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88 CHAPTER 5: IP ROUTING PROTOCOL OPERATION
fails, the BDR becomes 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 must be elected. Although it also takes a long time, it does not
affect the route calculation.
Note that:
■ 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.
■ The DR is based on the router interface in a certain segment. Maybe a router is
a DR on one interface, but it can be a BDR or DROther on another interface.
■ DR election is only required for broadcast or NBMA interfaces. For the P2P or
P2MP interfaces, DR election is not required.
Perform the following configuration in VLAN interface view.
By default, the priority of the interface is 1 in the DR election. The value can be set
from 0 to 255.
Setting the Peer
For an NBMA network, some special configurations are required. Since an NBMA
interface on the network cannot discover the adjacent router through
broadcasting the Hello packets, you must manually specify an IP address for the
adjacent router of the interface, and whether the adjacent router is eligible for
election. This can be done by configuring the peer ip-address command. If
dr-priority-number is not specified, the adjacent router will be regarded as
ineligible.
Perform the following configuration in OSPF view.
By default, the preference for the neighbor of NBMA interface is 1.
Setting the Interval of Hello Packet Transmission
Hello packets are the most frequently sent packets. They are periodically sent to
the adjacent router for discovering and maintaining adjacency, and for electing a
DR and BDR. The user can set the hello timer.
Table 29 Setting 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
Table 30 Configuring the Peer
Operation Command
Configure a peer for the NBMA interface. peer ip-address [ dr-priority
dr-priority-number ]
Remove the configured peer for the NBMA
interface
undo peer ip-address