SVI with EIGRP

[Steve Coady]

Hello

I have a vlan he following config

interface Vlan505
  description NetworkServices
  no shutdown
  no ip redirects
  ip address 10.0.52.3/24
  no ip port-unreachable
  ip router eigrp My-EIGRP
  ip passive-interface eigrp My-EIGRP
  ip pim sparse-mode
  hsrp 0
    preempt delay minimum 60 reload 300
    priority 120
    timers  1  3
    ip 10.0.52.1

The 2 statements in bold above seem to contradict each other.

I am including vlan 505 in my Eigrp  process, but I won't accept or advertisements/

Why would I have both on the same SVI?

[Jon Marshall]

Steve

Apologies for being a bit vague in my answers today but I haven't used EIGRP on Nexus switches but they way I read it -

1) the first statement - "ip router eigrp My-EIGRP" starts EIGRP on that interface. It also means that the 10.0.52.0/24 subnet is advertised out

2) the second statement makes the interface passive which means no EIGRP neighbors will be formed on that interface.

However there may well be other interfaces on the switch which do form EIGRP neighborships with other devices. And these other devices would presumably need to know how to reach the 10.0.52.0/24 subnet.

That is why you have the first statement ie. you have to start EIGRP on the interface to get the 10.0.52.0/24 subnet advertised even though you are not advertising it to any neighbors reachable via the vlan 505 interface.

Does that make sense ?

Jon

[Steve Coady]

John

Thank you for the response.

If other devices need to know of this vlan, why not allow Eigrp advertise it?

It seems counter productive.

[Jon Marshall]

Steve

If other devices need to know of this vlan, why not allow Eigrp advertise it?

EIGRP will advertise it to any neighbors that are reachable via other interfaces ie. not vlan 505.

When you run EIGRP on an interface two things happen -

1) EIGRP starts sending hellos out of the interface to find neighbors

2) EIGRP includes the subnet of that interface in it's advertisements.

The passive command stops neighborships being formed but it doesn't stop the subnet being advertised into EIGRP.

Where I used it most was in the DC where we had a pair of 6500s connected via a trunk and running HSRP. So for each vlan there was an SVI on each 6500. The 6500s were also connected to a pair of WAN routers.

Now we obviously needed the vlan subnets to be advertised to the WAN routers so we needed to include them in the EIGRP configuration. But without the passive command each 6500 forms an EIGRP neighborship with the other 6500 for every vlan across the trunk link and as we had a lot of vlans that means a lot of neighborships which just weren't needed.

So we used the passive statement for the majority of the vlans/IP subnets. This meant neighborships between the SVIs for those vlans weren't formed across the trunk but the subnets were still advertised into EIGRP which they had to be.

That explanation may or may not have helped.

If it hasn't then please come back and i'll try and explain it differently.

Jon