Cisco Support Community
cancel
Showing results for 
Search instead for 
Did you mean: 
Announcements

Welcome to Cisco Support Community. We would love to have your feedback.

For an introduction to the new site, click here. If you'd prefer to explore, try our test area to get started. And see here for current known issues.

Purple

eigrp

We are having a problem where for some reason on a couple routers at a given site it seems to be summarizing a 10.x.x.x space at the /8 boundary and pointing it to null 0 . Anyone have an idea what might cause this ? We have "no auto summary" under the eigrp process on both routing devices but they still point this 10.0.0.0/8 to null 0 . The devices are 3550's running eigrp .

  • LAN Switching and Routing
5 REPLIES

Re: eigrp

I have not experienced this problem before.

But still I would like to try out few things if possible like clear ip eigrp neighbor, clearing ip routing table, disable eigrp process and enable it again, , just to see if it helps.

Green

Re: eigrp

That is a normal mechanism for EIGRP.

In addition to the real route, EIGRP adds a summarized route at the classfull boundry pointed to NULL.

Since there is a more specific route (the one you really want to use) it is always selected (longest match) and used.

If that route goes away, and there is no other path, any traffic destined for that path will be discarded (NULL0).

IF you do a search on the main site using "EIGRP Null" it will give you a list of config guides that describe the mechanism.

Good Luck

Scott

Purple

Re: eigrp

Ok , if thats the case why is that route the only one in the table pointing to null 0 ? Don't see any other instances of a route pointing to null 0 and there are other class A networks in the table and I don't see anything pointed to null 0 . For some reason this null 0 route seems to be getting propagated even though there are more specific routes for all other 10.x.x.x routes . So if you trace to nonexistent 10.x.x.x address it is following this route because it sees that summary for some reason . Not sure what to do with this , supposed we could use a distribute list to deny that route but would like to figure out an easier way .

Re: eigrp

Hi

I just doubt wheter any routing protocol would add a Null0 route unless there is a summarised route getting generated from the router and this is just done for preventing routing loop . Iam not sure how a Null 0 route is getting generated without any summary address. Just cross verify wheter there was any summary address added before and the route is not getting removed...

regards

vanesh k

Green

Re: eigrp

Please post a sanitized version of your "show run", "sh ip route", and "show ver"

It could be a couple things:

Check that there is an

"IP classless" statement

Check which routes are listed in your "ROUTER EIGRP {as number}" command

Are you using a wildcard mask in your network statement?

I believe you said that you are using the "no auto-summary" .... verify that too

Are there any other routing protocls active?

Multiple instances of subnetworks below the 10.0.0.0/8 network would be advertised in the single summarization ... any traffic starting with a "10." would be sent to that router ... where a longer match would be found and that path would be used.

Please post the outputs and we'll take a look.

Good Luck

Scott

143
Views
10
Helpful
5
Replies