Ways to set administrative distance

[Kevin Dorrell]

Does anyone else agree with me that it would be really really useful if you could set the AD from a route map? That way you could set it according to a tag, for example. That would make maintaining border routers a lot easier, especially where you have multiple border routers between two IGP domains and you want to prevent route feedback.

Does anyone know any way to set the AD of a route according to a tag? The only way I can see to set the AD is according to the address of the route, or according to the advertising router.

Kevin Dorrell

Luxembourg

[pciaccio]

There are other ways to route around the bush.. For example using EIGRP you can tweek the metrics to allow one route to being more appealing then the other. you can do the same for pretty much all the routing protocols. The AD is set for what it is. The metrics are the variable that you can change to your liking. You can create floating static routes that allow for a prefered route with a lower metric then the default AD. These are the only ways I know of...

[Kevin Dorrell]

I agree that playing with the metrics you can usually achieve the result you want, but I was thinking more in terms of the border routers between two IGPs, for example RIP and OSPF. There, you have to arbitrate between routes offered by two different IGPs. Usually, that is no problem: the RIP routes go out the RIP side, and the OSPF routes out the OSPF side.

The problems really start to make themselves felt when there are two points of contact between the two IGPs. In that case, routes redistributed from RIP to OSPF in one border router have a nasty habit of re-entering the other border router, which then uses the OSPF domain to access its RIP routes. THat is the sort of situation where it would be useful to adjust the AD of specific routes.

Kevin Dorrell

Luxembourg

[mark.edwards]

Hi Kevin. Totally agree with what you are saying. In that scenario I would manipulate the AD of routes being redistributed to fix the AD problem. You will have to set the AD and tie it to an access-list which specifically matches the redistributed routes.

This should fix the issue on the 2 (or more) redistribution points. I also would tag routes to stop them looping round the IGP domains.

[Kevin Dorrell]

Thanks.

I think you are right: I will probably be stuck with specifying the actual routes in an access list, which is what I was trying to avoid.

The only situation where you can avoid this is OSPF, where filtering on "advertising router" is actually useful because it refers to the other border router, not the upstream neighbor. Thus you can say: "any (external) routes originating in the other border router, set their AD to 150." Not so useful, though, if the other border router also has internal routes.

Kevin Dorrell

Luxembourg

[sundar.palaniappan]

Kevin,

See the responses below.

1. Does anyone else agree with me that it would be really really useful if you could set the AD from a route map?

Agree, it would be nice to have that option as it would make configuration task easier. You can tag the routes at the point of redistribution and modify the distance on the learning routers based on the tag.

2. Does anyone know any way to set the AD of a route according to a tag?

I didn't think you could do this. Anyway, I checked a router running 12.4(7)image and don't see support for route maps either with distance command.

R3# config t

Enter configuration commands, one per line. End with CNTL/Z.

R3(config)#router rip

R3(config-router)#distance 250 1.1.1.1 0.0.0.0 ?

<1-99> IP Standard access list number

<1300-1999> IP Standard expanded access list number

WORD Standard access-list name

R3(config-router)#distance 250 1.1.1.1 0.0.0.0

R3(config-router)#do show ver | i Version

Cisco IOS Software, 3600 Software (C3640-JS-M), Version 12.4(7), RELEASE SOFTWAR

E (fc6)

ROM: System Bootstrap, Version 11.1(20)AA2, EARLY DEPLOYMENT RELEASE SOFTWARE (f

c1)

HTH

Sundar