From: James (james@towardex.com)
Date: Mon Aug 23 2004 - 22:43:59 GMT-3
Hi,
(sorry for top posting!)
The reason is b/c administrative distance only works on a protocol basis,
even if you use an access list to be specific of what you want.
The problem you are seeing is that you see a prefix (i.e. 1.1.1.0/24) duplicated
from two different routers using same ospf protocol. So your distance command
even with ACL specification will apply distance the same on prefix heard from
both routers, aka not accomplishing your needs.
The solution is to set ospf cost metric on the specified routes heard from
a specific ospf adjacency. There are various ways to do this, you can set
cost while redistributing prefixes into ospf, or set cost on a circuit (which
would not accomplish your even/odds goal though), or set cost while
summarizing them in, etc...
It is still doable, but setting cost like this on specific per-prefix basis
in OSPF is a bit pain in the butt. In my opinion, distance/path vector based
routing protocols are easier to deal with for this type of scenario, such as
RIP, EIGRP using offset lists or BGP using attribs.
HTH,
-J
On Mon, Aug 23, 2004 at 04:18:02PM -0700, Edwards, Andrew M wrote:
> All,
>
> I'm struggling with getting this to work in a development environment.
>
> I have four routers in a single OSPF area. There is connectivity
> between all routers in a full mesh. (e.g. distribution to core
> configuration). So, from any one core router there are 2 equal cost
> routes to all distribution subnets.
>
> I have setup the distribution routers to split the load on even and odd
> subnets. I have fixed the spanning tree root bridge selection and HSRP
> to reflect this in the distribution layer.
>
> What I want to do now is setup the OSPF process on the core routers such
> that even routes prefer router 10.10.10.25 and odd routes prefer router
> 10.10.10.9.
>
> I have decided to try and use distance for this:
>
> So, I match the source router-id for the 10.10.10.25 router and
> increment the distance of odd routes sourced from it. This should leave
> even routes at default distance.
>
> Next, I match the source router-id for the 10.10.10.9 router and
> increment the distance of even routes sourced from it. This should
> leave odd routes at default distance.
>
> What I expect is that the even routes would prefer 10.10.10.25 and odd
> routes would prefer 10.10.10.9. What I get in the routing table though
> is weird.
>
> I end up AGAIN with two equal cost paths to the either subnet (even or
> odd). The cost is the same as before (expected) and the distance for
> BOTH sources has incremented (not expected).
>
> Show ip route
>
> 10.10.100.0/24 [112/2] via 10.10.10.9
> [112/2] via 10.10.10.25
>
> Maybe I'm not implementing the distance command effectively or there is
> another way... I'm trying to stay away from PBR at this point and use
> the routing protocol... any ideas?
>
> Configs below:
> ------------
>
> CorerouterB:
>
> Int g1/1
> Ip address 10.10.10.10 255.255.255.248
>
> Int g1/2
> Ip address 10.10.10.26 255.255.255.248
>
> Router ospf 1
> Router-id 10.10.10.26
> Network 10.10.10.0 0.0.0.255 area 0
> Distance 112 10.10.10.9 0.0.0.0 20
> Distance 112 10.10.10.25 0.0.0.0 10
>
> Access-list 10 remark match odd subnets
> Access-lsit 10 permit 0.0.1.0 255.255.254.255
>
> Access-list 20 remark match even subnets
> Access-list 20 permit 0.0.0.0 255.255.254.255
>
>
> DistributionrouterA:
>
> Int g3/1
> Ip address 10.10.10.9 255.255.255.248
>
> Router ospf 1
> Router-id 10.10.10.9
> Network 10.10.10.0 0.0.0.255 area 0
>
>
> DistributionrouterB:
>
> Int g3/1
> Ip address 10.10.10.25 255.255.255.248
>
> Router ospf 1
> Router-id 10.10.10.25
> Network 10.10.10.0 0.0.0.255 area 0
>
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-- James Jun TowardEX Technologies, Inc. Technical Lead Network Design, Consulting, IT Outsourcing james@towardex.com Boston-based Colocation & Bandwidth Services cell: 1(978)-394-2867 web: http://www.towardex.com , noc: www.twdx.net
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