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Afroz Ahmad

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Category: 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.

Today while i was doing lab i was stuck in the scenario where i have to do unequal cost load balancing in the eigrp domain.After many hours i finally got the desired result , thought of sharing this , kindly tell me if i am wrong anywhere.

One of the most important factors in choosing EIGRP as an IGP in the Network is the ability of  doing unequal cost load balancing.To enable this feature into Eigrp we need to issue “Variance” command under the Eigrp process.In order for path to be considered for unequal cost load balancing it must be a feasible successor with a metric less than or equal to the successor’s metric times the Variance.

Feasible Distance= Local Distance + Advertised Distance

A path whose advertised distance is lower than the feasible distance of the successor is deemed as feasible successor

Note:-Only routes that are feasible successors can be used for unequal cost load

The formula for Eigrp Metric calculation:-

Metric = [k1 * bandwidth + (k2 * bandwidth)/(256 – load) + k3 * delay] *[k5/(reliability + k4)]

The “k” values are derived from the metric weights command, where K1 and
K3 are 1 by default and all other values are 0. This essentially means that only
bandwidth and delay are taken into account. “Bandwidth” is the inverse
bandwidth in Kbps times 107 (107/BWKbps). “Delay” is delay in tens of microseconds (DLYusec/10). These values are added together and then scaled
by a factor of 256. The composite metric is therefore represented by default as:

Metric = (107/BWKbps + DLYusec/10) * 256

Note:-Try to use Delay instead of Bandwidth because by changing bandwidth and QoS is running in the network , it will effect the traffic drastically in the network.

In order to achieve the any desired ratio suppose we want to distribute traffic in a ratio of 3:1 among the successors and feasible successors, and we are using delay we can set our equation like BW1 and Delay1 for the one path and BW2 and Delay 2 for other path.Now the equation will look like this.

3*[107/BW1 in Kbps + Delay1 in usec/10) * 256]=(107/BW2 in Kbps + Delay2 in usec/10) * 256

Here in the above formula watchout for BW1 ,Delay1,BW2 and Delay2.

By taking BW1 and BW2 as it is from the interface , we can find the end result something like :- Delay1=something times Delay2 ,now we can put the value of Delay 2 to find out Delay 1.

Lastly we can apply this Delay to the interface and check and verify our configuration by :-

sh ip route [destination network address] | include frame|share

Check the ratio in the traffic share count to see the exact result between both the paths.Also dont forget to include the Variance command  in the  Eigrp process.

sh ip eigrp topology [destination network address]