EIGRP Autonomous system mismatch detection

Unanswered Question
Oct 4th, 2007


It is possible to detect situation when two neigbour routers involved in EIGRP routing are configured by mistake with different AS number ?

I tried this situation practically. Two routers are connected together via Serial link network. One router has AS 1, other AS 10. I try to detect AS mismatch.

First I check what EIGRP packet are comming

debug ip packet detail show source <my neighbour IP address> destination Ip protocol type 88. These packets are EIGRP Hello packet.

I try to go more deeply into details.

debug eigrp packets

I see only ongoing EIGRP Hello packets. But I don't see any incoming packet from my neighbour (which has different AS number). It seems, because of different AS number router silently drop eigrp packet.

Other debug eigrp command also doesn't show any info about AS difference.

Any proposal?

Cisco IOS 12.4 (16)

Best Regards,

Tomas Chmelevski

I have this problem too.
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royalblues Thu, 10/04/2007 - 23:21

I dont think you will get a AS mismatch with EIGRP in the debugs

When you do a hello debug, you should see the AS highlighted.

*Mar 1 00:03:21.691: EIGRP: Sending HELLO on Serial2/0

*Mar 1 00:03:21.691: AS 100, Flags 0x0, Seq 0/0 idbQ 0/0 iidbQ un/rely 0/0

*Mar 1 00:03:26.191: EIGRP: Sending HELLO on Serial2/0

*Mar 1 00:03:26.191: AS 100, Flags 0x0, Seq 0/0 idbQ 0/0 iidbQ un/rely 0/0

You should then be checking whether this is the correct AS that has been configured



1pipantom2 Thu, 10/04/2007 - 23:50


Yes I see sending Hello message and my AS number.

But how to be sure, what remote router has wrong AS number and this is the reason why EIGRP peer doesn't work ?

Let suppose what I doesn't have Telnet access to remote router and the only way I can troubleshoot is to debug incoming messages.


Kevin Dorrell Fri, 10/05/2007 - 00:11

I think if you don't see the hellos from the other router, then it isn't talking EIGRP on this link.

If you have mismatched AS, your router will see the hello, it will look for an EIGRP process with that AS, it will not find one, so it will drop the packet. I'm not 100% sure whether you will see it in the debug, but I think you would.

I understand you can see your hellos using the debug, right?

Kevin Dorrell


1pipantom2 Fri, 10/05/2007 - 00:34

Hello Kevin,

Yes if remote router has differnt AS number than local one I don't see incoming Hello message from remote router.

Remote router actually sent EIGRP Hello. I can see this with debug eigrp packet on remote router.

If I set the same AS (as it should be)on both router I see incoming and outgoung Hello on both routers.

But IF I set different AS,only messages I can see are outgoing Hello messages (no incoming).

Now two router are on the table and I can connect to both to check what is happening.

But in real life how to detect if remote Router by mistake has different AS number than local one? Suppose, that there is no telnet connectivity to remote router.


Kevin Dorrell Fri, 10/05/2007 - 00:50

OK, thanks for confirming that.

I guess what is going on here is that the debug eigrp packet is part of the EIGRP process. But if the router cannot bind the incoming Hello to the EIGRP process (because the AS is wrong), then the process will not see it.

How can you read the remote AS if has been set wrong? That's a good question. Maybe debug ip packet would do it, but you might have to trawl through a lot of data. Filter on the interface first, debug interface S0/0.

Kevin Dorrell


royalblues Fri, 10/05/2007 - 00:56

Unfortunately i dont think there exists a way to check this.

I do not see any opcode options either that can indicate this.


1pipantom2 Fri, 10/05/2007 - 02:16

Thank all for reply,

At this point all have the same opinion, that is not possible to detect mismatched EIGRP AS number.

debug ip packet detail only show protocol type for EIGRP (88). So we only know, that EIGRP packets are coming in. But AS number is uknown.


Kevin Dorrell Fri, 10/05/2007 - 02:48

OK, it doesn't tell you which AS is configured. But at least it allows you to say that there is or isn't EIGRP at the other end of the link, which could be useful in itself. It's a pity you cannot get a hex dump of the IP packet.

Kevin Dorrell


Surya ARBY Mon, 12/03/2012 - 08:26

Hi all.

At this point all have the same opinion, that is not possible to detect mismatched EIGRP AS number.

Is it the final point ? I also try to detect AS number mismatch with a device on which I don't have any access...


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