H3C S9500 Series Operation Manual page 262

Operation Manual – Routing Protocol
H3C S9500 Series Routing Switches
4.2.13 Setting the Interface Priority for DR Election
On a broadcast or NBMA network, a designated router (DR) and a backup designated
router (BDR) must be elected.
The priority of a router interface determines the qualification of the interface in DR
election. The router with the priority of 0 cannot be elected as the DR or BDR.
DR is not designated manually. Instead, it is elected by all the routers on the segment.
Routers with the priorities larger than 0 in the network are eligible "candidates". Votes
are hello packets. Each router writes the expected DR in the packet and sends it to all
the other routers on the segment. If two routers attached to the same segment
concurrently declare themselves to be the DR, choose the one with higher priority. If the
priorities are the same, choose the one with greater router ID. If the priority of a router is
0, it will not be elected as DR or BDR.
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 4-22 Set the interface priority for DR election
Configure the interface with a priority for DR
election
Restore the default interface priority
Operation
4-20
Chapter 4 OSPF Configuration
Command
ospf dr-priority priority-num
undo ospf dr-priority