EIGRP Stub Router Advertising:

Lessons Learned:

EIGRP Stub router advertisements

Reduces size of eigrp query domain

-Stub routers do not receive query messages

Process level (eigrp stub (arguments)

-arguments controls what router advertises

-defaults to connected routes and summary routes

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Stub routes in the topology are normally routes that have only upstream reachability and do not have reachability to any other destinations. Basically the edge of the routing topology.

Normally the Stub routers only have reachability to their own local networks. In case there’s a failure in the topology, the query messages that are not bound to a summary will have to be flooded throughout the topology. So if the network has 1000 routers the query will need to be send to 1000 routers.

For Stub routers – they do not need to receive query message for routes that they cannot possibly reach through any other manner. If the upstream routers loses one of its routes it will send the query message out all hops. This is where the EIGRP stub router comes in.

The idea behind the eigrp Stub router advertisement is when we for adjacency with the remote peers we advertise ourselves as an EIGRP Stub router. This will prevent the upstream neighbors for sending us query messages for prefix’s that are coming from other portions of the topology.

So during the initial adjacency establishment the Stub router will advertise itself as a Stub.

Configuration:

Before we configure the Stub routers we need to run on some debugs for the before and after.

R1#debug eigrp packets query reply

This will show queries from upstream neighbor when a route goes away. The Stub router should never be receiving these anyway.

To correct this we need to configure the edge router as a stub:

EX:

R2(config-router)#eigrp stub

We can now verify from the upstream router’s perspective with the following command.

R1#sh ip eigrp neighbors detail

IP-EIGRP neighbors for process 500

H   Address                 Interface       Hold Uptime   SRTT   RTO  Q  Seq

                                            (sec)         (ms)       Cnt Num

1   192.168.12.2            Fa0/0             13 00:01:32   34   204  0  17

   Version 12.4/1.2, Retrans: 0, Retries: 0, Prefixes: 1

   Stub Peer Advertising ( CONNECTED SUMMARY ) Routes à it is advertising itself as a stub router and connected routes that it is locally generating

   Suppressing queries

This means the if there are other routed coming from downstream below the Stub router, they will not be able to be advertised upstream form the Stub.  

Typically there should not be any routers past the stub router.

You can tell the Stub router to leak prefixes

With the default options this is now possible – the arguments by default are connected and summary. Means we can advertise our local routes and a summary route.

If we do a SH IP ROUTE form the upstream neighbor, we will still see the routes that are forwarded from Stub. This just now means the Stub router can no longer be used for transit.

We could also create a static route on the stub router and then redistribute the static in Eigrp in the stub.

It should then show up as an EIGRP internal route upstream.