EIGRP offset-list instruction

[Brent Rockburn]

I've tried to look through docs and what not here on the cisco site but haven't found too much.

Can someone please send me some good info on configuring offset-lists for EIGRP?

Thanks,

BR

[Peter Paluch]

Brent,

Offset-lists are actually a protocol-independent feature available for all distance-vector protocols. What exactly are you interested in? The offset lists themselves are quite simple to use and there is no special logic behind them.

Have you read these articles?

http://www.cisco.com/en/US/tech/tk365/technologies_tech_note09186a00800c2d96.shtml#modifycompositemetric

http://www.cisco.com/en/US/docs/ios/iproute_eigrp/command/reference/ire_i1.html#wp1035554

Best regards,

Peter

[Jon Marshall]

Brent

Key thing to know is that all you are doing with an EIGRP offset is modifying the metric of the route. So when you do a "sh ip route" for example you might see the following routes -

D       10.1.3.0 [90/2221056] via 10.1.1.2, 00:25:27, Serial0.201
D       10.1.2.0 [90/2195456] via 10.1.1.2, 00:25:27, Serial0.201
D       10.1.3.0 [90/2221056] via 10.1.1.2, 00:25:27, Serial0.201
D       10.1.2.0 [90/2195456] via 10.1.1.2, 00:25:27, Serial0.201

in between the [ ] brackets is the 90 which is the AD of EIGRP internal routes and the metric. So you simply add a number (whatever that number is) to the metric to increase the value. The trick with offset-lists is to make sure that you can increase the metric enough so that all routers within your network are affected the same way ie. if you want to influence traffic to go

R1 -> R2 -> R3

but traffic currently goes

R1 -> R4 -> R5

then you can could use an offset-list on R5 to increase the metric which then increases the metric of R4's route so that R1 prefers to go via R2 & R3.

Attached is a Cisco doc with an example of using offset-lists to influence EIGRP traffic (apologies if this is one of the docs you have already read) -

http://www.cisco.com/en/US/tech/tk365/technologies_tech_note09186a00800c2d96.shtml#modifycompositemetric

Jon

[Brent Rockburn]

Thanks jon,

I guess my question then on R5 when configuring the offset-list would I increase the metric outbound or inbound does it matter? What's the best practices here?

[Jon Marshall]

b.rockburn wrote:
Thanks jon,
I guess my question then on R5 when configuring the offset-list would I increase the metric outbound or inbound does it matter? What's the best practices here?

From the example i gave you would increase it outbound. But that is what i was trying to explain about making sure all routers agree if you see what i mean.

For example you could also apply metrics inbound on R1 so that it prefers R2. But if R2 then has a link to R3 and R5 it might still see a better metric via R5 and send the traffic there which is not what you want. So you need to be aware of the topology of your L3 devices and apply the offset-list, either inbound or outbound, to where it will affect the overall path.

Jon