BGP TO OSPF REDISTRIBUTION

[grew00028]

While redistributing BGP routes to OSPF, I see some routes as TYPE 3 LSA and majority of routes as Type 5 LSA in OSPF database. Why I am seeing Type 3 LSA?? They should all be Type 5 right?

Can some one suggest anything?

Thanks

[Giuseppe Larosa]

Hello grew00028,

all routes originated by BGP redistribution are translated to type 5 external LSAs in OSPF

However, if any prefix  (subnet, and mask) is already present in the OSPF domain as internal route the type 3 inter area LSA is preferred over the type 5 LSA and you will see an O IA route in the routing table.

This is probably what happens in your network.

You can check the ospf database with

show ip ospf database external <prefix>

the external LSA may be in the database but not installed in the routing table because it is overriden by an internal inter area route.

Hope to help

Giuseppe

[grew00028]

Hi Giuseppe Larosa,

As soon as I remove the redistribute command under OSPF the Summary routes are no more in database. IF I am learning them from somewhere else as internal routes, they should not go away with the Redistribute command right?

Correct me if I am wrong

I have attached the database. All I want is that the routes in the Summary should be learnt as External routes. from two routers 9.168.28.10 and 9.168.28.2 respectively.

they are being learnt as type 5 from 9.168.28.2 but not from 9.168.28.10

[Giuseppe Larosa]

Hello grew00028,

>> As soon as I remove the redistribute command under OSPF the Summary routes are no more in database. IF I am learning them from somewhere else as internal routes, they should not go away with the Redistribute command right?

This is correct.

However, here we have two different OSPF routers with Router-IDs 9.168.28.2 and 9.168.28.10.

Is redistribution of BGP routes configured on both routers, or only on OSPF RID 9.168.28.2 ?

Can you post the OSPF configuration of both routers?

>> they are being learnt as type 5 from 9.168.28.2 but not from 9.168.28.10

we need to unrderstand why this is happening.

Hope to help

Giuseppe

[grew00028]

We have Two Sites Site A and Site B. Sites have Two Physical Redundant Paths Primary and Secondary. Each site has two CE's and Two PE's. The CE's connect to Customer Multi Layer Switch. We have MPLS running in between on the Service Provider.

Yes, Redistribution is confirmed on both the routers.

Attached is the config. I have different OSPF process IDs on both the routers. Can that be a problem???

[Giuseppe Larosa]

Hello grew00028,

>> I have different OSPF process IDs on both the routers. Can that be a problem???

In an MPLS L3 VPN with OSPF used as PE-CE protocol the answer is yes.

the OSPF process-id if different from remote site should lead to standard behavior of external routes.

iF the OSPF process-id is the same as in remote site this can lead to redistributed routes as O IA if they were internal routes in origin site (see note below)

This is a sort of emulation of OSPF protocol over service provider backbone.

All the OSPF related information including the OSPF domain id are carried as BGP extended communities in order to make it possible on remote site PE nodes to rebuild OSPF LSAs like they had travelled over an OSPF backbone and not over multi protocol BGP.

Edit

see

https://tools.ietf.org/html/rfc4577

Note: from RFC about comparison of OSPF domain identifiers

>>

If the route is from the same OSPF domain as the OSPF instance into
   which it is being redistributed, and if it was originally advertised
   to a PE as an inter-area or intra-area route, the route will
   generally be advertised to the CE as an inter-area route (in a type 3
   LSA).

This should be your case. IF you want to have only external routes change the OSPF process-id on the device that generates the O IA routes when redistribution occurs.

Hope to help

Giuseppe

[grew00028]

Hi,
Will make the change soon and will keep you posted, I think process-id is the problem. Thanks!

[grew00028]

Made the changes for the ospf process, it did not fix the problem but there's a change  in Database now, the routes it was learning as LSA 5 from from 9.168.28.2 previously are now also learned as LSA 3 type?

[Giuseppe Larosa · Accepted solution]

Hello grew00028,

what changes have you done? and in which device?

However, if now both routers are generating Type 3 inter area routes as a result of redistribution of MP BGP into OSPF,  this can solve your issue as now both routers are generating the same type of routes and both can be installed by other routers if they see equal cost paths.

Hope to help

Giuseppe

[grew00028 · Accepted solution]

Hi Giuseppe Larosa,

Thanks for your help, everything is working fine now.