What Routes does OSPF ASBR summarize

[mastram4u]

Hi All,

I have configured summary-address on my ASBR in which BGP routes are being redistributed into ospf process 100.

On ospf process 100, I have configured summary address 10.x.x.x.

Does the summary-address look for the 10.x.x.x only from the redistibuted routes on this router or the entire OSPF domain?  I have few prefixes in 10.x.x.x in OSPF 100, which from other routers in the OSPF domain.

Thanks for the help

Ramesh

[Reza Sharifi]

Hi Ramesh,

Since you are summarizing at the ASBR, all redistributed routes from BGP to OSPF will be summarized, as long as they are contiguous

HTH

Reza

[mastram4u]

Thanks.

The issue i am facing is , when the BGP goes down, the summary route does not get removed from the routing table.

I could see the BGP peers to be down, but on the OSPF process, i have multiple O E1 / O E2 / O IA routes in 10.x.x.x subnet in OSPF routing table.

[lamav]

Ramesh:

Please post the BGP and OSPF configs as well as the RIB entries that you think you shouldnt be seeing.

Downstream routers (not the ASBR) should show the summary route as an OE2 route in the RIB. And you should lose it on the downstream routers when the ASBR no longer has the BGP-learned prefixes in its RIB.

[mastram4u]

As per my understanding , OSPF will summarize even if one prefix in the range being summarized exists i.e. ( if the summary-address is 10.0.0.0/8, i will see this summary route even if I have one subnet in that range exists in its routing table). Is this a correct understanding?

Please find attached the scenario diagram.The situation is

• When the BGP is up between R1 and R2 , the summary address of 10.0.0.0/8 is seen in OSPF domain on all the devices.
• When BGP goes down between R1 and R2, since the BGP prefixes are not being received on R2, i thought that the summary address will get withdrawn , forcing the traffic through R5 to R6.
• However, even when the BGP between R1 and R2 goes  down, the routing in the OSPF domain still points to R2, impacting the traffic flow

[JoeKeegan3]

The summary-address command will summarize any external routes from redistribution into the summary address.

In your scenario you have two sources of external routes from redistribution), one source is BGP and the other sources is connected routes.

So when BGP goes down between R1 and R2, R2 still has external routes in that range, from the redistributed connected routes, and continues to advertise the summary.

You may want to try using network statements to get the connected routes into OSPF instead of redistributing connected.

[lamav]

In addition to Joe's comments, you must keep in mind that the ASBRs are redistributing 10.10.x.x addresses that it learns from BGP and summarizing them into a 10.0.0.0/8. I am assuming that the route table entries you show in the OSPF core reflect the state of routing affairs when the BGP neighborships are down. If so, that makes sense because I dont see the OE2 10/8 summary or the individual 10.10 addresses.

[mastram4u]

Thanks Joe / Lamav.

Couple of points to add.

• I have shown only a part of the prefixes available on the routing tables to make the scenario simple
• The prefixes shown in the core are the ones available when the BGP peering is up. These prefixes are available from the other routers ( for e.g. R4) which has BGP peering with different ASN and injecting those routes.
  

My earlier understanding was that the summary-address command will summarize only looking at the redistributed routes on its own external peering.i.e. R2 wil summarize only based on the routes redistributed from R1 BGP session and not looking at the OSPF domain.

Now, I think my understanding is incorrect. So, in the given scenario, my route summarization will never work as long as I have routes from other routers (e.g. R4/R1). Is this correct?

[lamav · Accepted solution]

Ramesh:

A summary address must be advertised judiciously so as not to create a routing blackhole. An ASBR will inject a summary route into the OSPF domain according to the configuration of the summary address command. So, if a 10/8 summary address is configured under the OSPF process as part of the summary-address command AND at least one component network within that advertised range exists in the ASBR's RIB, that 10/8 will be advertised.

One other thing that I think you're not thinking about is the fact that routers select outgoing interfaces when routing traffic based on the longest-prefix match The summary route will be used when a more specific match is not available.

Lastly, keep in mind that once the summary-address command is configured, a matching route to null0 is autonmatically installed in the ASBR's RIB to prevent routing loops.

HTH

Victor

[mastram4u]

Victor,

Now my understanding is clear. Thanks for the explanation.

In the topology diagram, I had indicated few routes in the OSPF domain just to indicate that these routes exist even when the summary address is configured. I had considered the longest-prefix match rule and the null route rule.