OSPF LSA Generation and SPF calculation

[zafar_118]

Hello,

OSPF routers genereate LSA when either there is a topolgy change or every 30 minutes.

Assuming that there is no topology change for 30 minutes, router will generate a LSA. Does this LSA has same sequence number as the one generated 30 minutes earlier? When the post 30 min LSA is received by upstream router, does it re-run the SPF algorithm?

[Rolf Fischer]

Hi,

periodic (sometimes refered to as 'paranoid') flooding is a last resort mechanism to remove old (invalid) LSAs from the LSDB after 1 hour: After 30 minutes routers originate a new instance of their self-originated LSAs.

The sequence number has to be incremented for every new instance of a LSA.

The SPF algorithm doesn't have to re-run in this case because the LSA content hasn't changed.

From RFC 2328:

If the LS age field of one of the router's self-originated LSAs reaches the value LSRefreshTime [30 minutes], a new instance of the LSA is originated, even though the contents of the LSA (apart from the LSA header) will be the same.

(...)

Whenever a new instance of an LSA is originated, its LS sequence number is incremented, its LS age is set to 0, its LS checksum is calculated, and the LSA is added to the link state database and flooded out the appropriate interfaces.

(...)

When comparing an LSA to its previous instance, the following are all considered to be differences in contents:

(...)

• The body of the LSA (i.e., anything outside the 20-byte LSA header) has changed. Note that this excludes changes in LS Sequence Number and LS Checksum.

HTH

Rolf