Changes to routing table with multilink PPP

I've converted a T1 connection between two sites to use multilink PPP,
in preparation for putting in a second T1 between the sites. As part of
this change I moved the IP address (172.29.1.25, with a /30 netmask) from
the serial line (serial1/0) to the multilink interface (multilink1).

This caused a change in the routing table that I didn't expect. Before
the multilink config, the relevant routing table entries looked like this:

172.29.0.0/30 is subnetted, 9 subnets
C 172.29.1.24 is directly connected, Serial1/0

The other 8 subnets on 172.29.0.0 were also /30s. But after the switching
to the multilink the routing table now looks like this:

172.29.0.0/16 is variably subnetted, 10 subnets, 2 masks
C 172.29.1.24/30 is directly connected, Multilink1
C 172.29.1.26/32 is directly connected, Multilink1

So apparently what's happening is that not only is the route for the
network on the multilink interface itself being added to the routing table
(as I expected), but also another route for the IP address of the router
on the *other side* of the multilink interface (172.29.1.26).

Is this expected behavior? This is on IOS 12.3(17a), by the way. If it's
not expected behavior, any ideas why it would be happening?

- John

 

this is default behaviour for PPP IPCP.

to disable this behaviour:

int <>
no peer neighbor-route

 

Yes, that was it, thanks. I see a lot of references to it now that I know
what to look for (so often the case with Cisco).

One point for anyone reading this in the future: adding these statements
to the config is not enough to get rid of the peer's /32 route once it's
infested your routing table--you have to bounce the interface.

- John