EGIRP showing "not on common subnet" for a non-neighbor router

Unanswered Question
Nov 18th, 2008

I have 4 routers connected one after another: R1 -- R2 -- R3 -- R4. EIGRP is running between R1 and R2, R2 and R3, R3 and R4. They have the same EIGRP config:

router eigrp 1

network 192.168.1.0

The neighboring interfaces are /30 as the following:

R1: 192.168.1.1/30 --- R2: 192.168.1.2/30;

R2: 192.168.1.5/30 --- R3: 192.168.1.6/30;

R3: 192.168.1.9/30 --- R4: 192.168.1.10/30;

However on the R3 router, EIGRP is showing "neighbor 192.168.1.1 no on common subnet" and "neighbor 192.168.1.2 not on common subnet".

R3 is not nighboring with those interfaces at all. What can be the issue here?

Thanks

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Richard Burts Tue, 11/18/2008 - 07:59

Gary

It would help us if you would describe the topology of the network. What kind of interfaces are these?

From the error message it seems that R3 is receiving EIGRP messages from R1 and R2. If you do show cdp neighbor on R3, what neighbors does it see?

HTH

Rick

gwhuang5398 Tue, 11/18/2008 - 12:39

Hi Rick:

Thanks. R3 is not seeing R1 as its CDP neighbor. That's what confusing me. Why the neighbor Hello between R1 and R2 has anything to do with the Hello between R2 and R3 or R3 and R4.

Here is the interface info (R1,R2 are 3845 routers,R3,R4 are L3 3750.)

R1 (Gi0/0): 192.168.1.1/30 --- R2 (Gi0/0): 192.168.1.2/30;

R2 (Gi0/1): 192.168.1.5/30 --- R3 (Int vlan 10): 192.168.1.6/30;

R3 (Int vlan 20): 192.168.1.9/30 --- R4 (int vlan 20): 192.168.1.10/30;

Thanks

Richard Burts Tue, 11/18/2008 - 13:13

Gary

Thanks for the additional information. Here are a few more questions:

- is cdp running? does R1 see R2 (and only R2) as a neighbor? Does R3 see CDP neighbors and if so what neighbors?

- what is R1 Gi0/0 physically connected to?

- what is in vlan 10 on R3?

- are you continuing to receive the error messages?

HTH

Rick

gwhuang5398 Tue, 11/18/2008 - 18:12

Hi Rick:

- CDP is running. R1 sees R2 as its EIGRP neighbor, but not CDP neighbor (the two are not directly physically connected). R3 sees R2 as its CDP neighbors. Connectivity between R3 and R4 are similar to R1 and R2.

- R1 Gi0/0 physically connected to a layer 2 switch managed by a local metro ethernet provider. The layer 2 metro ethernet link is between R1 and R2.

- On R3, int vlan 10 is only a routing interface, just having the port connected to R2.

- I have used "no eigrp log-neighbor-warning" because the error messages were too annoying. EIGRP was running fine even when messages kept showing. I'm not sure if the messages are still coming right now.

Thanks

-

l.mourits Wed, 11/19/2008 - 05:31

EIGRP uses multicast hello packets in order to communicate to other EIGRP neighbors. EIGRP generates the not on common subnet error message when it receives an EIGRP hello packet sourced from an IP address on a subnet that is not configured on its receiving interface.

In your config you configured eigrp to distribute to network 192.168.1.0/24 while your neighboring interface is configured for 192.168.1.8/30, hence a different subnet then the configured one. Obviously the multicast is forwarded, otherwise you would not see the message.

See below doc for more possible causes (allthough I think you'll be fine if you reconfigure the router eigrp to network 192.168.1.x with a 255.255.255.252 subnetmask on all three routers.

http://www.cisco.com/en/US/tech/tk365/technologies_configuration_example09186a0080093f09.shtml

HTH,

Leo

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