OSPF Summarisation with discontiguous subnets

Unanswered Question
Oct 12th, 2006
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Can anyone advise me in the following situation?

We have 4 routers connected in a 'diamond' as follows

R1 at site 1, R2 at site2, R3 at site 3, R4 st site 4.

R1 connects to R2

R1 aLso connects to R3 via a seperate link

R2 connects to R4

R3 also connects to R4 via a seperate link

R1 is in area 0

R4 is in area 1

R2 and R3 are the ABRs between areas

The idea is that if any link fails the sites will remain connected

once ospf recalculates.

Under normal circumstances traffic between site 1 and site 4 should be

routed via R2 and only go via R3 in the event of a failure in between R1

and R2.Traffic between site 1 and 3 should be routed direct between R1 and R3

and go via R4 and R2 in the event of a failure betwwen R1 and R3

The problem we have is suboptimal routing between site 1 and site 4.

Traffic between R1 and R4 is being routed by a higher cost path (via R3)

rather than the lower cost path via R2. The reason I believe is summarization

on the ABRs.

R2 is summarising into area 0

R3 is not summarising so is advertising all subnets in area 1 to area 0

As the summarised routes and the non-summarised routes are all in the routing

table on R1 the path being chosen is based on longest prefix rather than cost and hence

highest cost path is being used?

I can adjust the summary addresses but we have discontiguous subnets at site 3 and 4


site 4:,,,, -

site 3:,,,,, -

Options as I see it:

Re-addressing subnets to allow a single summary from R2 and R3 into area 0 for appropriate

subnets off these routers

Using numerous area 1 range commands on r2 and r3 to summarise networks appropriately from

each router

Distribute list on R1 to prevent certain subnets advertised by r2 or r3 from being added

to the routing table on r1

Can anyone advise if my assumptions are correct and suggest a solution?


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Overall Rating: 3 (2 ratings)
Richard Burts Thu, 10/12/2006 - 04:53
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If R2 is summarizing and R3 is not then that certainly explains why traffic is routed via R3. You are certainly correct that the routing decision considers longest match first thing and only compares cost when it has more than one path for the particular prefix that it is considering.

In terms of options I certainly agree that re-addressing the network so that subnets are contiguous and can be efficiently summarized would solve the problem and be good in the long run. Since we do not know details of the topology of your network we can not assess the amount of effort required to do this. But it probably is the cleanest and most efficient routing solution.

It seems to me that another option is to remove summarization from R2. If both R2 and R3 are advertising detailed routes then you should be able to manipulate the metrics to achieve the routing solution that you need. Since we do not know details about the topology of your network we can not assess the impact of doing this. But given the little that we do know (4 routers, 2 areas) I am not sure that the impact would be great.

I think that you might also consider an alternative of configuring summarization on R3 equivalent to the summarizatino done on R2. If both routers are ABRs between area 0 and area 1 they should both know the same routes. So I do not see that you would lose any connectivity if you did summarize on both routers.

I like the alternative of the distribute list the least. One thing I dislike about it is that I believe that it could cause lack of connectivity. If you use the distribute list to block some routes from R3 and then there is a problem between R1 and R2 the result would be that R1 would not have routes to at least some of the destinations in area 1.



ddavenport-dcc Thu, 10/12/2006 - 05:52
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Thanks Rick

Looks like re-addressing in the favorite then depending on the effort involved which I'll have to assess. If its too much in the short term I'll go the re-summarisation route and avoid distribute lists just in case!



caiyu19771110 Fri, 10/13/2006 - 06:32
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before deciding which protocol you want to use, the first thing is design your network with continuous ip address space which is important to all routing protocols. and then you can summarize your network properly.

as you mentioned, your network seems not huge, so you can disable summarization feature.

on the other hand, as you know, OSPF is link state protocol, so distribute list is not suitable to it comparing with distance vector routing protocol such as RIP or EIGRP.


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