eigrp weights

Unanswered Question
Sep 16th, 2009

I'm trying to figure out how to modify the EIGRP within our network and need some guidance. Basically, is there any way within EIGRP to force traffic into one router rather than another? I just need to know how to properly set the weights so that although the network is learned from two different routers via EIGRP the traffic will prefer one router over the other.

I have this problem too.
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Jon Marshall Wed, 09/16/2009 - 07:37

Matt

You can either

1) filter the eigrp routes so you only receive routes from the router you want but this means you lose any redundancy

OR

2) you manipulate the delay on the interfaces so that one path is preferred over the other. This is the recommended way to influence eigrp metrics

Jon

Richard Burts Wed, 09/16/2009 - 07:55

Matt

In addition to the possibility of altering the interface delay to change the calculated metric of EIGRP there is another possibility that you might consider. You can configure an offset list. An offset list will add a configurable amount to the EIGRP metric, making that advertisement less attractive. An offset list can be configured for specific interfaces and it can alter the metric of inbound updates or of outbound updates, so you have some flexibility of where to implement the offset list. The offset list can use an access list to identify the routes to alter, so you do not need to alter every route advertised if you only care about certain routes.

HTH

Rick

YANGCCIE4 Wed, 09/16/2009 - 16:16

Hi, rburts,

your way looks fresh to me

as you said,

An offset list will add a configurable amount to the EIGRP metric,

i did the first step,

router eigrp 100

offset-list 0 in 2

no auto-summary

eigrp router-id 1.1.1.1

I am wondering how to add a configurable amount to eigrp metric,

could you please more specific,

Thank you very much,

Giuseppe Larosa Wed, 09/16/2009 - 08:03

Hello Matt,

EIGRP weights need to match in all routers so they cannot be manipulated.

As Jon has explained you can use delay because EIGRP metric is cumulative on delay or if you want a more selective effect you can use offset-list that are able to add delay only to specific prefixes out/in an interface.

see

http://www.cisco.com/en/US/partner/docs/ios/iproute/command/reference/irp_eig2.html#wp1012264

or

http://www.cisco.com/en/US/docs/ios/iproute/command/reference/irp_eig2.html#wp1012264

Hope to help

Giuseppe

mpozorski Wed, 09/16/2009 - 13:31

Thank you all so much, I will do some research and see what would work the best for us.

mpozorski Wed, 09/16/2009 - 13:48

I don't quite understand the offset list. It looks like you need to create an access list and refer to that in the offset list correct? I don't see how I would do that when the router will be learning the routes via eigrp when a new location connects to it. Perhaps the interface weight would make more sense to me. Does anybody have any examples of placing a weight on the interface?

Thanks,

Matt

Jon Marshall Wed, 09/16/2009 - 14:05

Matt

You apply an offset list to an interface either in or out.

So lets say you have 3 routers.

R1 (s0/0) is connected to R2 (s0/0)

R1 (s0/1) is connected to R3 (s0/0)

R1 is receiving a route to the same network eg 192.168.5.0/24 from both R2 & R3 but you want the traffic to go via R2.

So you could write an acl that matches the route

access-list 1 permit 192.168.5.0

then you can either apply this offset list to R3 s0/0 interface outbound towards R1 or you can apply it inbound on s0/1 on R1.

As for changing the delay you can simply modify the delay under the interface configuration eg

int fa0/0

delay

If you are unsure of the value, from the example we used above

R1

sh int s0/0

sh int s0/1

the output of the above commands will show you the existing delay. You can then increase that delay on the s0/1 interface so the s0/0 path is preferred.

Jon

mpozorski Wed, 09/16/2009 - 14:11

Thanks Jon - That clears it up for me, I think I'll be able to get it in place one of these nights and do some testing. I really appreaciate the help.

Matt

Richard Burts Thu, 09/17/2009 - 04:16

Matt

Note that using an access list with the offset list is optional. If you do not use an access list (when you use 0 in the place of the access list number) then the added amount is added to all routes learned and processed by the offset list.

If you do use the access list it will identify the routes to which you want to add. If you are bringing up new sites and that adds new routes that need to have the metric increased then you would need to update the access list as part of bring up the new site.

HTH

Rick

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