BDR are determined in a network, unless they become invalid, any new routers joining or exiting
would not cause re-election.
As shown below, on a network of five routers, ten adjacency relations need to be established if
one between every two routers, but only seven adjacencies are required if DR and BDR are
introduced. To conclude, on a network of N routers, N*(N-1)/2 adjacencies are required in
general, but the adjacencies required will be (N-2)*2+1 if DR and BDR are introduced. Therefore,
the more routers on the network, the more significant the advantages will be.
Figure 10-33 Diagram of DR/BDR Adjacency Relation
DR or BDR is determined by the interface priority and router ID. First of all, whether a router
could be the DR or BDR on a network is decided by its interface priority. The one of highest
priority would be elected as DR or BDR; while if all the interfaces are of the same priority, it
would then be decided by the router ID. In conclusion, DR or BDR is the feature of a certain
interface of the router which indicates the status of the router in a network segment rather than
the features of the router on the network. Every network segment needs to elect a DR and a
BDR to synchronize the routing information. The configuration of router interface parameters
needs to be done on the basis of network planning.
OSPF Working Process
In the following, we would take the example of two routers initiating interface OSPF protocol to
introduce the working process of OSPF routing protocol in the Ethernet model.
1)
The router interface initiates the OSPF protocol, and then the interfaces in the same
network segment would discover neighbors by sending Hello packets. If the interfaces are
connected on the same public data link, and the area IDs, authentication information,
network subnet, Hello data interval and neighbor router dead-interval are all matched, the
two routers would put each other in its neighbor table.
2)
If the receiver discovers its own ID on the neighbor table of the Hello packet, a successful
mutual communication would be established. And then they will elect DR and BDR
according to such parameters as the interface priority and the router ID, while if DR and
BDR already exist in the network, they will be accepted.
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