advice

[Tareq Emad Al Asfoor]

Hi,

Any one can advice me ... If i have 4 routers   (  A > B > C > D ) the ospf area between ( A & B) on area 0  and the area between ( B & C ) on area 0 and the area between ( C & D ) on area 1 .. 

I know the summary route can done in ABR or ASBR  switch ..  So what is the best solution to send some thing like summary from Router B to Router A .. 

I will be thankful for any advice .. 

Kind Regards, 

[Peter Paluch · Accepted solution]

Tariq,

What you request is not possible with OSPF. You can summarize only on ABRs and ASBRs, just as you have correctly noted yourself. You cannot summarize inside an area. You could configure, say, a static route on router B to Null0 that would represent the summary network and redistribute it into OSPF, but that would not suppress (filter out) the subnets you would want to summarize.

Is there any specific reason you want to do this, or is this merely a lab exercise?

Best regards,
Peter

[Tareq Emad Al Asfoor]

Thanks Peter .. Nope, This is merely a lab exercise .. :)

If i using the static null0 the router B will still advertise the small subnet .. So This will not useful .. :(

Kind Regards

[dukenuk96]

Hi

in this case I would suggest you on router B:
1. Create static route (that summary you would like)
2. Manually redistribute this static route into OSPF process
3. Remove IP interfaces from OSPF

Also note that after this router B will become ASBR and will use Type 5 External LSA to advertise that static route.

[Tareq Emad Al Asfoor]

Thanks .. But if i removed the Ip interface from OSPF this will remove the peering between the router A and B .. I need Router A still see router B as OSPF .. 

[dukenuk96]

You do not need to remove interfaces from OSPF which are used for peering between routers, only those one which you want to summarize.

[Tareq Emad Al Asfoor]

mmm .. Thanks .. I will be thankful if you can support me with small command line example ... Thanks in advance  

[dukenuk96]

Well... it's difficult without actual configs, but I'll try.

Let's assume you have e0/0 and e0/1 interfaces used for communication between rotuers with addresses 172.16.1.1/30 and 10.1.1.1/30. Also you have interfaces e0/2 and e0/3 for users usage with addresses 192.168.0.0/24 and 192.168.1.0/24. So OSPF config for your situation will look like this:

ip route 192.168.0.0 255.255.254.0 Null0

router ospf 123
 network 172.16.1.1 255.255.255.255 area 0
 network 10.1.1.1 255.255.255.255 area 0
 redistribute static subnets

There maybe much more params and filtering options, but I think at this moment that will be enough for you. Next you should see similar output on neighboring router:

Router#sh ip os data

<...>

Type-5 AS External Link States

Link ID ADV Router Age Seq# Checksum Tag

192.168.0.0 10.1.1.1 12 0x80000001 0x00a3a7 0

Router#sh ip ro

<...>

O E2 192.168.0.0/23 [110/20] via 10.1.1.1, 00:00:08, FastEthernet0/0

192.168.0.0/32 is subnetted, 1 subnets

[Tareq Emad Al Asfoor]

Cool , This mean in Router A  when i do  ( show ip route ) i should see only one row in the routing table with subnet ( 192.168.0.0./23 ) .. Thanks mate .. 

I will check and test this and give you feedback about the result .. Thanks so much :) ..

But i'm still have small question .. Let we assume we have another static route but not on Null0 .. So , Here The  (redistribute static subnets ) will distribute them also or only for the static that is on Null0  ?  Thanks in advance 

Kind Regards, 

[dukenuk96 · Accepted solution]

All statis routes will be redistributed, not just Null0, you can configure filtering using route maps to select which exact routes you want to redistirbute. It just a good practice to create Null0 routes in such cases to prevent routing loops.

[Tareq Emad Al Asfoor]

Thanks Mate .. I  will test this and give you the feedback .. 

Kind Regards,