OSPF: Difference between revisions
→R&S Quick Notes
Line 499:
;Route Selection Hierarchy - E1, E2, N1, N2
{{UC}}
= Withdraw OSPF Routes =
*Router LSA:
When a link fails, the connected OSPF routers send an updated LSA which omits the now shutdown link from it’s list of connected links.
This new LSA has an incremented sequence number which replaces the old LSA in the LSDB of all routers in that area.
OSPF will kick off a new SFP run, calculate a new topology and remove any routes associated with that link.
*Summary LSA
If no summarisation done on ABR, it will create a Type-3 Summary LSA for every Type-1 stub-network prefix.
The ABR senda a new Summary LSA with updated sequence number flagged as unreachable by setting it’s 24-bit metric field to all Ones called LSInfinity with decimal value of 16777215.
The LSA requests that it be flushed from the receiver’s LSDB by setting it’s LSAge to MaxAge (3600 seconds).
*External LSA
When the link goes down the ASBR will send an updated Type-5 LSA for the prefix with an updated sequence number with link cost of LSIninity and LSAge to MaxAge.
Type-5 prefix withdrawal is simpler as Type-5 LSAs have domain-wide flooding scope.
As such the poisoned LSA will immediately be flooded to all routers in the OSPF domain, regarded as unreachable and flushed from the receivers LSDB and routing tables.
= R&S Quick Notes =
| |||