IGP criteria for advertisement/redistribution

Hi all,


Just trying to make sure I am correct in my understanding of the rules for each IGP about when a route can be advertised to a neighbor or redistributed as shown below (I am sure I have read about these somewhere but now that I am trying to double check i can't find difinitive references for each IGP so maybe someone can point me to them) . Reading the redistribution blog post http://blog.ine.com/2008/03/17/understanding-redistribution-part-iii/ I can see that in order for an EIGRP route to be advertised to a neighbor it has to be either connected or in the routing table, just being in the topology tale is not suficient.But is that also tru for redistribution or does it merely need to be in the topology table?


                        Neighbor Advertisement Criteria                      Redistribution

EIGRP :           route
in routing table                                        route is in topology table

                       or route is connected

RIP:                 route in routing table                                       route is in rip database

                       or route is connected

OSPF:              route
is in link state database                          route is in link state database






  • If you redistribute you can redistribute from connected, static or from other protocols. If the route is connected then it is always available as long as link is up. If it is static it is also available unless outgoing interface is down. If you redistribute from another protocol the route must exist in the RIB from that protocol or be a connected interface of that protocol.

    That is my understanding but as it is way too late I might not be thinking straight :)

  • Hi there,

      I think i get your point. This is related to distance vector/advance distance vector protocols(RIP,IGRP,EIGRP) and link-state protocols(OSPF,IS-IS) behavior. Let's fist consider native arouting protocols advertisments, through network commands or by enabling the protocol at interface level, for those which support. In this case, DV will advertise only best path from the topology table but ONLY if the router used it actively, meaning that path shows up in the routing table as well. Link-state, will flood the whole LSDB table (of course with LS rules, like intra-area,inter-area,external,etc), regardless if it uses the prfix actively in the routing table or not.

      When you speak about redistribution, thins are the same in all IGP's, that is when you redistribute from protocol A to protocol B, only what exists in the routing table and is learned from protocol A, ends up being also advertised in protocol B.

    Hope this is the answet you were looking for! Good luck with your studies!

  • Thanks for the answers.

    Yes I am trying to make sure my understanding of both when a neighbor will and won't advertise a route to a neighbor, and when a route will or won't be redistributed when going from one IGP to another, are solid.


    If I have an OSPF route in the link state table, but not in the routing table (because of a matching lower AD route), would this route still be redistributed to another IGP? For instance if this OSPF process was then redistributed into another OSPF process would the second OSPF process advertise this external route throughout it's routing domain (given the usual OSPF rules)?


    I suppose I should just try these things out, but I guess my origial intention was to see if there was some reference documents/blogs which summed up the various rules around this.




  • Hi,

       You can find the answer in my previoius post, but however: so at redistribution, routing protocols ONLY take what's in the routing table, so in your case the answer is NO.

    Good luck with your studies!

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