Henry's Points

Briefly outline in here that OSPF has two ways to do summarization

For Internal Area: area <#> range <IP> <Mask>

The summarization has to be done on ABR. It actually said that, for area <#>, we are going to summary as this IP with this Mask and advertise it out by default. The summarization will be advertised as LSA Type 3 to the whole areas. The summarization will not suppress sub network, which is different with summary-address in OSPF and ip summary-address in RIP and EIGRP.

For External Area: summary-address <IP> <Mask>

The summarization has to be done on ASBR. (This command will do nothing if configured on ABR). It actually said that, all external route can be summrized as <IP> with <Mask>. It will be advertised to all OSPF area as LSA type 5. The advertising router is ASBR for the whole area, and most important, it will advertise back to IGP like RIP or EIGRP when doing mutual redistribution, since it will generate a discard route in the local routing table – “O    10.10.0.0/21 is a summary, 18:32:09, Null0″. The redistribution from OSPF to RIP, as an example, will do two steps.

1. show ip route ospf to see all OSPF generated route.
2. list all connected interface that advertised by OSPF

Therefore, we need route-map when advertise OSPF to RIP or EIGRP to filter out this summary route.

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