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New Member

who can help to summarize the routes?

R1 is the OSPF hub router of R2 and R3

on R2, there are some P2P routes (/30), IP range from 10.1.1.1~63

on R3, there are some P2P routes (/30), IP range from 10.1.1.65~254

on R2 and R3, under OSPF process, do redistribute connected subnet and

R1 will see all specific routes with /30 mask

how can I summarize route on R2 and R3 in result of only two routes shown on R1?

the two routes may like 10.1.1.0/26 and what????

thanks

9 REPLIES
Hall of Fame Super Silver

Re: who can help to summarize the routes?

If R1, R2, and R3 are in the same area then it will not be possible to summarize the routes. If you configure OSPF so that the interfaces of R1 are in area 0, and R2 link to R1 is area 0, and R2 P2P are in another area (maybe area 2) then R2 will be an ABR (Area Border Router) and the ABR can use the area 2 range command to summarize the P2P routes and send one summary to R1. Similarly if R3 has its link to R1 in area 0 and its P2P in area 3 then it is also an ABR and can use the area 3 range command to summarize its P2P links and send one summary to R1.

HTH

Rick

Bronze

Re: who can help to summarize the routes?

Rick,

Using your suggestion would it be possible for R3 to send one summary route? Please correct if I'm wrong but would the summary for 10.1.1.64-255 require two summary statements; 10.1.1.64/26 and 10.1.1.128/25?

Ryan

Hall of Fame Super Silver

Re: who can help to summarize the routes?

Ryan

Good catch. You are correct. I had assumed that R2 and R3 had similar size group of subnets. But that is not what the question actually said when I read the fine print. So yes to summarize the addresses given would need two area range statements.

HTH

Rick

New Member

Re: who can help to summarize the routes?

thanks for you guys input

acutally, the orignial question is that only 2 routes can be learned on R1 regardless OSPF area because I am redistributing the connected routes (not using area range command)

on R2, it is sure that the routes can be summarized to 10.1.1.0/26

on R3, it is for sure the routes can be summarized to two routes, but on R1 will have 3 summarized routes

is there any possible the R3 can summarize like this way

10.1.1.64 255.255.255.64

mathmeticly, it is working, but can not apply on cisco router (which cause incontigous mask)

thanks again for your input

Bronze

Re: who can help to summarize the routes?

No, I don't beleive it is possible to summarize 10.1.1.64-255 into one route. I'm not even sure that 255.255.255.64 is a valid mask.

To summarize 10.1.1.64-255 two routes are needed; 10.1.1.64/26 and 10.1.1.124/25.

10.1.1.64 = x.x.x.01000000 (64 binary) w/26

Due to the high order bits being zero and one the highest summarized subnet or network that could be inluded (when all 1's are on in "host" portion) would be 10.1.1.127.

10.1.1.127 = x.x.x.01111111

10.1.1.128 = x.x.x.10000000 w/25

yeilds 10.1.1.255 as the highest summarized network.

I don't really see any other way if doing this...

Hall of Fame Super Silver

Re: who can help to summarize the routes?

Ryan is quite correct here. There is not any way to summarize 10.1.1.64-255 in one route. The best that can be done for this is summarization into 2 routes, as Ryan has pointed out.

While discontiguous masks can be used in access lists there is a requirement in subnetting and in summarization that the mask be contiguous. So while the 255.255.255.64 mask is interesting it is not a valid mask for sumarization.

HTH

Rick

New Member

Re: who can help to summarize the routes?

thanks again for your input.

actually, I found a good way to summarize them to 2 routes

10.1.1.1~63 can be summarized to 10.1.1.0/26

10.1.1.64~255 can be summarized to 10.1.1.0/24

since R1 will learn two routes, so even summarized to 10.1.1.0/24 is not very exact, but anyhow, it works, R1 knows how to get detailed routes

you guys did not think it carefully ^_^

Bronze

Re: who can help to summarize the routes?

Clever way to take advantage of classless routing!

In normal operational network this will work; however if an interface in the 10.1.1.0/26 residing on R2 goes down I believe there is potential for a routing loop.

For example let?s say network 10.1.2.0/24 interface on R2 state changes to down. Any traffic destined to that network will be sent to R2 from R1. R2?s route table will not have any entry for the downed network but it will have a summary route for 10.1.1.0/24 w/ next hop of R1 (R1 rx?d the summary route from R3). The packet will be sent right back to R1 and because of the summary route 10.1.1.0/26, it will have no choice but to send it back to R2; thus creating a loop.

Unfortunately I don?t have any routers accessible to test my ?theory?. While the configuration will work I would strongly advise against it in this particular situation. In the big scheme of things what?s one more summary route (10.1.1.64/26 + 10.1.1.128/25) if it prevents potential undesirable routing?

New Member

Re: who can help to summarize the routes?

R1---R2---10.1.1.0/26

R1---R3---10.1.1.0/24

can you explain again where the loop is from, I did not see any loop

thanks

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