Hello Sathish
First of all, let me clarify Rene’s statement here.
- LSA Aging Timer: Each LSA in OSPF does indeed have an aging timer, represented by the link-state age field. The maximum age for an LSA is 60 minutes. When the age of an LSA reaches 60 minutes without being refreshed, it is considered stale and removed from the link-state database.
- Refreshing LSAs: OSPF routers will refresh LSAs periodically before they reach the maximum age, typically every 30 minutes. This is the 30 minutes that Rene mentioned in the lesson. This means that although an LSA has a validity of 60 minutes, it is re-advertised and refreshed every 30 minutes to prevent it from expiring.
- Sequence Number: When the router that originated the LSA resends it (due to refresh or a topology change), it increases the sequence number to ensure the newer LSA is recognized by other routers, avoiding confusion with older ones.
Now, the refreshing of the LSA can only be achieved by the router that generated the LSA! The timer is measured within that router, and when 30 minutes elapse, it resends the LSA.
For a router that has received an LSA from the originator (or from another router that has already received it), the LSA’s age field starts counting up. If the LSA isn’t refreshed or updated within 60 minutes (its maximum age), the LSA will expire on that router and be removed from its link-state database. It doesn’t ask for an update, or inform any other router that the LSA has expired, it simply removes it from the database. Does that make sense?
I hope this has been helpful!
Laz