Re: Question on flushing AS-external LSAs

[Acee Lindem <acee at CISCO.COM>]

Hi Mukesh,
See inline.
Mukesh Hira wrote:

> Hi,
>
> I'm looking for an answer to how OSPF would behave in the following scenario.
>
> Say an ASBR advertised some AS-external LSAs and then crashed. Now, OSPF (at least v2) only allows a router to flush LSAs that it originated. Given that the router that originated these AS-external LSAs has crashed, would these AS-external LSAs stay in the Link State Database of all routers in the OSPF domain for upto an hour until they get aged out? Would these not be flushed?
>  
The AS-external LSAs would stay in the database for up to an hour 
(MAXAGE) and would not be
flushed until the aged out.

> Let's assume that the forwarding address in the AS-external LSAs was set to the IP address of one of the interfaces on the ASBR. When SPF is run after the Router LSA (or Summary LSA if the ASBR is in another area) to this IP address gets flushed, would this forwarding address not being reachable anymore cause all these AS-external routes to be deleted from the routing table of all nodes?
>  
Yes. When the LSA is flushed this would happen. However, the crash of 
the ASBR would
not cause the Router-LSA to be flushed. However, it would cause the ASBR 
to be
unreachable and trigger an SPF calculation which would cause the AS 
external routes to
be removed from each router's routing table.

> What if this ASBR that crashed was one of two ASBRs advertising the same AS-external routes, and has the higher router ID, so that the other ASBR flushed its AS-external LSAs. When this ASBR with higher router ID crashes, what triggers the other ASBR to re-inject its AS-external LSAs into the network? 
This isn't specified in the protocol but I would mark LSAs that suppressed
self-originated LSA orginations due to duplication and check for 
reachability each
time the LSA is processed in an AS external SPF calculation. If the 
LSA's ASBR
is unreachable then there is no reason to suppress self-origination.

> Would the event of forwarding address for the LSAs heard from the other ASBR not being reachable anymore trigger it to inject its own AS-external LSAs in the network?
>  
I think there are problems trying to implement it this way since the
forwarding address is a /32 and the corresponding intra-area or 
inter-area OSPF
route generally is not.

Hope this helps,
Acee

> Thanks in advance
> - Mukesh
>
>