EIGRP and PPP encapsulation

[fsoffia]

hallo,

I configured EIGRP in a network of routers interconnected by leased lines. On serial interfaces I used encapsulation PPP and not default HDLC.

Doing "sh ip route" I see on some routers the routes /32 about the ip addresses on the PPP links.

If I use HDLC this behaviour disappear.

Some one can explain me why ?

best regards

francesco

[Harold Ritter]

This is normal behavior for a ppp interface to install a host route for its peer address in its RIB. Configure "no ppp neighbor-route" if you don't want it to be installed.

Hope this helps,

[Harold Ritter]

Oups, the command is actually "no peer neighbor-route" not "ppp neighbor-route".

Hope this helps,

[itsnavee4]

hi guyz,

im not sure if you ppl are still alive on this community

but i have a question,

why wud you need "no peer neighbor-route" command?

extra /32 routes can cause any problem?

[Peter Paluch]

Hello,

The /32 route present with PPP is used to set up routing towards the PPP neighbor in the case that the neighbor is not on the same subnet as the local router. Such addressing is allowed with PPP exactly because of the IPCP (IP Control Protocol) negotiation during the PPP link negotiation whereby the peers mutually inform themselves of their IP addresses. The /32 route is never harmful in case the peers are on the same subnet, and is necessary in case the PPP peers are not on the same subnet.

I am not aware of any common circumstances in which it would be beneficial to use the no peer neighbor-route command. Perhaps somebody else could provide a scenario in which it would be called for but I have personally never needed it.

Best regards,

Peter

[Manouchehr]

If you are running PPPoFR hub and spoke topology where you don't want your spokes to establish adjacency between them in that case you should turn off peer neighbor-route with "no peer neighbor-route"

HTH

-----------------------

Manouchehr

[Peter Paluch]

Hi Manouchehr,

I am sorry but this does not make sense to me. EIGRP adjacencies are not established based on the routing table but based on mutual reachability on a per-link basis, i.e. either based on reachability provided by link-local broadcasts, multicasts or by targeted sessions if neighbor commands are used.

Perhaps you could provide an example that would elucidate your point. Thanks!

Best regards,

Peter

[Manouchehr]

Hi Peter,

Yes you are right about it, the Multicast address used by EIGRP is link local in scope with TTL=1 which is not routed beyond the local link.

Best Regards

Manouchehr

[scottpickett]

Hi Peter,

I think I have found an example use case for disabling the "no peer neighbor-route" command:

Two routers create a /32 route for their PPP peer.

If running EIGRP, the two routers then advertise the peer /32 and the PPP link (/24 or whatever) to anyone who will listen. 

But they do not advertise their own /32.

In topologies where there are two redistribution points, the /32 route can make it's way around the network and, due to longest match logic, get installed in a location where it creates suboptimal routing.

It's probably a very rare scenario where it would cause a real problem but I'm looking at some minor reachability issues in a lab right now because of it.

Scott