EIGRP update process

[atef yamin]

Hello guys and girls,

I am having a little issue with eigrp udpate process. I have set up a lab on gns3 and am running few routers on it. i have configured a loopback interface on  the router "A" and have turned on eigrp network command on that interfaces. its all good so far.

now when i apply the shutdown command under one of the loop back interface and start capturing packets on wireshark i see that the query process starts. that is fine too

but when I apply no shutdown on the same interface this is what i see on wireshark:

From Router "A" to Router "B" ---->Update : (the router A informs the Router B about the route that just came up)

From Router "B" to Router "A" ---->  Ack : (the Router B acknowledges the update)

From Router "B" to Router "A" ----> Update ( the neighbor udpates back!!! and says destination unreachable (for the same network that it just received the update for))

From Router "A" to Router "B" ---->Ack : ( Router A acknowldeges the update)

no other activity in terms of update takes place and in the same time Router B is able to ping the loopback interface of Router A !!!

I guess my question is why do i see the second update ? and if Router B is unable to reach the network as it mentions in the second update why on earth it is able to ping it then? (the route does show under topology and routing table).

Thanks guys

[Peter Paluch]

Hello,

My personal suspicion is that the B sends back a poisoned reverse, i.e. it advertises the loopback network back to router A as unreachable, as A is the next hop for B towards the loopback network (have a closer look at the metrics in the Update packet from B to A).

Please let us know if that is not the case - I'll then try to lab it up during the day.

Best regards,

Peter

[atef yamin]

Hello,

Thanks for the reply. I think that might be the case. I am at work and hence cant see the metrics. I will check it once i get to home.

Thanks

[atef yamin]

Hi,

I did check the metric and the Delay has been upped and the hop count is now "1". I searched some more and found a website that was showing exactly what i am experiencing. However it does not shed any light on it.

http://www.kimiushida.com/bitsandpieces/articles/packet_analysis_eigrp/index.html

It happens in the packet 8.

I still dont get it why it sends the update. seems strange since no where in the eigrp process there is a mention of such update packet.

Really appreiate if someone could explain this to me.

Thanks

[Peter Paluch]

Hi,

This process may be dependent on your network topology and types of links used. Can you post a picture of your network please? Also please indicate where is the loopback you are activating/deactivating, and what is the link you are capturing packets at.

Thanks!

Best regards,

Peter