ospf route/path manipulation

[davidjinx]

hi there,

I'm having some issues getting our OSPF route selection to work properly.

From two separate data-centres (HK and AMS) we're redistributing BGP routes in to OSPF and tagging different subnets via route-maps.

The tags are because we have some sites we need to route via HK, and some we need to route via AMS. We want to use the tags to identify the route we don't want, and drop it in favour of the remaining one that we do.

The OSPF sites (all are area 0) pick up the routes and tags fine when just from one source.

Here's a test example:

--

R3#sh ip route 192.168.100.0

Routing entry for 192.168.100.0/24

  Known via "ospf 1", distance 110, metric 20

  Tag 200, type extern 2, forward metric 1

  Last update from 3.3.3.1 on FastEthernet0/0, 00:00:04 ago

  Routing Descriptor Blocks:

  * 3.3.3.1, from 1.1.1.1, 00:00:04 ago, via FastEthernet0/0

      Route metric is 20, traffic share count is 1

      Route tag 200

--

R3#sh ip route 192.168.100.0

Routing entry for 192.168.100.0/24

  Known via "ospf 1", distance 110, metric 20

  Tag 100, type extern 2, forward metric 1

  Last update from 2.2.2.2 on FastEthernet0/1, 00:00:00 ago

  Routing Descriptor Blocks:

  * 2.2.2.2, from 2.2.2.2, 00:00:00 ago, via FastEthernet0/1

      Route metric is 20, traffic share count is 1

      Route tag 100

--

When I put in a distribute-list I can also successfully match & drop the tagged route.

However, when both hit R3 at the same time, something strange seems to happen - we get both routes but the tags get made the same:

--

R3#sh ip route 192.168.100.0

Routing entry for 192.168.100.0/24

  Known via "ospf 1", distance 110, metric 20

  Tag 100, type extern 2, forward metric 1

  Last update from 3.3.3.1 on FastEthernet0/0, 00:00:35 ago

  Routing Descriptor Blocks:

    3.3.3.1, from 1.1.1.1, 00:00:35 ago, via FastEthernet0/0

      Route metric is 20, traffic share count is 1

      Route tag 100

  * 2.2.2.2, from 2.2.2.2, 00:08:24 ago, via FastEthernet0/1

      Route metric is 20, traffic share count is 1

      Route tag 100

--

As you see, they both say tag 100 instead of 100 & 200 when only one route is received. This stops us identifying and dropping one of them.

Any ideas? Is this normal behaviour?

thanks a lot

David

[Ryan Curry]

David, this is only speculation since we don't tag the same route differently in our environment, but I suspect what you're seeing is how the router views the route that it wants to send out.  Even though you have two routes (same route, different tags) coming in, the router will need to send this out to its neighbors as a single route statement (to get to 192.168.100.0/24 go through me) which will only use a single tag.  Which one it chooses I couldn't say why, but I suspect that if you want to change this tag to 200, you may need another route-map to reset the tag to southbound neighbors.

Again, speculation, but I hope it helps (TAC may be able to help as well).