EIGRP neighborship question

[Krishnendu AR]

I am having difficulty in understanding the EIGRP neighborship in the below given scenario:

Router#sh ip ei nei             

IP-EIGRP neighbors for process 100

H   Address                 Interface       Hold Uptime   SRTT   RTO  Q  Seq

                                            (sec)         (ms)       Cnt Num

2   10.220.96.2              Fa0/0             11 10:06:12    1   200  0  44

1   10.220.96.3              Fa0/0             10 10w1d       1  1518  0  7593

0   10.220.96.1              Fa0/0             12 21w4d       4   200  0  29787

All the neighbors are learnt from inter fa0/0

Lets have a look at neighbor of interface fa0/0

Router#sh cdp neighbors fa0/0 det

-------------------------

Device ID: Switch

Entry address(es):

  IP address: 10.220.96.1

Lets log in to 10.220.96.1 and chk its eigrp neighbors

Switch#sh ip ei nei

IP-EIGRP neighbors for process 100

H   Address                 Interface       Hold Uptime   SRTT   RTO  Q  Seq Type

                                            (sec)         (ms)       Cnt Num

0   10.220.96.3              Vl3               13 09:58:04    1  5000  0  7593

2   10.220.96.2              Vl3               13 09:59:42    1   200  0  51

4   10.220.97.5              Gi1/0/24          13 21w4d       9   200  0  149736

3   10.220.97.1              Gi1/0/23          14 21w4d       6   200  0  149737

1   10.220.96.6              Vl3               11 21w4d       1   200  0  7640

10.220.96.3 and 10.220.96.2 are 10.220.96.1's neighbors.

Im confused how these 2 (10.220.96.3 and 10.220.96.2) have become eigrp neighbors of Router thru inter fa0/0 even though its not connected via fa0/0 of Router

[Talha Ansari]

Hi,

I think all the eigrp neighbors of the router may be connected in vlan 3 of the switch ... and the switch also is a part of the eigrp AS.

Show ip eigrp neighbors will always list down the layer 3 interface... so the router reaches to the other eigrp neighbors in the same broadcast domain via the switch connected on fas 0/0 and at the same time also forms eigrp neighborship with the switch.

Regards,

Talha

[Krishnendu AR]

How can the router from a neighborship with a device passing through another L3 device. In our case its like this:

Router ----> L3 Switch---> Connected to multiple router devices

How can the router pass thru L3 Switch and from neighborship with other devices.

Krishna

[Richard Burts]

Krishna

Perhaps it will help if we remember that EIGRP neighbor formation uses multicast packets (224.0.0.10). The router is physically connected to the L3 switch but the connection is part of VLAN 3 and VLAN 3 forwards multicast packets from 10.220.96.2 and 10.220.96.3 through the layer 3 switch to the router. This is how the EIGRP neighborship is formed through the VLAN.

HTH

Rick