OSPF: Difference between revisions
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“ip ospf database filter all out” – Breaks RFC, Stop sending LSA’s, but still receive LSA’s |
“ip ospf database filter all out” – Breaks RFC, Stop sending LSA’s, but still receive LSA’s |
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</pre> |
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= Withdraw OSPF Routes = |
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*Router LSA: |
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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. |
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This new LSA has an incremented sequence number which replaces the old LSA in the LSDB of all routers in that area. |
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OSPF will kick off a new SFP run, calculate a new topology and remove any routes associated with that link. |
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*Summary LSA |
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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. |
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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. |
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The ABR does this by sending a new Summary LSA with and updated sequence number. |
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The prefix is flagged as unreachable by setting it’s 24-bit metric field to all ones. |
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This is called LSInfinity and has a decimal value of 16777215. |
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The LSA requests that it be flushed from the receiver’s LSDB by setting it’s LSAge to MaxAge (3600 seconds). |
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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. |
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*External LSA |
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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. |
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Again it will set the link cost to the 24-bit LSIninity and the LSAge to MaxAge. |
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Type-5 prefix withdrawal is simpler as Type-5 LSAs have domain-wide flooding scope. |
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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. |
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= Concept Recap = |
= Concept Recap = |