Summarization in OSPF

Answered Question
Jun 3rd, 2008

There is network scheme on the attached picture. Summarization of routes from area 0 (10.0.1.0, 10.0.2.0, 10.0.3.0, 10.0.4.0) into 10.0.0.0/8 is configured on router B (review the attached config). They appear as individual prefixes in "Summary Net Link States (Area 5)" on router B, but not as whole 10.0.0.0/8 (review the show ip ospf database). There is virtual-link between routers B and I. If I remove this virtual-link, all work properly and I see only one summary prefix in area 5. But with virtual-link I see 4 individual prefixes in area 5. How this virtual-link can influence the summarization?

I have this problem too.
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Correct Answer by Harold Ritter about 8 years 6 months ago

Just as a follow-up, The scenario I described above is not valid as router I would not generate a type 3 LSA for 10.0.1.0/24 in area 5 as it is the transit area.

Also, if you want the "area range" on router B to take effect despite of the virtual-link (i.e. disable the behavior stated in RFC2328), you can use the command "no capability transit", as described in the following document:

http://www.cisco.com/en/US/docs/ios/12_0s/feature/guide/ospfatc.html

Regards,

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solpandor Wed, 06/04/2008 - 01:33

Hi

As far as Im aware, a virtual link should have no influence on the summarized routes, but looking at router B's config, your summarization doesnt look right, the subnet mask should be 255.255.248.0 (experienced forum members please correct me if im wrong) not that this answers your question but just as an observation. Also, please post routing table of B.

kondratev Wed, 06/04/2008 - 02:34

I use the whole network 10.0.0.0/16 only in area 0. So it need not to configure longer mask than 16.

I've added in attachements routing table for router B.

Kevin Dorrell Wed, 06/04/2008 - 02:04

I shall have think about this interesting problem, but here are my initial thoughts:

What is happening here is that the virtual-link is extending area 0 into router 'I' by tunelling through area 5. That is what virtual links do. So router 'I' will have a full database for area 0.

So router 'I' has areas 0, 5, and 20, and it will be acting as an ABR for areas 5 and 20. It is router 'I' that is injecting the prefixes into area 5 on behalf of router B. Interesting though that the "advertising router" is 100.2.

Kevin Dorrell

Luxembourg

kondratev Wed, 06/04/2008 - 03:31

You're right. I've add logical scheme to this post (as I understand it) and 'show ip ospf database' on routers I and F. So, ABRs can't inject the summary prefix in area 5, can they? And what is more one of ABSs is injecting individual prefixes as summaries!

royalblues Wed, 06/04/2008 - 04:04

If you have more than one ABR between an area and the backbone area, sending a summary (type 3) LSA with the explicit network information into an area ensures that the shortest path to destinations outside the area is selected.

If you summarize the addresses, suboptimal path selection may occur. This might be the reason why the individual prefixes are also appearing along with the summary

Narayan

kondratev Wed, 06/04/2008 - 04:12

Do you want to say that I must suppress advertisements of that prefixes into area 5 on router I (with distribute-list)?

Kevin Dorrell Wed, 06/04/2008 - 04:24

Unfortunately, distribute lists are not so useful in OSPF becauise they only affect the local router.

If you really want 'I' not to inject the more specific routes, put the area 0 summary on 'I' as well as on 'B'.

Kevin Dorrell

Luxembourg

kondratev Wed, 06/04/2008 - 04:33

I'm so sorry! I've already done it, but forgot to attach new config. The issue remains (all posted 'show ip ospf database' I've got with this new config).

Attachment: 
Harold Ritter Wed, 06/04/2008 - 05:04

Andrew,

This is normal behavior for the area range not to work with a virtual link configured, as per RFC2328, section 12.4.3:

"If an area is capable of carrying transit traffic (i.e., its TransitCapability is set to TRUE), routing information concerning backbone networks should not be condensed before being summarized into the area. Nor should the advertisement of backbone networks into transit areas be suppressed. In other words, the backbone's configured ranges should be ignored when originating summary-LSAs into transit areas."

For more information, please refer to RFC2328:

http://www.ietf.org/rfc/rfc2328.txt?number=2328

Regards,

Harold Ritter Wed, 06/04/2008 - 05:36

Kevin,

This happens to me on a daily basis :-)

I should explained what could be the end result if this rule didn't exist. Let's take Andrew's network as an example.

Because of the virtual-link configured between router B and I, both are seen as ABRs for area 5, which is the transit area for the VL. Both of these routers see all the non-summarized prefixes from area 0. If router B was configured to summarize these prefixes into area 5 and router I was not, this would mean an automatic routing loop.

For instance, traffic from router I to 10.0.1.0/24 would follow the route learnt via the VL (i.e. NH = router F). Router F on the other hand, would have a route of 10.0.0.0/16 via router B and a route of 10.0.1.0/24 via router I and traffic would therefore be returned to router I.

Regards,

Correct Answer
Harold Ritter Wed, 06/04/2008 - 07:30

Just as a follow-up, The scenario I described above is not valid as router I would not generate a type 3 LSA for 10.0.1.0/24 in area 5 as it is the transit area.

Also, if you want the "area range" on router B to take effect despite of the virtual-link (i.e. disable the behavior stated in RFC2328), you can use the command "no capability transit", as described in the following document:

http://www.cisco.com/en/US/docs/ios/12_0s/feature/guide/ospfatc.html

Regards,

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