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What if we dont know the AS number of the Neighbouring Router in EIGRP…

Here i have taken a simple topology R1 is connecting directly over Serial 0/0 link to BB1 , and as we know we dont have much access to Backbone routers in LAB , what if we dont know the EIGRP AS number of Neighbouring router.

R1 ——S0/0———BB1

The ip address used between R1 and BB1 is in range of

Lets start R1 by configuring it in AS 100 (wild guess) .

R1(config-if)#router eigrp 100
R1(config-router)#no auto

It seems we are not lucky so the neighborship is still down , lets see the output of “sh ip protocols”.

R1#sh ip protocols
Routing Protocol is “eigrp 100”
Outgoing update filter list for all interfaces is not set
Incoming update filter list for all interfaces is not set
Default networks flagged in outgoing updates
Default networks accepted from incoming updates
EIGRP metric weight K1=1, K2=0, K3=1, K4=0, K5=0
EIGRP maximum hopcount 100
EIGRP maximum metric variance 1
Redistributing: eigrp 100
EIGRP NSF-aware route hold timer is 240s
Automatic network summarization is not in effect
Maximum path: 4
Routing for Networks:
Routing Information Sources:
Gateway         Distance      Last Update
Distance: internal 90 external 170

Here also it seems everything fine , so here starts our troubleshooting we will use Extended Access list 101 to limit the output of “debug ip packet details 101”

R1(config)#access-list 101 permit ip any host

R1#debug ip packet detail 101
IP packet debugging is on (detailed) for access list 101

*Mar  1 00:07:39.131: IP: s= (local), d= (Serial0/0), len 60, sending broad/multicast, proto=88
*Mar  1 00:07:39.219: IP: s= (Serial0/0), d=, len 60, rcvd 2, proto=88

The output of the debug shows that we indeed sending and receiving Eigrp updates on IP protocol 88 port.But why our neighbor is still down and answer is we are in wrong EIGRP AS , so the Question is how do we figure out the correct EIGRP AS number of the neighbouring router BB1.

Here is the trick.

We use a hidden command in debug called as “DUMP” feature it will show the HEX characters received from the neighbouring router.

R1#debug ip packet detail 101 dump
IP packet debugging is on (detailed) (dump) for access list 101
*Mar  1 00:12:05.643: IP: s= (Serial0/0), d=, len 60, rcvd 2, proto=88
07DFE7F0:          0F000800 45C0003C 00000000
07DFE800: 015847D5 C8C8C802 E000000A 0205DF27
07DFE810: 00000000 00000000 00000000 00000FA5

07DFE820: 0001000C 01000100 0000000F 00040008  …………….
07DFE830: 0C040102                             ….
*Mar  1 00:12:07.747: IP: s= (local), d= (Serial0/0), len 60, sending broad/multicast, proto=88

Here in the first highlighted part E000000A , if we translate this into binary it comes the multicast address of the EIGRP , then we move five steps ahead of this HEX character we see 00000FA5 and this is our neighouring router AS number , we convert this HEX number into binary it comes “4005” and that is our AS number of the BB1.

Lets not assume and configure R1 with AS 4005 and see what results we see.

R1(config)#no router eigrp 100
R1(config)#router eigrp 4005
R1(config-router)#no auto
*Mar  1 00:23:39.883: %DUAL-5-NBRCHANGE: IP-EIGRP(0) 4005: Neighbor (Serial0/0) is up: new adjacency

So now it is confirmed that the BB1 AS number is 4005.

Afroz Ahmad
Afroz Ahmad

"Hey there, network enthusiasts! My name is Afroz, and I've been a CCIE for over 14 years. I work as a Network Designer, and my true passion is teaching others about the industry and sharing my knowledge through my blog and courses. I know the struggles of navigating the complex world of networking, but I firmly believe that teaching makes you a better learner and reinforces understanding. So, whether you're just starting out or a seasoned veteran, join me on this journey of learning and discovery, it will be worth it, and who knows, you might even have some fun along the way!"

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