EIGRP Queries- Stub Routers

EIGRP Queries occur when you lose a route to a prefix (subnet) and there is no FS (feasible successor) in the topology table. The queries are sent to all non EIGRP stub neighbors to find an alternative route for the lost prefix.

Each neighbor needs to acknowledge and reply the query messages. Once the query is sent a timer starts and after 3 minutes if the neighbor does not respond back, the neighbor is dropped and this is called SIA (Stuck in Active).

In newer IOS releases, after 90 seconds a SIA query is sent to the neighbor to acknowledge that it's still waiting to hear back from other neighbors. The bigger your network, the longer this can potentially take and convergence can be slowed down.  The router that lost the route cannot update its routing table until all devices have responded back even if a neighbor advises it about an alternative path. See how this can become a problem?

This is why it's important to have a feasible successor when possible or implement load balancing as it provides instantaneous convergence (Load Balancing). As already mention, implementing stub neighbors prevent queries from being sent to them as they don't route to other networks.

Other methods that you can use to improve convergence times when queries are sent is route summarization, the neighbor device will instantly respond back to the query with a NO message stating it does not have an alternative path because it only knows a summary of the route.

Diagram 1

How to configure EIGRP stub neighbors:

Assume all these devices are already neighbors with EIGRP defaults. On R2 you would need to change the following. No changes are required on R1 and this will reset your connection so don't do this in a real network intraday.

config t

!

router EIGRP 10

EIGRP stub

end

!

By default connected and summary are selected when you type in EIGRP stub.

R2(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

  <cr>

Before EIGRP Stub:

Note: You only see two neighbors, I did not configure the third neighbor....

R1#show ip EIGRP neighbors detail

IP-EIGRP neighbors for process 10

H   Address                 Interface       Hold Uptime   SRTT   RTO  Q  Seq

                                            (sec)         (ms)       Cnt Num

1   10.10.12.2              Fa0/1             13 00:00:14 1276  5000  0  4

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

0   10.10.10.2              Fa0/0             13 00:00:14 1281  5000  0  4

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

R1#

After EIGRP Stub:You can see it clearly states it's a stub and it's suppressing queries and what routes types are being advertise from the neighbor.

R1#show ip EIGRP neighbors detail

IP-EIGRP neighbors for process 10

H   Address                 Interface       Hold Uptime   SRTT   RTO  Q  Seq

                                            (sec)         (ms)       Cnt Num

0   10.10.10.2              Fa0/0             11 00:01:37   78   468  0  7

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

   Stub Peer Advertising ( CONNECTED SUMMARY ) Routes

   Suppressing queries

1   10.10.12.2              Fa0/1             13 00:03:00 1276  5000  0  4

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

R1#