Weird static to OSPF redistribution problem

[tommyboay]

Hi all,

I have two 7604 routers running IOS 12.2(33)SRB5a as ASBRs in my network. They both connect to a common AS via DS3 lines.

FR01 is considered my main circuit, FR02 is the backup. But I have some heavy disk sync traffic I want to route via the backup unless that circuit is down.

Here is the remote AS config on both routers. We summarize the routes and only change the redistribute metric to distinguish main from backup circuit :

router ospf 1000

  log-adjacency-changes

nsf

summary-address 10.4.0.0 255.254.0.0 tag 33

summary-address 172.18.0.0 255.255.0.0 tag 33

summary-address 10.5.212.0 255.255.255.0 tag 34

redistribute ospf 93 metric 23000 subnets route-map EU_US

passive-interface default

Here is the local AS config on both routers (static redistribution only occurs on FR02).

router ospf 93

log-adjacency-changes

auto-cost reference-bandwidth 10000

nsf

area 2 stub no-summary

area 3 stub no-summary

area 4 nssa no-redistribution no-summary

area 5 nssa no-redistribution no-summary

area 6 nssa no-redistribution no-summary

area 7 stub no-summary

area 8 stub no-summary

area 9 stub no-summary

area 12 stub no-summary

area 13 stub no-summary

area 22 stub no-summary

area 23 stub no-summary

area 24 stub no-summary

summary-address 10.2.0.0 255.254.0.0 tag 1000

summary-address 172.16.0.0 255.255.0.0 tag 1000

redistribute static subnets

redistribute ospf 1000 metric 23000 subnets route-map US_EU

(Please note that I omitted quite some output.)

Here are my routemaps (basically using tags to avoid redistribution loops)

route-map US_EU deny 10

match tag 33

!

route-map US_EU permit 20

set tag 1000

!

route-map EU_US deny 10

match tag 1000

!

route-map EU_US permit 20

set tag 33

And finally I have my static route for the specific disk sync subnet :

track 2 interface Serial3/0/0 line-protocol

ip route 10.2.34.0 255.255.255.0 10.2.244.85 track 2

When I enable the static redistribution on my FR02 router, ALL the traffic toward the remote AS is redirected via FR02.

See here on a network part of the remote AS that should NOT be lead to FR02 :

agfr01cswt02#show ip route 10.2.200.0

Routing entry for 10.2.0.0/15

  Known via "ospf 53", distance 110, metric 20, type extern 2, forward metric 45

  Last update from 10.5.55.3 on Vlan55, 00:10:10 ago

  Routing Descriptor Blocks:

  * 10.5.55.3, from 10.5.0.134, 00:10:10 ago, via Vlan55

      Route metric is 20, traffic share count is 1

When I remove the static redistribution, it comes back to FR01 :

agfr01cswt02#show ip route 10.2.200.0

Routing entry for 10.2.0.0/15

  Known via "ospf 53", distance 110, metric 18000, type extern 2, forward metric 20

  Last update from 10.5.55.3 on Vlan55, 00:00:36 ago

  Routing Descriptor Blocks:

  * 10.5.55.3, from 10.5.0.133, 00:00:36 ago, via Vlan55

     Route metric is 18000, traffic share count is 1

How could a static redistribution possibly influence all my routes ?

[Peter Paluch]

Hello Thomas,

I may be shooting blindly, as I do not have a good overview of your network yet, but what I find interesting is the following:

With the redistribution on FR02 activated:

agfr01cswt02#show ip route 10.2.200.0

Routing entry for 10.2.0.0/15

  Known via "ospf 53", distance 110, metric 20, type extern 2, forward metric 45

  Last update from 10.5.55.3 on Vlan55, 00:10:10 ago

  Routing Descriptor Blocks:

  * 10.5.55.3, from 10.5.0.134, 00:10:10 ago, via Vlan55

      Route metric is 20, traffic share count is 1

Without the redistribution on FR02:

agfr01cswt02#show ip route 10.2.200.0

Routing entry for 10.2.0.0/15

  Known via "ospf 53", distance 110, metric 18000, type extern 2, forward metric 20

  Last update from 10.5.55.3 on Vlan55, 00:00:36 ago

  Routing Descriptor Blocks:

  * 10.5.55.3, from 10.5.0.133, 00:00:36 ago, via Vlan55

     Route metric is 18000, traffic share count is 1

This suggests that both FR01 and FR02 are redistributing and summarizing these routes. However, the FR02 redistributes these routes with the E2 metric set to 20 (the default value) or higher while the FR01 is redistributing them with the E2 metric of 18000 or higher. The E2 metric is the first tiebreaker when two routers redistribute the same set of external E2 routes, and with these values, FR02 naturally wins.

