Reflooding of MaxAge LSA

[ShahriarBasiri]

Dear community,

According to RFC2328 (OSPFv2) part 12.1.1 "When an LSA’s age first reaches MaxAge, it is reflooded." then in part 14 it also says that: "As a router ages its link state database, an LSA’s LS age may reach MaxAge. At this time, the router must attempt to flush the LSA from the routing domain. This is done simply by reflooding the MaxAge LSA just as if it was a newly originated LSA".

Can anybody explain these statements? Does it mean whenever that a LSA in any router reaches MaxAge it should flood the MaxAge LSA in network so that other routers delete the LSA from their databases? why not just letting other routers delete the LSA when they also reach the MaxAge?

Any help is appreciated,

Shahriar 

[Giuseppe Larosa]

Hello @ShahriarBasiri ,

in a working OSPF domain no LSA should reach the MAxAge ( one hour ), because the LSA owner should refresh it before every 30 minutes in Cisco OSPF implementation (LSA Refresh Time).

LSA refresh occurs in normal conditions ando the forced flashing of a MaxAged LSA should not happen.

At each LSA refresh the LSA sequence number is incremented.

The sequence number is treated as a signed 32 bit integer and the starting point is not 0 but 0x8000000 when there is a wrapping in the sequence number the LSA is actually discarded by generating it with MaxAge set.

for a better explanation see the following thread

https://community.cisco.com/t5/routing/ospf-lsa-aged-to-maxage/m-p/1512788

That should be the only rare case when flushing a MaxAge LSA is needed as the Sequence number wrapping starts again from 0x80000000 and must be accepted as newer by all routers.

It is really rare to see this happens because it would require a lot of time to reach this kind of situation.

Sending an LSA with MAXaAge will help all the routers to purge the LSA as soon as possibile this should be the reason behind this.

Hope to help

Giuseppe

[ShahriarBasiri]

Dear @Giuseppe Larosa 

Thanks for your answer,

It is true that sequence number wrapping is done by flooding a MaxAge LSA but It seems that rfc2328 refers to another case here: "As a router ages its link state database, an LSA’s LS age may reach MaxAge. At this time, the router must attempt to flush the LSA from the routing domain. This is done simply by reflooding the MaxAge LSA just as if it was a newly originated LSA". I guess it is relevant to situations when an entry in LSDB expires and every node can reflood the MaxAge LSA.!?

[Giuseppe Larosa]

Hello @ShahriarBasiri ,

an LSA should not reach MaxAge however in case it happens the MaxAge LSA can be re-flooded with the scope to have all routers to purge it from LSDB.

It is a really rare case because each router has to refresh the LSA it generates every 30 minutes (in Cisco implementation).

Hope to help

Giuseppe

[ShahriarBasiri]

Hello @Giuseppe Larosa 

So in the rare case, every node can re-flood that MaxAge LSA!? Why not just letting other routers delete the LSA when they also reach the MaxAge?

[Giuseppe Larosa]

Hello @ShahriarBasiri ,

OSPF loop free capabilities is based on the fact that on a given area all routers have the same entries in the LSDB, without the forced floodling of MaxAged LSA there would be a small transition time in which one LSA could exist or not on each router in an area and this is not a good thing.

So the flooding is used to force convergence of all routers on purging the expired LSA ( Max Aged LSA)

Hope to help

Giuseppe