I have a OSPF running over VPLS network (single VLAN/ Broadcast multiaccess), more often than not the link to current OSPF DR chokes due to congestion and and it drops OSPF packets, and the whole network goes down.
I'm planning to shift the OSPF DR and BDR to other routers which experience less congestion, so that the rest of the network will be stable even if the said link experiences congestion.
I will configure priority on both the routers intended to be DR (200) and BDR (180),
So my question is, what is the safest way to shift the OSPF DR and BDR to another routers,
Does 'clear ospf database' command need to be run on all the routers participating in the OSPF domain simultaneously? or single command on the DR should do the trick?
How much time will the network require to reconverge?
In a multiaccess network, you would have to clear the adjacencies from all the router's once after the priority is changed. The convergence time completely depends on the number of routers in a given segment, more the number more the time.
I suppose there is not much we can do here when it comes to changing DR as this will be disruptive for the time the reconvergence happens. Changing priority as you mentioned and re-establishing neighborships is the only way to change DR/BDR for a given segment.
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This command forces the router to send out all LSAs in its database (learned and self-originated) after setting them to MAXAGE. Here is what happens when this command is issued:
1) The router marks all LSAs as MAXAGE and sends them out.
2) The actual originators of those LSAs now receive these LSAs and re-originate them.
3) Effectively you're refreshing the OSPF database of the whole area.
Here is a small piece of documentation from Juniper's website:
(Optional) Discard all entries in the link-state advertisement database. All link-state advertisements are set to MAXAGE and are flooded. The database is repopulated when the originators of the link-state advertisements receive the MAXAGE link-state advertisements and reissue them.
Since OSPF DR election is non-preemptive, and that the election takes place by exchange of Hello packets, there is no other way than to set the new priorities and trigger re-election by clearing the adjacencies.
As rightly pointed out, the time to reconverge will depend on the number of routes.
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