When summarizing networks, the metric of the summary network is set to the lowest metric from among all routes that were summarized. Apparently, on FR02, the lowest metric among static redistributed routes was 20, and that metric was also inherited by the summarized address 10.2.0.0/15.

What puzzles me slightly is that I do not see any tags in the show ip route output although there should be some tags visible (you are tagging the summary route with the tag 1000). I do not know for now if that is an important indication or not.

I am not going to suggest a solution yet - just please be so kind to review my thoughts here and do correct me if any of my observations is incorrect. Thank you!

Best regards,

Peter

[tommyboay]

Hi Peter,

You are correct on your thoughts. I was trying to summarize to avoid info flood at first. Let me give some more details.

AGFR01RTR03 <-> AGFR01CSWT01/02 <-> AGFR02CSWT01 <-> AGFR02RTR03 are my backbone devices

AGFR01RTR03 and AGFR02RTR03 are ASBRs where the config outputs were taken from.

AGFR01CSWT01/02 and AGFR01CSWT01 are ABRs where the route outputs were taken from.

AGFR01RTR03 and AGFR02RTR03 are summarizing the routes from remote AS with the same config. Only the metric differs :

#AGFR01RTR03

summary-address 10.2.0.0 255.254.0.0 tag 1000

summary-address 172.16.0.0 255.255.0.0 tag 1000

redistribute ospf 1000 metric 18000 subnets route-map US_EU

#AGFR02RTR03

summary-address 10.2.0.0 255.254.0.0 tag 1000

summary-address 172.16.0.0 255.255.0.0 tag 1000

  redistribute ospf 1000 metric 23000 subnets route-map US_EU

Both routers also have strictly identical OSPF configuration for the remote AS excepting their router-id obviously.

Your comment about the tag sounds right : I really believe something is not working correctly in the route redistribution and mapping. How would possibly my static redistribution modify the metric provided in my ospf redistribution otherwise (moving from 23 000 to 20 on FR02) ?

In there is any output I can provide to help, please let me know.

Tom

[Peter Paluch]

Hi Tom,

Thank you for the update.

Let us focus on the on the router FR02. There are two redistributions performed on this router:

1. From OSPF process 1000 into OSPF process 93, using seed metric of 23000
2. From static routes into OSPF process 93, using default seed metric of 20

The redistribution of static routes into OSPF is performed without setting any explicit seed metric, hence using E2 metric of 20. If there is a static route falling under the 10.2.0.0/15 scope, it will be redistributed into OSPF 93 with the E2 metric set to 20, and during network summarization, it will "pull down" the metric of the entire summary route down to 20. That would explain why the summary route is advertised using a suspiciously low default E2 metric.

Can you please verify if the FR02 does have a static route falling under the 10.2.0.0/15 scope that is indeed redistributed into OSPF 93?

Best regards,

Peter

[tommyboay]

Hi Peter,

I confirm : The destination network I want to redistribute via FR02 is 10.2.34.0/24. It is the only static route existing on that device. So if I understood correctly, the summarization takes the lowest metric from the networks it covers ? Should I modify the static redistribution to match the ospf redistribution ? Will my /24 route still exist so that It would be preferred over the summary route ?

Sorry to ask all these questions. I'm realizing I should review my summarization courses

Thanks,

Tom

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[Peter Paluch · Accepted solution]

Hello Tom,

So if I understood correctly, the summarization takes the lowest metric from the networks it covers ?

Correct for Cisco routers. All IGP routing protocols on Cisco routers behave according to this rule when performing route summarization.

Should I modify the static redistribution to match the ospf redistribution ?

Yes, I recommend doing that. It will cause the summary route carry the increased E2 metric of 23000.

Will my /24 route still exist so that It would be preferred over the summary route ?

The /24 route will remain to exist in the routing table of the FR02 and will of course be used. The summarization and redistribution never influence the routing table on the router that performs these two activities - they influence only the routing tables of other routers. Of course, because of the summary-address command, other routers will not see the /24 route but rather the /15 summary. I am not sure if this is what you want.

No reason to apologize, Tom, these forums are exactly for asking questions and - hopefully - getting answers.

Best regards,

Peter

[tommyboay]

Thanks very much Peter. It is clear now.

As you suspect, this is not my goal. I want to inject the 10.2.34.0/24 route inside my local AS to force related traffic to use the FR02 backup link when it is up (I use the track function conditioning the static route). I guess i'll have to break my summarization to kick that network out of it. If you have any other idea, I'd be curious to hear it.

