EIGRP default route disappears

[spfister336]

We are using EIGRP between a central site and several remote locations. Everything has been going fine for years, but recently we had a strange problem. One particular remote site had trouble getting out to the internet. During troubleshooting, I noticed that while the sites's switch had all of its normal routes for the rest of the network, the default route had gone missing. Doing a 'clear ip eigrp 1 neighbors soft' brought it right back, but I'm being asked to discuss preventative actions.

We had a power brownout at the central site late last week, but the issue wasn't happening until early this week. I'm not sure how to explain it, or come up with any prevention.

[balaji.bandi]

we need more information in related to the configuration of what device and IOS code running.

what do you see in the Logs?

[spfister336]

This is a 4506e running 15.2(2)E5a. I don't think I found anything in the logs that was related before doing the clear command

[Richard Burts]

You have described a very strange situation. It might help us identify possible prevention if we knew more about your situation. How is the central site connected to the remote sites? Is it metro ethernet? dedicated circuit? MPLS? something else? Is there any kind of remote site to remote site connectivity or does each remote site connect only to the central site?

Also when you say that the remote had its normal routes but not the default, can you help us understand what is normal routes? Also can you tell us how the default route is generated for EIGRP?

Do any of the sites use the eigrp stub feature?

[spfister336]

It is strange. I can't remember seeing this issue before. The remote sites are all connected to the central site via metro Ethernet (AT&T ASE). For the most part, all traffic goes from the remote site to central. Not a whole lot of site to site connections happen.

Each site has a handful (6 or 7) of VLANs. The routes are summarized to two and advertised to the central site.

Some sites have 'eigrp stub connected summary' enabled, including the site that had the issue

The default route is generated by redistributing a static route at the central site.

[Giuseppe Larosa]

Hello @spfister336 ,

>> Doing a 'clear ip eigrp 1 neighbors soft' brought it right back

As noted by @Richard Burts  EIGRP provides different ways to generate a default route so in order to understand what is happening it would be important to discover the EIGRP next-hop(s) that provides the default route and then the EIGRP configuration of that device(s).

In short EIGRP can advertise the prefix 0.0.0.0/0 or it can flag a network with a special flag so that downstream routers will install a default route. The latter method should be the so called ip default network.

Hope to help

Giuseppe

[Richard Burts]

You tell us that the remote site had all the normal routes for the rest of the network but did not have the default route. Did you notice what was the next hop for those routes? And did you by any chance do a show ip eigrp neighbor while the problem was happening?

I can not explain all of the issue, but I do have an idea about parts of the issue. It is significant that the sites are connected to the central site by Metro Ethernet. You think of the network as the remote sites advertising their subnets to the central site. But if the sites are in a subnet (vlan) that connects to the central site then the remote site EIGRP advertisement goes not only to the central site but also to each of the other EIGRP neighbors in the subnet. So if there were to be a problem with the EIGRP neighbor relationship of that remote site and the central site, then the remote site could have routes to the rest of the network because it sees the other remote sites as neighbors, but be missing the default route because it is not communicating with the central site.

[spfister336]

I'm pretty sure the next hops for the routes was correct, even when the default route was gone. I don't think I did a show ip eigrp neighbor, though. 

Yes, I understand that that switch is exchanging routes with all of its eigrp neighbors. I'm just being asked to come up with a reason that the default route was missing.