July 2012

Sunday, July 1, 2012

OSPF LSA and Packet Format

There are 5 types of OSPF packet types, all are used in the normal operation of OSPF:-
1. Hello:- Discovers neighbors and build adjacency between them.
2. Database Description:- Check for database synchronization between routers.
3. Link State Request:- Requests specific link-state records from routers to routers.
4. Link State Update:- Sends specifically requested link-state records; LSU may contain more than one LSA.
5. Link State Acknowledgement:- Acknowledges the other packet types.

To ensure an accurate database, OSPF use Sequencing, Checksum & Age. OSPF floods (refresh) each LSA every LSRefreshTime (30 min). Each time a record is flooded, the sequence number is incremented by one & This LSA (refresh) will reset the record MaxAge timer (60 min) when it receives a new LSA update. An LSA (record) will never remain in the database longer than the MaxAge timer (60 min / 3600 sec) without a refresh. And this is used to maintain & insure LSDB synchronized across the Area. Sequence number range: InitialSequenceNumber (0x80000001) to MaxSequenceNumber (0x7fffffff). If the present sequence number is MaxSequenceNumber and a new instance of the LSA must be created, the router must first flush the old LSA from all databases. This is done by setting the age of the existing LSA to MaxAge (defined later in this section) and reflooding it over all adjacencies. As soon as all adjacent neighbors have acknowledged the prematurely aged LSA, the new instance of the LSA with a sequence number of InitialSequenceNumber may be flooded, only the router that originated the LSA can prematurely age it. The checksum is a 16-bit integer calculated using a Fletcher algorithm. The checksum is calculated over the entire LSA with the exception of the Age field (which changes as the LSA passes from node to node and would therefore require recalculation of the checksum at each node). The checksum of each LSA is also verified every five minutes as it resides in the link-state database, to ensure that it has not been corrupted in the database.