Route Summarization question

Unanswered Question
Jul 2nd, 2008

I have a router which has OSPF area0 and and eBGP configured. I have the subnets such as 10.22.33.0/24 available through my OSPF and they are E2 / E1 routes. Also, I will be receiveing huge number of prefixes in 10.x.x.x range from the eBGP.

I would like to have a summary route for 10.0.0.0/8 to be available throughout the ospf domain, when redistributed from eBGP. If I confiugre summary-address 10.0.0.0 255.0.0.0 will this install a summary route in my ospf domain?

If yes, what will happen when my eBGP goes down and the component subnets withdrawn from BGP but still available on the OSPF domain. Will the summary route get withdrawn?

I have this problem too.
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lee.reade Wed, 07/02/2008 - 02:25

Hi,

The summary-address is derived from the redistributed bgp routes, which are already in the ospf database, therefore if ALL of these routes disappear from bgp, hence they will not be in the ospf database and the summary-address will not be created.

If however, there is a single 10.x.x.x network in the ospf database, perhaps the network for your bgp connection, then the summary will still be generated, and you will have a black hole routing issue.

HTH,

Cheers

LR

mastram4u Wed, 07/02/2008 - 02:37

Hi LR,

Thanks for the quick post. There will not be any 10.0.0.0/8 route in the ospf domain. So my understanding from your post is that there will not be any problem.

Is there a way that I can add some metric to these routes? I will have to originate them from two different routers for backup.

However, for my clarification, does OSPF not take into consideration the E1/E2 routes already available under OSPF?

Can you please refer me to any document which talks about redistributions in detail?

Regards

Ramesh

lee.reade Wed, 07/02/2008 - 02:53

HI,

Correct, you will have no problem with the redistribution.

With regard to the metrics, well do you want to have some routes go one to one router, and the others go to the other one?

You can either set the metric in the redistribute command or you can use route-map, however since you will be summarising then it would prob be easiest for you to it at the point of redistribution.

Im not quite sure what you mean about ospf taking into account the routes already with e1/e2.

Note that ospf will prefer e1 routes to e2 when selecting a best path.

HTH,

Cheers,

LR

ps, have a look here for some excellent redistribution info;

http://blog.internetworkexpert.com/category/ccie-routing-switching/interior-gateway-routing/

mastram4u Wed, 07/02/2008 - 03:04

Thanks for all the help you are providing..

I will have the 10.0.0.0/8 advertisements from two routers, part of the same ospf domain. I should be having higher metric from the backup router so that in case the primary router fails, the route to 10.0.0.0/8 prefers the backup path.

I have the configuration " redistribute bgp 65100 metric 500 subnets route-map BGPOSPF" on the backup router. The route-map calls the access-list which permits the prefixes.

Do I need to permit 10.0.0.0/8 on the access-list?

If I got you correctly again, the summarization will happen first, then it will go through the access-list on the redistribution command and decide whether to redistribute or not? Is this a right understanding?

lee.reade Wed, 07/02/2008 - 03:11

Hi,

You dont actually need to have anything in the access-list, and you dont really need a route-map either, you could do it via "redistribute bgp 65100 metric 500 subnets" this would redistribute all bgp routes into ospf with metric 500.

If you want to import only 10.x.x.x routes, say you have 20.x.x.x also in bgp, then yes use route-map BGPOSPF permit 10

match ip address 1

access-list 1 per 10.x.x.x 0.255.255.255

You have the order of operations incorrect, summarisation will only be done if the external routes are in the ospf database, therefore the order is;

bgp-ospf via redistribute-summary-address-

HTH,

Cheers,

LR

mastram4u Mon, 07/07/2008 - 20:01

Hi LR,

Another follow-up question.

Is it possible to advertise selective smaller subnets alongwith the summary route when I use the summary-address command?

Regards

Ramesh

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