2 NSSA ABRs and summarization

[jmcoffey]

I have two related questions about having a NSSA area with two (2) ABRs and external route summarization. I hope someone can give me a definite answer and thank you in advance for your time.

All the external routes in this NSSA area are RIP routes and in the range of 111.222.x.x/16. There are approx. 8 ASBRs in this NSSA area and 2 ABRs attached to area 0. My question - Can and how do I configure the ABRs to summarize all these external RIP routes to the backbone area with one advertisement to area 0? If I have 2 ABRs attached to this NSSA area which router will actually advertise this summary route? In other words, I want to aggregate all the various type 7 external routes being injected into the NSSA as one type-5 summary advertisement to the rest of the network so to reduce the number of type-5 external route advertisements in the OSPF database in the backbone routers.

I have done all the summarization I can do at the ASBRs in this NSSA area. This is an older network in which clustering of IP addresses for proper aggregation at ASBRs was not originally accomplished optimally.

Thanks!

Jim Coffey

[thisisshanky · Accepted solution]

Hello Jim,

In a NSSA, all RIP routes would appear as ON2 routes..(or ON1)...inside the NSSA. These routes will be automatically converted by the NSSA ABRs. So in this case, both would convert eash ON2 or ON1 route into a OE2 or OE1 route.Now regarding summarization, you can summarize them at both the ABRs, into a single route. so that even if one of the ABR goes down, the other is available...

once you get the external routes on the ABR, use "area range " to summarize the routes into a singleone..

area-id = area id of NSSA

range should be the summarized range.

Hope thathelps

[jmcoffey]

Thanks, that helps :-) One more question...

When both ABRs are operating properly, which one will advertise this summary route? (possibly the one with the higher router ID?) Or will both ABRs advertise it at the same time? Can I set a default cost?

[thisisshanky]

Both will advertise the summary, if you summarize them at both the ABRs.

You can set a cost for the summary if you want...

area

< summar-mask> cost

Choose which ABR u want as primary and set a lower cost for that summary route.

Hope that helps!

[jmcoffey]

What I have now is a normal OSPF area with 8 ASBRs attached (ppp links) to two ABRs and many (approx 50) RIP routers attached to the ASBRs . The ABRs also have RIP circuits/networks/routers attached to them. This area only uses a single class B address range BUT the "original design" did not summarize any of the external RIP ip addresses at the ASBRs because the ip addresses at the ASBRs are not continous and therefore can not be summarized there (at the ASBRs).

At this point I have a area range command on the ABRs and it has sucessfully summarized all the "OSPF" router links to one type-3 LSA distributed to the rest of the network. But I still have a 100 or so type-5 LSAs flooding the rest of the network. This is the reason I want to convert this area to NSSA so that I can summarize the external RIP routes (type-7) at a NSSA ABR to ONE(1) type-5 LSA along with the one type-3 LSA.

The 'area range' command will summarize all the OSPF routes in a area. But I ALSO want the ABRs to summarize ( and translate) all the EXTERNAL type-7 LSAs to one AS external type-5 to the rest of the OSPF domain. Which probably calls for a 'summary-address' statement. Correct? What I have now are approx. 100 type-5 LSA going thoughout the network from the this normal area which I want to convert to NSSA to eliminate all these external routes.

Also according to RFC1587 ... I quote

Translating Type-7 LSAs Into Type-5 LSAs

This step is performed as part of the NSSA's Dijkstra calculation

after type-5 and type-7 routes have been calculated. If the

calculating router is not an area border router this translation

algorithm should be skipped. All reachable area border routers in

the NSSA should now be examined noting the one with the highest

router ID. If this router has the highest router ID, it will be the

one translating type-7 LSAs into type-5 LSAs for the NSSA, otherwise

the translation algorithm should not be performed.

Which seems to means to me that only one ABR (the highest router-ID) does type-7 to type-5 translations. If this is the case then a second (lower router-ID) ABR in a NSSA area would never see any incoming traffic and would only do so if the primary ABR failed. Am I correct?

Basically, the main question is if BOTH or only ONE ABR would generate these summary LSAs....

I am still wondering :-(

Thanks,

Jim Coffey

jmcoffey@charter.net

[maneesh]

Dear Jim,

If there are multiple ABR's for a NSSA area, the router with the highest router ID is the one responsible for tranlating type 7 LSA into Type 5 and flood it to area 0. Since the choice is dependent only upon one router, it is the only router to perform conversion between LSA types and generate the summary route to area 0.

This can be found out at the following url

http://www.cisco.com/en/US/tech/tk648/tk365/technologies_tech_note09186a0080094a88.shtml

Hope this helps.

Best Regards

Maneesh

[jmcoffey]

Thanks for the reply

Since only one NSSA ABR distributes a summarized AS external type-5 LSA, then the second (lower router-ID) ABR in a NSSA area would never see any incoming external traffic and would only do so if the primary ABR failed. All incoming external traffic would come to the primary ABR.

To try to overcome this poor external routing, my question now is, can I configure a static route on the lower router-ID NSSA ABR to distribute a type-5 LSA?

For example, if the LSA from the "primary" NSSA ABR had a advertised cost of 50 for a summarized address range of 111.222.0.0/24 and I configured a static route for the SAME address range and metric cost on the "secondary" NSSA ABR, would the "secondary" ABR distribute this static route to the backbone?

My idea here is to get the static type-2 cost equal to the cost of the type-2 LSA distributed from the primary ABR/ASBR. If the external routes are both type 2 routes and the external costs to the destination network are equal, then the path with the lowest cost to the ASBR is selected as the best path.

The cost advertised by a type-2 external LSA is the same thoughout the OSPF domain. A type-1 external LSA cost will also take into account the cost to the ASBR and add the external cost to it and will vary thoughout the OSPF domain.

IF this is all true then this would overcome the poor external network routing of a NSSA area with two ABRs?

I believe the primary ABR determines the metric cost of the summarized LSA it sends to the backbone by using the largest cost to a N2 network and adding one(1).

[maneesh]

Everything in your message above seems logical enough, except for one thing. When you ask whether the static routes configured on the secondary NSSA ABR will be advertised to the backbone. The answer is no.

You will have to configure redistribution again on the secondary ABR making it a ASBR. However while doing this the following points need to be noted: -

1) Redistribution will cause this to be flooded back to NSSA area as a Type 7 LSA. So you need to use the area filter-list command to prevent it from being propagated back to NSSA area.

2) If you are redistributing the static route summarizing the NSSA type 7 routes as a E2 route, then care needs to be taken the primary ABR is also receiving type7 E2 routes so that it also summarizes the routes as E2 route into E0.

3) One more interesting point is when the primary ABR goes down and your secondary ABR takes over, which is now also the ASBR, there will be two type 5 routes flooded to the backbone. I am not sure about this.

The best way will be to test it in a lab setup. Do let me know the results if you do test it !!

Best Regards

Maneesh