OSPF route distribution with summary on upstream router

Answered Question
Dec 2nd, 2008
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Having some issues with a simular setup:


4 routers connected in series:


RTR A >> RTR B >> RTR C >> RTR D


How do I only show the summary in RTR A routing table?


------------

RTR A


fastethernet 0/1

description link to RTR B

ip address 10.1.1.1 255.255.255.252


router ospf 1

network 10.1.1.0 0.0.0.255 area 0


>>>>>>Routing table contains summary address:


O IA 10.250.0.0/23 VIA 10.1.1.2


>>>>>>As well as the individual advertisments:


O E2 10.250.0.0/24 VIA 10.1.1.2

O E2 10.250.1.0/24 VIA 10.1.1.2


------------

RTR B


fastethernet 0/1

description link to RTR A

ip address 10.1.1.2 255.255.255.252


fastEthernet 0/2

description Link to RTR B

ip address 10.2.2.1 255.255.255.252


router ospf 1

network 10.1.1.0 0.0.0.255 area 0

network 10.2.2.0 0.0.0.255 area 1

network 10.250.0.0 255.255.252.0 area 1 <<<<<<<<<<< If I add this the summary address apears as shown in RTR A table.


area 1 range 10.250.0.0 255.255.254.0


------------

RTR C


loopback 1

ip address 10.250.1.1 255.255.255.0


fastEthernet 0/1

description Link to RTR A

ip address 10.2.2.2 255.255.255.252


fastEthernet 0/2

description Link to RTR B

ip address 10.2.2.5 255.255.255.252


router ospf 1

redistribute connected subnets

network 10.2.2.0 0.0.0.255 area 1


------------

RTR D


loopback 1

ip address 10.250.1.1 255.255.255.0


fastEthernet 0/1

description Link to RTR B

ip address 10.2.2.6 255.255.255.252


router ospf 1

redistribute connected subnets

network 10.2.2.0 0.0.0.255 area 1


Correct Answer by lejoe.thomas about 8 years 5 months ago

Hi,

From OSPF's perspective, since D and C are redistributing routes, they are treated as external routes (those connected segments). So to configure summarisation use summary-address command under ospf process on both D and C. This will cause both A and B to see the summarised routes. Please note, not correctly summarising on C and D would lead to ambiguity because Router B will not be able to make the difference (because the first two octets are the same for the connected segment on C and D)


The best solution is to use the network command and make those interfaces on which you dont want to run ospf passive.




If you would like only A to see summarised routes, you must move it into another area and make it a stub area(no type 5 LSAs). By doing so, it will no longer see those external routes at all rather have default route to the ABR for all external routes.


HTH

Lejoe

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lejoe.thomas Tue, 12/02/2008 - 19:19
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Hi Daniel,


This is probably because those routes are external routes, in which case summary-address command must be used. Area range commands are used to summarise I.A routes not external routes.


Instead of redistributing connected segments on C and D, use the network command in ospf process on C and D. Then use the area 1 range command on router B, ofcourse you'll have to remove the line

network 10.250.0.0 255.255.252.0 area



HTH

Lejoe


Daniel Graham Tue, 12/02/2008 - 20:10
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I appreciate your reply, I have tried that and it works, I would prefer not to run ospf on c and d 10.250.x.0 interfaces.


Is there anyway to get around that?


Thanks -

Correct Answer
lejoe.thomas Tue, 12/02/2008 - 20:19
User Badges:
  • Silver, 250 points or more

Hi,

From OSPF's perspective, since D and C are redistributing routes, they are treated as external routes (those connected segments). So to configure summarisation use summary-address command under ospf process on both D and C. This will cause both A and B to see the summarised routes. Please note, not correctly summarising on C and D would lead to ambiguity because Router B will not be able to make the difference (because the first two octets are the same for the connected segment on C and D)


The best solution is to use the network command and make those interfaces on which you dont want to run ospf passive.




If you would like only A to see summarised routes, you must move it into another area and make it a stub area(no type 5 LSAs). By doing so, it will no longer see those external routes at all rather have default route to the ABR for all external routes.


HTH

Lejoe

ralphcarter Tue, 12/02/2008 - 20:39
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If you dont want to run ospf on C & D, then why dont you create a static route on Router B and "redistribute static subnets" into ospf. This will make Router B an ASBR which will allow you to summarize if needed, meaning if you use multiple static routes.


Another option...



Ralph

www.techsnips.com


Daniel Graham Wed, 12/03/2008 - 20:00
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Thanks for the option, because of the amount of static routes I would have I would rather have OSPF handle them.


Thanks,

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