OSPF database unnumbered interface

[Sam-CCNP]

Hi,

I was doing some labs and configured an unnumbered interface on a GRE tunnel that borrows the loopback IP. When I looked at the router LSA I saw that the point-to-point link's IP address was 0.0.0.18 and was wondering where it got it from. Other than being a public IP, I don't know anything about it. Does anyone have any ideas?

Link connected to: another Router (point-to-point)
(Link ID) Neighboring Router ID: 0.0.0.4
(Link Data) Router Interface address: 0.0.0.18
Number of MTID metrics: 0
TOS 0 Metrics: 1000

interface configuration is as follows:

!
interface Tunnel0
ip unnumbered Loopback1
ip ospf 1 area 0
tunnel source 34.1.1.3
tunnel destination 34.1.1.4
end

R3#sh ip int brief
Loopback1 33.3.3.3 YES TFTP up up
Tunnel0 33.3.3.3 YES TFTP up up

Thanks in advance for any help.

[M02@rt37 · Accepted solution]

hello @Sam-CCNP 

In your case, you configured the Tunnel0 interface as unnumbered, using the IP address of Loopback1. When OSPF advertises this link in LSAs, it uses the pseudo IP address 0.0.0.18 to represent the point-to-point link without assigning a real IP address to it.

The pseudo IP address 0.0.0.18 is just a placeholder used in OSPF LSAs to represent the unnumbered point-to-point link. OSPF routers on either end of the tunnel use the actual IP address of the Loopback1 interface to establish OSPF adjacency and exchange routing information.

This is a common behavior in OSPF when using unnumbered interfaces, and the pseudo IP address is not an actual routable IP address. It's simply a way for OSPF to identify and represent the link in LSAs without assigning a separate IP address to the point-to-point link itself.

We can find the same things on that post:

https://community.cisco.com/t5/vpn/flexvpn-and-ospf-issue/td-p/2296821

[Sam-CCNP]

Okay, that makes sense. It is a point-to-point link and there are no hosts on there, so there's no need to calculate a route to it (similar to prefix suppression). Which, I guess, is why the command cannot be applied to a multiaccess network.

[MHM Cisco World]

I will check this case

But in normal we dont do that for

vti tunnel.