Peer Neighbor route

This is a useful feature for PPP links where the two ends are not on the same subnet. The most common scenario would be in the use of IP Unnumbered links (although the feature comes from the dial environment). In the scenario of two routers connected via a PPP link with nothing but a loopback (/32) and unnumbered link between them :

R4# sh ip int brief
Serial1/1 155.1.254.4 YES TFTP up up
Loopback10 155.1.254.4 YES NVRAM up up

R5#sh ip int brief
Serial1/1 155.1.254.5 YES TFTP up up
Loopback10 155.1.254.5 YES NVRAM up up

you would expect only one /32 route in each routing table, but :

R5# show ip route
155.1.0.0/32 is subnetted, 2 subnets
C 155.1.254.4 is directly connected, Serial1/1
C 155.1.254.5 is directly connected, Loopback10

R4#sh ip route
155.1.0.0/32 is subnetted, 2 subnets
C 155.1.254.4 is directly connected, Loopback10
C 155.1.254.5 is directly connected, Serial1/1

we’ve actually learnt the route from the other end via PPP. It only learns the route of the other end of the link – not remote connected networks. This behaviour is enabled by default, but can be disabled using R5(config-if)#no peer neighbor-route.

This can be debugged using deb ppp negotiation.