EIGRP neighborship

[ranjipremandrewp]

Suppose there are two routers that run EIGRP and are interconnected by an L2 switch. Is there a way that neighborship can be formed between these routers.If yes, how?

[chrihussey]

As long as the router interfaces are connected to the switch on a common VLAN they should be able to peer.

[ranjipremandrewp]

I am sorry, but could you please let me know what should be the configuration on the switchports that connect the routers

[chrihussey]

In its simplest form the default configuration of the ports should allow it to work. Out of the box they are in VLAN 1. Provided there are no cabling issues, the router interfaces are configured in the same L3 network, once you connect the routers to the switch it should work. You could define the ports as access ports  and enable portfast to avoid the spanning tree delay and allow for immediate processing of packets:

!
interface GigabitEthernet1/0/2

  switchport mode access
 spanning-tree portfast
!

Short of that it all depends on your environment. For example if it needs to be in a dedicated VLAN across the switch or if you are trunking between the routers and switch, etc.

Your question is somewhat broad and there are multiple options so it is tough to provide specifics. 

[Leonardo Pena Davila]

A condition to EIGRP to form a neighborship  is that the interfaces that directly connect two routers are at the same ip address network. So if you have a L2 switch between these routers you have to make sure that the ports of the L2 switch and interfaces in both routers share the same ip segment. How you can accomplish this task?

1.- You have to configure the interfaces on your L2 switch in mode access to the same VLAN 

R1 (Ge0/0)----> Sw(G1/0/1 Vlan 1) Sw(G1/0/2 Vlan 1) --->R2(Ge0/0)

R1 Ge0/0 ip address 10.0.0.1/32

SwG1/0/1 swichtport mode access Vlan x

SwG1/0/2 swichtport mode access Vlan x

R1 Ge0/0 ip address 10.0.0.2/32

on your switch you have to get up the interface Vlan x.

Now if you have a SVI on your switch other than VLAN 1, you have to put the ports that connect to the routers in access mode this VLAN.

HTH

Lp

[Joseph W. Doherty]

As the others have noted, yes they can, they just need to be within the same L2 domain.

BTW, they can logically peer interconnected via multiple switches too.

This also can be done with other routing protocols.  Some, though, like OSPF make certain assumptions when you connect OSPF routers via Ethernet, whether via a direct physically link or via one, or more, switches.  With OSPF, if there are the only two (logical) devices on the L2 interconnect, you can "tell" OSPF it's a logical p2p connection, and OSPF will peer a bit differently.

[Tausif Gaddi]

If two routers are running eigrp with different AS number, Router will not communicate with each other and no entry will be found in their routing tables.