EIGRP Load Balancing on Multiple unequal cost Links

[nagesh]

hi

We presently have 4 E1 Links in a Multilink Bundle with EIGRP running between the 2 routers, these links have unequal costs (latency) We are facing the following problem.

1.Whenever one of the links continiously flaps in a Bundle EIGRP Neigbour dies and MLPP bundle goes down and it doesn't come up unless we take out the flapping circuit from the MLPPP.

2. Since we are running VOIP on this link, Voice calls suffer due to the change in MLPPP change.

We are planning to replace the MLPPP with IP CEF with EIGRP.

Please suggest the solutions for the following.

1.Will IP CEF work well with EIGRP

2.How can we achieve best Load Balancing among 4 links with EIGRP

3.What would be the CPU Load if we use EIGRP Load balancing.

4.Will EIGRP remain stable in case of a single link flapping

[Richard Burts]

Nagesh

Here are answers to your questions:

1) yes IP CEF does work well with EIGRP.

2) Assuming that the serial links are defined with the same bandwidth and the same delay then the metrics calculated over the 4 links should be equal. In this case load sharing takes place automatically. If for some reason the metrics are not equal over the links you would either need to manipulate the metrics to make them equal or you would need to configure EIGRP with variance. Variance would probably be the preferred option.

3) EIGRP load balancing (or more properly load sharing) does not impose any extra CPU load.

4) If a single link flaps EIGRP will lose a neighbor and will need to make appropriate updates in the EIGRP topology table and in the routing table. There should be minimal impact from this and routing should continue over the three surviving links.

HTH

Rick

[nagesh]

Thanks for response.

Could you also send us a document which outlines the configuration needed for optimum performance of voip using both EIGRP and cef.

would be a great help.

[royalblues]

Hi Nagesh,

There is no additional configuration required as long as the metric for all the links is same.

If CEF is disabled for any reason either globally or on the interface level u can enable it by issuing the following commands.

(config)#ip cef

(config-if)#ip route-cache cef

I am still searching for a document on the cisco site about the interoperatability between EIGRP & CEF

Narayan

[nagesh]

hi Rick

Need some more help on this, could you send me the sample configs for this setup, below is the setup

1. We have 4 E1 links connected to a Router at the distant end, these links are in a Multilink PPP bundle

2. These 4 E1s are used for both Voice(VOIP) and Data

3. We are running EIGRP for Dynamic Routing

4. We are facing problems whenever a link goes down in a MLPPP bundle.

5.We would like to split these E1 links to an indiviual links, do Load sharing between these 4 links.

Suggest us the best way to do this and it would be a great help if you could send me the sample configs

Thanks once again

/\/agesh

[Richard Burts]

Nagesh

What you will do is pretty straightforward. You will remove the multilink commands from each of the E1 interfaces, you will make sure that each E1 interface has the default bandwidth configured, you will assign an IP address (and subnet) to each E1, you will have network statements under router eigrp for each E1 interface, and you will ensure that CEF is enabled.

The config might look something like this:

ip cef

interface serial 0/0

ip address 192.168.2.1 255.255.255.252

ip route-cache cef

interface serial 0/1

ip address 192.168.2.5 255.255.255.252

ip route-cache cef

interface serial 1/0

ip address 192.168.2.9 255.255.255.252

ip route-cache cef

interface serial 1/1

ip address 192.168.2.13 255.255.255.252

ip route-cache cef

router eigrp 100

network 192.168.2.0 0.0.0.3

network 192.168.2.4 0.0.0.3

network 192.168.2.8 0.0.0.3

network 192.168.2.12 0.0.0.3

This will run IP on each of the E1 interfaces, will enable EIGRP on each interface, EIGRP will learn the remote routes on each E1 interface, EIGRP will have 4 routes to each remote destination, EIGRP will load share on the 4 E1 links, and if one of the E1 links flaps then EIGRP should continue to forward traffic on the 3 surviving E1 interfaces.

Obviously you will need to put your own IP addressing scheme into this and your own EIGRP AS number.

HTH

Rick

[nagesh]

Rick

Thanx for the response, We are going to configure this tomorrow and test the working of this setup

Thanx a lot again for the help

/\/agesh.

[Richard Burts]

Nagesh

I am glad that my response was useful. Let us know how it turns out.

HTH

Rick

[nagesh]

hi Rick

We have successfully implemented EIGRP Loadbalancing on 4 serial Links. Now the traffic is distributed across the 4 links.

Thank you very much for giving us a valuable suggestion on this

/\/agesh.

[royalblues]

Hi Nagesh,

good to hear that the problem has been solved.

regards

Narayan