EIGRP Topologie Question

[barney.pause]

Hello,

In a lab, I have a router with 2 EIGRP adjacencies. Both neighbors are advertising a route for the 10.254.254.1/32 subnet. One of them has an internal route and the other is an external route (redistributed from OSPF).

With both adjacencies up, when I do a sh ip ei topo I only see the internal route:

Remote2#sh ip ei topo  10.254.254.1/32

EIGRP-IPv4 Topology Entry for AS(100)/ID(10.254.254.4) for 10.254.254.1/32

  State is Passive, Query origin flag is 1, 1 Successor(s), FD is 156160

  Descriptor Blocks:

  10.100.1.1 (FastEthernet1/1), from 10.100.1.1, Send flag is 0x0

      Composite metric is (156160/128256), route is Internal

      Vector metric:

        Minimum bandwidth is 100000 Kbit

        Total delay is 5100 microseconds

        Reliability is 255/255

        Load is 1/255

        Minimum MTU is 1500

        Hop count is 1

        Originating router is 10.254.254.1

If I shutdown the interface on which I receive the internal route and do another sh ip ei topo, I now see the external route:

Remote2#sh ip ei topo  10.254.254.1/32

EIGRP-IPv4 Topology Entry for AS(100)/ID(10.254.254.4) for 10.254.254.1/32

  State is Passive, Query origin flag is 1, 1 Successor(s), FD is 412160

  Descriptor Blocks:

  10.1.3.1 (FastEthernet1/0), from 10.1.3.1, Send flag is 0x0

      Composite metric is (412160/409600), route is External

      Vector metric:

        Minimum bandwidth is 100000 Kbit

        Total delay is 15100 microseconds

        Reliability is 255/255

        Load is 1/255

        Minimum MTU is 1500

        Hop count is 1

        Originating router is 10.1.3.1

      External data:

        AS number of route is 100

        External protocol is OSPF, external metric is 20

        Administrator tag is 0 (0x00000000)

Why the external route does no appear in the sh ip ei topo when both links are up?

Thank you

[Peter Paluch · Accepted solution]

Hi Barney,

Most probably, this happens because if the internal route is available, it is propagated to the router performing OSPF-to-EIGRP redistribution, but on that router, the EIGRP route is more preferred (the AD=90 as opposed to OSPF's AD=110) and is placed into the routing table. Therefore, the redistributing router does not redistribute this route from OSPF to EIGRP because in its routing table, the route is not learned from OSPF in the first place.

Remember that in order for a route to be redistributed from protocol A to protocol B, it must first be present in the routing table as learned by protocol A, and will always stay that way, even after redistribution.

Please feel welcome to ask further, this is is a commonly confusing topic.

Best regards,

Peter

[barney.pause]

It makes sense, thanks for the clear explanation!