• All devices within same area must have same link state database.
    • Summarization cannot happen anywhere within an area.
  • Summarization can only occur between areas or at ASBR.
  • Internal summarization
    • Summarizes type 1 LSA into type 3 LSA.
    • ABRs
  • External summarization
    • Summarizes type 5 into type 5.
    • Summarizes type 7 into type 7.
    • Performed on ASBRs.
  • Local Discard/Null0
    • When summarizing, OSPF automatically creates local discard route.
    • If longest match IS the summary address, there’s something wrong.
      • There should always be a more specific route.
      • End result is that summary router cannot fallback to default.
      • Can be disabled with ‘no discard-route’.


In the image above we have five routers running OSPF and EIGRP. In OSPF perspective, Backbone area and area 146, with one external domain. This means there are two opportunities to summarize. Summarization can occur on the ABR or the ASBR. R5 has the following interfaces being advertised into OSPF.

These loopback IPs need to be summarized into OSPF Area 146, which can be accomplished with the following command:

This is under the router ospf process configuration, and area number is specifying what area the prefixes came from that are being summarized. On R3 we can now see that the is what’s being installed in the RIB.

Next step is doing a summarization on the ASBR via redistribution. On R1 there are the following interfaces being advertised into EIGRP AS 1.

Which can be seen in R2’s routing table.

Redistribution is the first step.

Now in R3’s routing table we’ll find E2 routes for the redistributed EIGRP routes outside of OSPF. is a transit network that can be ignored, but the routes starting in 10.100.x.x will now be summarized with the below command.

And now on R3 we’re receiving the summary /16 route for external/redistributed prefixes.

Something else to note- on the ASBR R2, the routing table has a null route for the

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