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Aggregate has lower admin distance?

bradleyordner
Level 3
Level 3

This might be simple, but I can't find an answer anywhere. 

 

I have two devices, they doing iBGP between themselves and participating in OSPF with each other and two other routers. Mutual redistribution is occuring on both BGP peers, restricted via route maps. 

 

The two BGP peers, they advertise the aggregate - 

 

aggregate-address 10.10.0.0 255.255.0.0 

 

One BGP host enters this into the routing table as the Null Route. The other host learns the route from OSPF and prefers this as AD is 110. 

 

I wondered, why with the aggregate command, does one pick BGP? If I remove the aggregate command on one BGP host, with a debug ip routing, we see the other host pick BGP as it says lower admin distance although it is 200 and it is learning it again from OSPF?

 

I guess without the aggregate command, there would be no summary to inject into OSPF but why does it say lower admin distance? Does it have to as a loop prevention mechanism?

 

Hope that explains it?

 

Brad

1 Accepted Solution

Accepted Solutions

Giuseppe Larosa
Hall of Fame
Hall of Fame

Hello @bradleyordner ,

the two ASBR nodes compete for injecting the summary route in OSPF.

 

>> One BGP host enters this into the routing table as the Null Route. The other host learns the route from OSPF and prefers this as AD is 110. 

 

if you look at the OSPF database on the second device you will see that the LSA type 5 is originated by the other one.

 

show ip ospf database external 10.10.0.0

 

The second device when receives the OSPF LSA generated by the other one installs it in the IP routing table.

Because only routes installed in IP routing table can be redistributed the second device does not generate its own LSA type 5.

 

The first device has actually a discard route to Null0 ( the static route to Null0) used for safety.

 

This is a race condition the first one to inject the route in OSPF wins and the other one suppresses its own LSA.

 

Hope to help

Giuseppe

 

View solution in original post

3 Replies 3

Giuseppe Larosa
Hall of Fame
Hall of Fame

Hello @bradleyordner ,

the two ASBR nodes compete for injecting the summary route in OSPF.

 

>> One BGP host enters this into the routing table as the Null Route. The other host learns the route from OSPF and prefers this as AD is 110. 

 

if you look at the OSPF database on the second device you will see that the LSA type 5 is originated by the other one.

 

show ip ospf database external 10.10.0.0

 

The second device when receives the OSPF LSA generated by the other one installs it in the IP routing table.

Because only routes installed in IP routing table can be redistributed the second device does not generate its own LSA type 5.

 

The first device has actually a discard route to Null0 ( the static route to Null0) used for safety.

 

This is a race condition the first one to inject the route in OSPF wins and the other one suppresses its own LSA.

 

Hope to help

Giuseppe

 

bradleyordner
Level 3
Level 3

Thanks, this makes sense.

I will do another test in the lab and see if I can capture this decision in
the debugs, just want to see if it says anything else. I only been doing
debug ip routing so far.

One other question, the remaining OSPF routers only have the one type 5
LSA, but when that fails the second host Ext LSA appears, but the stale
entry stays for sometime. Is that normal or should be removed from database
immediately? Think it might be lab bug.

Hello @bradleyordner ,

 

>> One other question, the remaining OSPF routers only have the one type 5
LSA, but when that fails the second host Ext LSA appears, but the stale
entry stays for sometime. Is that normal or should be removed from database
immediately? Think it might be lab bug.

 

It depends on how you emulate the fault however if it is single area each type 5 LSA can be checked with the ASBR Router LSA and even if the router has had no time to purge its own generated LSAs the neighbors will update their own Router LSA and Network LSA so that the now isolated ASBR can be ignored and also the LSA type 5 it has generated.

 

So this should not be an issue.  In OSPF only the owner of an LSA ( the advertising router) can purge it from database by sending a new copy with age = Max Age otherwise they are removed when reaching MAx Age= 3600 seconds

 

Hope to help

Giuseppe