout the entire Autonomous System; all other types of link state advertisements are specific to a single area. However, AS external link advertisements 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. As with summary link advertisements, if separate paths exist based on TOS, separate TOS costs can be included in the AS external link advertisement. The encoding of TOS in OSPF link state advertisements is described in Section 12.3. If the T-bit of the advertisement's Options field is clear, no non-zero TOS paths to the destination exist.

If a router advertises an AS external link 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).

For an example of AS external link advertisements, consider once again the AS pictured in Figure 6. There are two AS boundary routers: RT5 and RT7. Router RT5 originates three external link advertisements, for networks N12-N14. Router RT7 originates two external link advertisements, for networks N12 and N15. Assume that RT7 has learned its route to N12 via EGP, and that it wishes to advertise a Type 2 metric to the AS. RT7 would then originate the following advertisement for N12:

              ; AS external link advertisement for Network N12,
              ; originated by Router RT7

              LS age = 0                  ;always true on origination
              Options = (T-bit|E-bit)     ;TOS-capable
              LS type = 5                 ;indicates AS external link
              Link State ID = N12's IP network number
              Advertising Router = Router RT7's ID
                     bit E = 1            ;Type 2 metric
                     TOS = 0
                     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 EGP information with the non-OSPF router RTX. RTA must then originate AS external link advertisements for those destinations it has learned from RTX. By using the AS external link advertisement'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 EGP peer RTX. The resulting AS external link advertisement 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 = (T-bit|E-bit)          ;TOS-capable
              LS type = 5                 ;indicates AS external link
              Link State ID = DefaultDestination  ; default route
              Advertising Router = Router RTA's ID
                     bit E = 1            ;Type 2 metric
                     TOS = 0
                     metric = 1
                     Forwarding address = RTX's IP address

In figure 16, suppose instead that both RTA and RTB exchange EGP information with RTX. In this case, RTA and RTB would originate the same set of AS external link advertisements. These advertisements, 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 external advertisements, 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 advertisements (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 advertisements (i.e., same destination, cost and non-zero forwarding address), then the advertisement originated by the router having the highest OSPF Router ID is used. The router having the lower OSPF Router ID can then flush its advertisement. Flushing a link state advertisement is discussed in Section 14.1.