3Com 8800 Configuration Manual page 187

3Com Switch 8800 Configuration Guide
If DR fails due to some faults, the routers on the network must elect a new DR and
synchronize with the new DR. The process will take a relatively long time, during which,
the route calculation is incorrect. In order to speed up this process, OSPF puts forward
the concept of BDR. In fact, BDR is a backup for DR. DR and BDR are elected in the
meantime. The adjacencies are also established between the BDR and all the routers
on the segment, and routing information is also exchanged between them. When the
DR fails, the BDR will become the DR instantly. Since no re-election is needed and the
adjacencies have already been established, the process is very short. But in this case,
a new BDR should be elected. Although it will also take a quite long period of time, it will
not exert any influence upon the route calculation.
Note the following:
The DR on the network is not necessarily the router with the highest priority.
Likewise, the BDR is not necessarily the router with the second highest priority. If a
new router is added after DR and BDR election, it is impossible for the router to
become the DR even if it has the highest priority.
DR is based on the router interface in a certain segment. Maybe a router is a DR
on one interface, but can be a BDR or DROther on another interface.
DR election is only required for the broadcast or NBMA interfaces. For the p2p or
p2mp interfaces, DR election is not required.
Perform the following configuration in interface view.
Table 19-19 Set the interface priority for DR election
Configure the interface with a priority for
DR election
Restore the default interface priority
By default, the priority of the interface is 1 in the DR election.
Use the ospf dr-priority and peer commands to set priorities with different usages:
Use the ospf dr-priority command to set priority for DR selection.
The priority you use the peer command to set indicates whether the adjacent
router is eligible for election. If you specify the priority as 0 during neighbor
configuration, the local router considers that this neighbor is not eligible for
election, thus sending no Hello packets to this neighbor. This configuration can
reduce the Hello packets on the network during DR and BDR selection. However,
if the local router is DR or BDR, this router can also send Hello packets to the
neighbor with priority 0 to establish adjacency relations.
Operation
19-19
Chapter 19 OSPF Configuration
Command
ospf dr-priority priority_num
undo ospf dr-priority