EIGRP questions

[ajith96935]

I am preparing for exam and I was doing some EIGRP lab and I have a few questions.I have created the topology which is attached . However since the link speeds are different I am not sure why the Eigrp neighbor relation formed as the k values change  

[Georg Pauwen]

Hello,

what exactly is your question ? It is unclear from your drawing which routers are supposed to form neighborships, as you have not indicated the common IP subnets. K values would become relevant in case of redundant paths. 

Please explain further what your issue is...

[ajith96935]

Thank You for the quick response Georg,I have four routers connected  via  switch routers R8 and R9 are connected via a Ethernet link and assigned with ips 10.0.0.1/8 and 10.0.0.2/8 respectively on their e3/0 interface.

The other two routers R10 and R11 connected via a Fast Ethernet link and assigned with ips 10.0.0.3/8 and 10.0.0.4/8 respectively on their F0/0 interface.

My question is how come the Eigrp neighbor ship is formed across the four routers as the K value of R8 and R9 is different from K value of R10 and R11. When I look at the show IP eigrp neighbor table I see the following

R8(config)#do show ip eigrp neighbors
EIGRP-IPv4 Neighbors for AS(100)
H Address Interface Hold Uptime SRTT RTO Q Seq
(sec) (ms) Cnt Num
2 10.0.0.3 Et3/0 10 03:36:01 140 1260 0 7
1 10.0.0.2 Et3/0 12 03:36:01 636 3816 0 7
0 10.0.0.4 Et3/0 11 03:36:01 233 1398 0 7


R9(config)#do show ip eigrp neighbors
EIGRP-IPv4 Neighbors for AS(100)
H Address Interface Hold Uptime SRTT RTO Q Seq
(sec) (ms) Cnt Num
2 10.0.0.1 Et3/0 12 03:37:14 276 1656 0 7
1 10.0.0.3 Et3/0 12 03:37:44 1781 5000 0 7
0 10.0.0.4 Et3/0 11 03:37:44 1417 5000 0 7

R10(config)#do show ip eigrp neighbors
EIGRP-IPv4 Neighbors for AS(100)
H Address Interface Hold Uptime SRTT RTO Q Seq
(sec) (ms) Cnt Num
2 10.0.0.1 Fa0/0 13 03:37:46 122 732 0 7
1 10.0.0.2 Fa0/0 13 03:38:16 118 708 0 7
0 10.0.0.4 Fa0/0 11 03:38:44 1625 5000 0 7

R11(config)#do show ip eigrp neighbors
EIGRP-IPv4 Neighbors for AS(100)
H Address Interface Hold Uptime SRTT RTO Q Seq
(sec) (ms) Cnt Num
2 10.0.0.1 Fa0/0 11 03:38:11 100 900 0 7
1 10.0.0.2 Fa0/0 11 03:38:41 120 1080 0 7
0 10.0.0.3 Fa0/0 11 03:39:08 1688 5000 0 7

[Richard Burts]

I have looked at your diagram and have read your post several times. I am still not clear about your question. You ask this " the K value of R8 and R9 is different from K value of R10 and R11" and my question is what would make their K values different? The bandwidths are different. But that does not change the K value. You seem to believe that the bandwidth IS the K value. But that is not the case. The K value is an attribute of the neighbor that is used in calculating the EIGRP metric. The K value is independent of the particular value of the bandwidth.

HTH

Rick

[ajith96935]

Hi Rick,

Thank you for the response. The reason for me stating that the k values of R8 and R9 are different from R10 and R11 is because my understanding is that the k value is calculated as

{10000000/lowest b/d in Kbps+cumulative delay/100} x256  and since the bandwidth for the E3/0 is different from F0/0 the K value would also change. Since the neighbor ship requirement in Eigrp states that the K value must be the same  then how is that the routers are forming neighbor ship ?

[Richard Burts]

I thought that this might be the case. The formula that you give is the formula for calculating the EIGRP metric, not a formula for K value. Here is the actual formula

• metric = ([K1 * bandwidth + (K2 * bandwidth) / (256 - load) + K3 * delay] * [K5 / (reliability + K4)]) * 256

As you can see there are K values (K1, K2, K3, K4, K5) embedded in the formula. They are used to help determine which parameters (bandwidth, delay, reliability, load) will actually be used in the calculation. The formula that you give is the simplified result of using the default K values.

In fact all 4 of your routers have the same K values, and have different calculated metrics.

Here is a link that has detailed discussion of EIGRP, including its use of K values.

https://www.cisco.com/c/en/us/support/docs/ip/enhanced-interior-gateway-routing-protocol-eigrp/16406-eigrp-toc.html#anc6

HTH

Rick

[paul driver]

Hello

Just like to add - These eigrp k values are used by the each router to locally calculate metric for best path to a particular destination prefix which it receives from it advertised neighbour(s) so they should match on each eigrp router to even establish a peering

Now are you saying you have different k value metrics applied? if so are you not incurinf eigrp neighbour errors?