OSPF: Difference between revisions
→R&S Quick Notes
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“ip ospf database filter all out” – Breaks RFC, Stop sending LSA’s, but still receive LSA’s
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= 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
Assuming no summarisation had been configured on the ABR, it will originate a single Type-3 Summary LSA for every Type-1 stub-network prefix it had installed in it’s routing table.
The ABR informs all backbone routers that it can no longer reach the prefix from the failed link and asks the those routers to flush the Type-3 LSA for that prefix from their LSDBs.
The ABR does this by sending a new Summary LSA with and updated sequence number.
The prefix is flagged as unreachable by setting it’s 24-bit metric field to all ones.
This is called LSInfinity and has a 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).
If there are other ABRs in the network, they will cascade the prefix removal by re-originating these ‘poisoned’ Type-3s into their connected non-backbone areas.
*External LSA
As soon as the associated link goes down the ASBR will send an updated Type-5 LSA for the prefix with an updated sequence number.
Again it will set the link cost to the 24-bit LSIninity and the 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.
= Concept Recap =
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