Type 5 Autonomous System External LSAs

Type 5 LSAs are generated by the ASBRs. These LSAs describe routes to destinations that are external to the AS. Type 5 LSAs are flooded everywhere, with the exception of stub areas.

External links are an indication of networks outside of the OSPF routing process in the AS. These outside networks can be injected into OSPF via different sources, such as static and redistribution. The ASBR has the task of injecting these routes into an AS. Figure 3-9 illustrates the operation of Type 5 LSAs.

Figure 3-9 Type 5 LSA Operation

External AS\

\ 1 1


Type 5 LSA—1 know about those

\ / \ 1

external routes and this default. /


1 \ i \ / ^

/ /

Consider a real-world example of how a Type 5 LSA might be used. In Figure 3-9, you see an AS that has a default route to the Internet. The ASBR knows about the default route, but in order for the rest of the network to know about it, the ASBR must inject the route into an advertisement. The advertisement that carries this default route information to the rest of the AS is the Type 5 LSA.

The link-state ID is the external network number. Type 5 LSAs also inform other routers about a default route if one is configured in your network. Figure 3-10 shows the format of Type 5 LSA packets.

110 Chapter 3: OSPF Communication

Figure 3-10 Type 5 LSA Packet Structure

0 12 3 0 1234567890 1234567890 1234567890 1

Network Mask




Forwarding Address

External Route Tag



TOS Metric

Forwarding Address

External Route Tag

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