3Com Switch 8800 Family Configuration Guide

322 CHAPTER 31: OSPF CONFIGURATION
Note: 3Com supports the broadcast domain only.
By default, OSPF determines the network type based on the link layer type. After
the interface has been configured with a new network type, the original network
type of the interface is removed automatically.
Setting the Interface
Priority for DR Election
On a broadcast 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 interfaces.
Perform the following configuration in interface view.
By default, the priority of the interface is 1 in the DR election.
Tabl e 290 Set the interface priority for DR election
Operation Command
Configure the interface with a priority for DR election ospf dr-priority priority-num
Restore the default interface priority undo ospf dr-priority