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Ospf abr & asbr

Discussion in 'Routing & Switching' started by Crazydave1990, Mar 25, 2013.

  1. Crazydave1990

    Crazydave1990 Bit Poster

    Yo guys - just a n00b question...

    Currently revising OSPF at the moment, and I've just learned about Area Border Routers, and Autonomous System Boundary Routers...

    Now, I know the Area Border Routers interconnect Areas - and that all ABRs must connect to Area 0 (the backbone) - I also know that each area has it's own routing topology, which is summarized by the ABR (I'm guessing because if the ABR didn't summarize, it's routing topology table would just be hench!)

    Am I right in saying, that even though all these routers have been divided into separate areas, they are all still part of the same Network?

    Also, with ASBRs, these are used to connect to other networks such as the Internet? or another network which could be running EIGRP, or RIP? Is this also correct?

    Many thanks :)
  2. cisco lab rat

    cisco lab rat Megabyte Poster

    In short Areas are filters, they control which type of LSA (OSPF Prefix info) are allowed in.

    You are correct, even though you have multiple areas this would still be a single network, i.e a single autonomous system under your administration, you have created areas to control the sort of LSA going into an Area

    For instance a Stub Area will prevent LSA Type 5's these are External LSA generated by the ASBR via Redistribution, a totally Stubby Area will prevent LSA type 3's and Type 5's and in place sticks in a Default Route

    Yes, ASBR are used to import prefixes from something else, i.e not via the Network statement or interface level command ("ip ospf "pid" area "n"), you can redistribute EIGRP, RIP, BGP, Connected, Static, etc in to OSPF and this will make the router an ASBR, but you can't have an ASBR on a STUB or Totally Stubby Areas

    Hope this help


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