EIGRP - Null Route Redistribution

Unanswered Question
Nov 16th, 2007

Hi All,

I have some questions about EIGRP redistributing a null route. I understand the concept of the null routes with route summarization but I am confused on the behavior in this case.

The goal was to not spit out 10.x.x.x stuff to the ISP router if it wasn't in the routing table. The problem was it redistributed that route into the routing table and told everyone to send 10.0.0.0/8 stuff to him who deposited it in the bit bucket. Changing the EIGRP network to 10.2.x.x 0.0.255.255 corrected my problem. My thought is it looked at the Null 0 interface as directly connected and had 10.0.0.0 255.0.0.0 as its IP address or something like that. With that, it put the 10.0.0.0/8 network in the routing table and dumped it to Null0 when it got there.

Setup:

- router connected to internet using gre/ipsec vpn tunnels to 4 other routers

- default route set to isp router

- internal LAN set to 10.2.x.1 255.255.254.0

- eigrp set to network 10.0.0.0

- null route created for 10.0.0.0 0.0.0.255 null 0

- no route redistribution set

Here is an upstream routers routing table when network 10.0.0.0 was in place. Yes, it does do some redistribution but this route was put there by the router with the null route.

Router1#sh ip route 10.0.0.0

Routing entry for 10.0.0.0/8, 636 known subnets

Attached (117 connections)

Variably subnetted with 6 masks

Redistributing via eigrp 1

D10.0.0.0/8 [90/12902400] via 10.2.x.173, 00:29:00, Tunnel192

Can someone enlighten me to why this happened? Why would it redistribute that?

Thanks for your help.

Jamie

I have this problem too.
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Richard Burts Fri, 11/16/2007 - 10:46

Jamie

I am not sure how completely it answers your question, but I believe that it would be helpful to differentiate between EIGRP "redistributing" and EIGRP "advertising". EIGRP redistributes when it learns some route from another source (another dynamic protocol or from a static route) and then advertises that route within EIGRP. These redistributed routes have administrative distance of 170 within EIGRP. What you have shown in your post is not a redistributed route but is a route advertised from a network statement with administrative distance of 90.

One way to fix it was to change the mask used on the network statement so that EIGRP did not match the null 0 interface. Another way to fix it would have been to create a static route for 10.0.0.0 with null 0 as the next hop, rather than assigning 10.0.0.0/8 as the address of the null 0 interface.

HTH

Rick

Edison Ortiz Fri, 11/16/2007 - 11:26

The behavior you are seeing is announcing a network within EIGRP with auto-summary turned on (default).

To correct this, you need to go into the router eigrp process and enter no auto-summary.

HTH,

jamie.honeycutt Fri, 11/16/2007 - 12:02

Thanks for the replies.

I forgot to mention, no auto-summary is enabled on the router in question. The issue is that it saw the static null route for 10.0.0.0/8 and advertised it out to peer routers. I apologize for saying redistributed....I meant advertised. Once I change the network statement to something other than 10.0.0.0/8, it doesn't advertise 10.0.0.0/8 out.

Also, I didn't apply any addresses to the Null 0 interface. The only thing I did was create a null route (ip route 10.0.0.0 255.0.0.0 null0).

I'm still lost on why it did this. It doesn't make sense that it advertised that null route out....unless any null route is automatically advertised out.

It was definately related to the network statement matching the route for 10.0.0.0/8

-Jamie

Richard Burts Fri, 11/16/2007 - 12:40

Jamie

I thought I understood in the earlier post that you had applied the 10.0.0.0 address to the null 0 interface. Thanks for the clarification that it was a static route with null 0 next hop.

I believe that the explanation of the behavior is that when there is a static route to an interface, and when the network in the static route matches the network statement in EIGRP then EIGRP will advertise that network. Your fix was to change the mask in the network statement so that it no longer matched the address in the static route.

It is not that any null route is advertised. But a static route to an interface where the network in the static route matches the network statement is what causes it to be advertised.

HTH

Rick

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