Ok, to recap, here was the problem. We have two PVCs from router A to
router B. We have eigrp running on the first PVC but not on the second, and
there are static routes forcing certain traffic to use the second PVC.
These are point-to-point frame relay links. Here are the relevant static
routes affecting the second PVC, and let's assume the loopback address at
the router B is 10.50.1.1:
ip route 10.50.1.1 255.255.255.255 10.1.1.2 50
We also have another static route for an entirely separate reason, but it is
involved here, as well:
ip route 10.0.0.0 255.0.0.0 Null0
We recently upgraded to 12.1 from 11.2. In 11.2, "no ip classless" was the
default. In 12.x, ip classless is the default.
We thought that if the second PVC went down, since the next hop was
unavailable, the router would pick the next available route with a higher
administrative distance. Since there is a valid route to router B over the
first PVC, we thought it would take this route. The odd thing was, it
wasn't and we didn't know why. Here's why:
Even though 10.1.1.2 was at the other end of a point-to-point link and shows
in the routing table as directly connected, the router never realized that
that route disappeared! When the PVC was down, if I did a show ip route
10.50.1.1, it would show as reachable via 10.1.1.2. Then a show ip route
10.1.1.2 would show that it was reachable thanks to the 10.0.0.0 route
pointing to null0. Isn't that freaky??
I assumed that the router would be smart enough to know that if a point to
point link is down, the remote IP address is truly unreachable, even if
there is valid supernet route; apparently, such is not the case. I did
some testing and proved to myself that this is what was happening.
So, since we don't require classless routing on this router, I've turned it
off. Now, the router will not use the 10.0.0.0 supernet route and it will
correctly decide that the point to point link is down. Then, it will use
the eigrp-learned route on the other PVC.
If someone understands why a router would choose a supernet route to reach a
directly attached down interface, please let me know. I'm sure this will
come up later!
Thanks,
John
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