Just rated your answer as it explains my issue.

Tom

Sent from Cisco Technical Support iPad App

[Peter Paluch]

Hello Tom,

I guess that a simple "hack" would work: while leaving the existing summary-address 10.2.0.0 255.254.0.0 command in place on FR02, add the following command to the OSPF 93 on FR02:

summary-address 10.2.34.0 255.255.255.0

This should effectively result in two summary addresses being advertised out of FR02 - 10.2.0.0/15 and 10.2.34.0/24.

Best regards,

Peter

[tommyboay]

Hello Peter,

That is interesting indeed !

One information I couldn't find so far in Cisco documentation : Would this summary-address depend on the availability of redistributed route(s) part of that 10.2.34.0/24 subnet ? The only thing I'm afraid of is that this solution would still keep the 10.2.34.0/24 route via the FR02 router no matter the state of the DS3 line.

Thanks,

Thomas

[Peter Paluch · Accepted solution]

Hello Tom,

Would this summary-address depend on the availability of redistributed route(s) part of that 10.2.34.0/24 subnet ?

Absolutely yes. If there is no network present that would fall under the configured summary, the summary itself will not be advertised.

Let me show you a very simple network topology: two routers, R1 and R2, connected via their Fa0/0 interfaces, running simple OSPF (activated directly on interfaces, no network commands used). R2 has two static routes configured:

R2(config-router)#do show ip route static

     10.0.0.0/8 is variably subnetted, 4 subnets, 3 masks

S       10.2.0.0/16 is directly connected, Null0

S       10.2.34.0/24 is directly connected, Null0

R2(config-router)#

Also, the R2 performs both redistribution and summarization of these static routes:

R2(config-router)#do show run | sec router ospf

router ospf 1

log-adjacency-changes

summary-address 10.2.0.0 255.254.0.0

summary-address 10.2.34.0 255.255.255.0

redistribute static subnets

R2(config-router)#

On R1, currently, these networks are learned via OSPF:

R1#show ip route ospf

     10.0.0.0/8 is variably subnetted, 3 subnets, 2 masks

O E2    10.2.0.0/15 [110/20] via 10.0.12.2, 00:03:32, FastEthernet0/0

O E2    10.2.34.0/24 [110/20] via 10.0.12.2, 00:03:32, FastEthernet0/0

Note that the 10.2.0.0/15 is a summary, and the 10.2.34.0/24 is advertised as-is, as the summary address is identical to the network address itself.

Now, I remove the static route 10.2.34.0/24 from the R2 and check the R1's routing table again:

R2(config)#no ip route 10.2.34.0 255.255.255.0 null0

R2(config)#do show ip route static

     10.0.0.0/8 is variably subnetted, 3 subnets, 3 masks

S       10.2.0.0/16 is directly connected, Null0

R2(config)#

R1#show ip route ospf

     10.0.0.0/8 is variably subnetted, 2 subnets, 2 masks

O E2    10.2.0.0/15 [110/20] via 10.0.12.2, 00:05:02, FastEthernet0/0

R1#

As you can see, the network has been removed from the R2's routing table, and the summary address was withdrawn as well as a result. The only summary address being still advertised is the 10.2.0.0/15 because R2 is still redistributing the network 10.2.0.0/16 that falls under the 10.2.0.0/15. If I remove that network from the routing table on R2 as well, there will be no summary addresses advertised at all:

R2(config)#no ip route 10.2.0.0 255.255.0.0 null0

R2(config)#do show ip route static

R2(config)#

R1#show ip route ospf

R1#

So the summarization is a very nice concept, actually: the summary address is advertised as long as there is at least a single route that can be summarized using that summary address. If, however, the last component of the summary address is removed, the summary address itself will cease to be advertised.

Best regards,

Peter

[tommyboay]

Once again, very clear answer. Thanks very much Peter. You answered all my doubts. I'm going to use your solution.

And if I understood correctly, I should not even need my static route with your solution : Since there is a 10.2.34.0/24 redistributed from OSPF remote AS, the auto-summary will work unless the dead timer is reached for that network taking out the more specific summary route but leaving the global 10.2.0.0/15 with the higher metric than the primary FR01 router. Hence rerouting the traffic to the primary link when necessary.

Best regards,

Thomas

[tommyboay]

Quick and last update : The summary route is working just fine. And I did not add static route and related redistribution. This is an awesome and straightforward solution. Thanks much Peter.

[Peter Paluch]

Hello Tom,

I am very glad to have helped. Please feel welcome any time to stop by here at CSC

Best regards,

Peter