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Those things DR election

DR action is one for database synchronization, the second is the representative virtual network node generates class 2 LSA.
After the wait time, proceeded to elect DR, higher priority router becomes the DR, or high RouteID become DR.
DR can not be preempted, once the elections are over, even with a higher priority router to your network, and we can seize become the DR, the same will not preempt the BDR. This is to ensure network stability and avoid shocks. (There is a special case of elaborate later)
After the election of DR if the current DR fails, the BDR becomes the DR, and re-elected BDR.
MA network can not BDR, but not without DR.

First, the elections DR / BDR process

Each router interface based on hearing Hello packets, generating a neighbor table, and maintain interfaces in three sets:

DR set: Hello learned by all routers DR set {}
BDR set: Helli learned by all routers BDR set {}
DRother set: the DR and BDR but non-eligible to become the DR and BDR routers set {}
Note that DR and BDR election is a local election in each router (with a priority candidate for the router and other state 2way own broadcast domain of non-zero)
Elections in different environments and router interfaces declare the order of time about the election results would be different.
A router running OSPF - OSPF interfaces up and the intake declared, Wait timer starts when the length of 4 times the hello interval.

Wherein a following router standing angle be described the election results DR　

If you receive a hello packet DR and BDR field is empty, and he is a neighbor 2way state of waiting for the election into DRother set of the elections DR, BDR. If the wait time ends receive Hello packets still in the DR and BDR field is empty, the local elections, the comparison (priority .route ID) to elect a BDR in DRother set, and then if you still do not receive the neighbor Hello packets DR text fields, will be elected as BDR DR publish the election results, followed by the election BDR, BDR election results released in the second hello.

If you receive a Hello packet DR and BDR field is non-empty, Wait timer stops, accept the election results for the current DR / BDR election. (However, if you receive a DR of itself, is not immediately stopped, continue to listen Hello continue to be elected).

There are a case where a special case of DR, BDR cause preemption follows:

This figure is a broadcast domain, configured OSPF both sides declared finished after waiting on both sides are each elected DR, then switch on the intermediate link, and the network has two DR, the BDR after the BDR, but after the first after hello interaction, we will conduct two DR PK, will be a priority for large or route id DR DR inside in two elections.

A hello network will receive the DR A non-stop direct election to accept the DR network advertised in the election wait time.
A network received hello in DR is A, will continue to be elected in the electoral wait time
If A is the DR has already received other routing a hello in their own non-DR DR These two fields will be bigger wins.