ip unnumbered and eigrp on cat9k

[ayee]

Is it possible to run ip unnumbered on ptp ethernet interfaces on Cat9K's and peer with eigrp?  Links seem to peer, but no routes are advertised and eventually eigrp goodbyes are received.

I have the following configuration:

9500-1:

interface loopback 8

ip address 1.1.1.1 255.255.255.255

!

interface te1/0/1.10

encapsulation dot1q 10 native

ip unnumbered loopback 8

!

router eigrp 5

no auto-summary

network 1.0.0.0

9500-2:

interface loopback 8

ip address 1.1.1.2 255.255.255.255

!

interface te1/0/1.10

encapsulation dot1q 10 native

ip unnumbered loopback 8

!

router eigrp 5

no auto-summary

network 1.0.0.0

[Georg Pauwen]

Hello,

both neighbors are not on the same subnet. Change the /32 masks to /30 masks:

9500-1:

interface loopback 8

ip address 1.1.1.1 255.255.255.252

9500-2:

interface loopback 8

ip address 1.1.1.2 255.255.255.252

[ayee]

I thought the reasoning behind ip unnumbered (aside from saving IP space) was that you can also use them to peer different subnets?

[Georg Pauwen]

Hello,

it is a specific EIGRP requirement that neighbors have to be in the same subnet, as far as I recall.

[ayee]

Okay, so dumb question--  If I wanted to keep going and add an R3--R4--R5--etc.  would I just change the loopback to a /24 instead?

I made it as far as R3 with a 1.1.1.3/24, but the moment i added 1.1.1.4/24 it stopped peering again.

[Georg Pauwen]

Hello,

actually, it doesn't work even when the loopback are in the same subnet. The initial adjacency will be formed, but no packets are exchanged because one router does not know how to reach the loopback of the other.

I think I remember that the only solution is to add static routes for the loopbacks, which obviously defeats the purpose of using a dynamic routing protocol. In your original design, you can keep the /32 addresses and add the lines marked in bold. The adjacencies will stay up that way.

9500-1:

interface loopback 8
ip address 1.1.1.1 255.255.255.255
!
interface te1/0/1.10
encapsulation dot1q 10 native
ip unnumbered loopback 8
!
router eigrp 5
no auto-summary
network 1.0.0.0
!
--> ip route 1.1.1.2 255.255.255.255 TenGigabitEthernet1/0/1.10

9500-2:

interface loopback 8
ip address 1.1.1.2 255.255.255.255
!
interface te1/0/1.10
encapsulation dot1q 10 native
ip unnumbered loopback 8
!
router eigrp 5
no auto-summary
network 1.0.0.0
!
--> ip route 1.1.1.1 255.255.255.255 TenGigabitEthernet1/0/1.10