CCNP ROUTE Study – EIGRP Stub Routers

Stub Routers in EIGRP have two purposes:

1. to prevent a router from advertising any routes it has learnt via EIGRP to neighbouring routers
2. to limit the scope of query messages when a route goes “active”

Here’s our network (all links 100Mbps, unless stated):

Going “active” means that a Successor route to a network has gone down, and if there is no Feasible Successor (FS) route in a router’s topology table, then the router must send query packets to all it’s neighbours asking if they have a route to the network in question.

In our network, we have decided that there is no point in router WEST advertising routes to networks behind router EAST, as any such routes would be overly long and slow. For the same reason there is no point in our backbone routers querying router WEST if a route to a network behind router EAST goes “active”.

When configuring a Stub Router, we have a few options:

WEST(config-router)#eigrp stub ?
  connected      Do advertise connected routes
  leak-map       Allow dynamic prefixes based on the leak-map
  receive-only   Set IP-EIGRP as receive only neighbor
  redistributed  Do advertise redistributed routes
  static         Do advertise static routes
  summary        Do advertise summary routes

If we just hit Enter, the default is to only advertise directly connected networks and summary routes (either manual or auto). Let’s do that on router WEST:

WEST(config)#router eigrp 1
WEST(config-router)#eigrp stub 

WEST(config-router)#
*Mar  1 01:12:07.847: %DUAL-5-NBRCHANGE: IP-EIGRP(0) 1: Neighbor 192.168.1.1 (Serial0/0) is down: peer info changed
WEST(config-router)#
*Mar  1 01:12:10.563: %DUAL-5-NBRCHANGE: IP-EIGRP(0) 1: Neighbor 192.168.1.1 (Serial0/0) is up: new adjacency

We can verify the config is correct with the show ip eigrp neighbors detail command on one of our backbone routers:

BB1#show ip eigrp neighbors detail
IP-EIGRP neighbors for process 1
H   Address                 Interface       Hold Uptime   SRTT   RTO  Q  Seq
                                            (sec)         (ms)       Cnt Num
0   192.168.1.2             Se0/0             13 00:01:38    2   200  0  20
   Version 12.4/1.2, Retrans: 0, Retries: 0
   Stub Peer Advertising ( CONNECTED SUMMARY ) Routes
   Suppressing queries

As we can see, neighbouring routers now know that router WEST is a Stub router only advertising connected and summary routes, so they will not send it any query messages.

I hope this has been a useful explanation.  Thanks for reading, and good luck with your CCNP studies!

Rich

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