3Com 7700 Configuration Manual page 90

82 C 5: R P
HAPTER OUTING
O
ROTOCOL PERATION
Set the Interface Priority for DR Election
The priority of the router interface determines the qualification of the interface for
DR election, a router of higher priority is considered first if there is a collision in the
election.
DR is not designated manually, instead, it is elected by all the routers on the
segment. Routers with priorities > 0 in the network are eligible candidates. Among
all the routers self-declared to be the DR, the one with the highest priority is
elected. If two routers have the same priority, the one with the highest router ID is
elected DR. Votes are the 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, the
one with the higher priority wins. If the priorities are the same, the router with
higher router ID wins. If the priority of a router is 0, it is not be elected DR or BDR.
If a DR fails, the routers on the network must elect a new DR and synchronize with
the new DR. The process takes a relatively long time, during which, route
calculation can become incorrect. To speed up this DR replacement process, OSPF
implements the BDR as a backup for DR. The DR and BDR are elected at the same
time. 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 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.
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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
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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
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P2MP interfaces, DR election is not required.
Perform the following configuration in VLAN interface view.
Table 27 Set the Interface Priority for DR Election
Operation
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. The value can be set
from 0 to 255.
Set the Peer
For an NBMA network, some special configurations are required. Since an NBMA
interface on the network cannot discover the adjacent router through
Command
ospf dr-priority priority_num
undo ospf dr-priority