EIGRP Neighbor Not On Common Subnet ?

[bravealikhan]

Hi,

I have crated a test lab, as below scenario: 

Two Routers' interfaces are connected to common switch, with 2 different Subnets, along with a 3rd Router. All Routes are in VLAN1.

When enable EIGRP process on R1 for 10.0.1.0 & 10.0.2.0 got the message "Neighbor not on common subnet", Why ?

appreciate any response.

Thanks

[marce1000]

 - Check if this thread can help you :

          https://community.cisco.com/t5/switching/eigrp-neighbor-not-on-common-subnet-why/td-p/3815051

 M.

[luis_cordova]

Hi @bravealikhan 

https://www.cisco.com/c/en/us/support/docs/ip/enhanced-interior-gateway-routing-protocol-eigrp/13674-15.html

Regards

[bravealikhan]

Thanks for support, so following is the issue? please correct me if I'm wrong.

If EIGRP receives a hello packet that is sourced from an IP address on a subnet that is not configured on the EIGRP receiving interface, EIGRP generates this error message:

timestamp: IP-EIGRP: Neighbor neighbor_IP_address not on common subnet for interface

 

[Giuseppe Larosa]

Hello @bravealikhan ,

the issue is caused by the emulated object called L2-S, the two ports e0/0 and e0/1 should be in differente VLANs, but they are in the same Vlan, so when the router sends out an EIGRP hello that is a multicast IP packet in a multicast ethernet frame the frame is received on one port and sent out the other port to the other router's interface (this is called flooding of multicast and broadcast frames at ethernet level).

As a result of this you see the error message.

Edit:

after reviewing your initial post you have written that all routers interfaces are connected to ports in Vlan 1.

If this is the case: you need  a single subnet shared by the two routers and only one interface of the router on the left should be connected to the device L2-S if this device is a L2 only LAN switch.

Your setup would be correct if the device L2-S would be a L3 switch having three Vlans and three L3 associated interfaces and would take part in EIGRP routing.

if this is not possible the only way is to share a common IP subnet between the two routers as explained above.

Hope to help

Giuseppe

[Martin L]

in order to communicate, 2 routers must share network.  just like we have to be on the "same page" or speak same language to communicate.

you have class A subnetted into many class C networks, aka /8 into /24.

10.0.1.0 & 10.0.2.0 and 10.0.3.0 /24 are not on same network; they would be with /8

Nice for EIGRP complains about this; others don't

Regards, ML
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