EIGRP Network Command

[GRANT3779]

Hi All,

If I have a Layer 3 switch with multiple SVIs - Lets say the following for example.8

Vlan18                 172.27.18.1     
Vlan19                 172.27.19.1     
Vlan20                 172.27.20.1     
Vlan21                 172.27.21.1     
Vlan22                 172.27.22.1     
Vlan23                 172.27.23.1

I have an EIGRP peering with a router (Ip address 172.27.18.2). I want to advertise the other 5 networks to this neighbor so I would add network statements for the above networks. When I do this, does this mean ALL my SVI interfaces will be sending EIGRP hellos? Does the network statement for each network, advertise the network AND "turn on" EIGRP hellos for the interfaces?

[Peter Paluch · Accepted solution]

Hello,

When I do this, does this mean ALL my SVI interfaces will be sending EIGRP hellos?

Yes, that is correct. Every interface falling under a network command in EIGRP (and in OSPF as well) will start sending and receiving Hellos and other messages of the particular routing protocol.

Does the network statement for each network, advertise the network AND "turn on" EIGRP hellos for the interfaces?

Exactly. A network command has three basic functions:

1. Allow sending of routing protocol packets over an interfaces falling under the network command
2. Allow processing of received routing protocol packets over an interface falling under the network command
3. Advertise the IP subnet on the interface falling under the network command to other routers

Of course, it does not make sense to send Hellos through those SVIs where only networks with end hosts and no routers are connected. That is why it is strongly recommended to use the passive-interface command in EIGRP for such networks to basically stop Function 1 and Function 2 above. An interface that falls under a network command but at the same time is configured as a passive interface does not send nor receive EIGRP packets. The IP subnet on that interface is still advertised to other neighbors over other non-passive interfaces, but the passive interface itself does not actively participate in EIGRP by sending and receiving EIGRP packets.

In typical enterprise networks where multiple multilayer switches have SVIs into the same set of VLANs, just one VLAN is typically designated as the inter-switch VLAN over which EIGRP or OSPF is running. Other SVIs are declared to be passive interfaces. Imagine you have 50 VLANs and 50 SVIs on two multilayer switches. It would not make sense for these two switches to establish 50 adjacencies to each other - this only adds complexity and slows down convergence while providing no real added reliability. Usually, either a specific VLAN just for the routing adjacency is created and other SVIs are configured as passive, or even a dedicated link configured as routed link (no switchport) is put between these multilayer switches and the routing protocol is run over it.

Best regards,
Peter

[Umesh Shetty]

Hi Grant ,

You are right by default the network command will advertise the subnet and start sending hellos on the interface. If you only need to advertise the subnet and do not want adjacencies you can add the passive-interface "int no" under the router eigrp process which will suppress the sending of hellos only and will also not process any incoming hellos. This is a good best practice to avoid unnecessary adjacencies on use facing vlans across the L2 Lan network.

Regards

Umesh