3Com 5500-SI Configuration Manual page 241

Configuring the Cost for Sending Packets on an Interface
You can control network traffic by configuring different message sending costs for
different interfaces. Otherwise, OSPF automatically calculates the cost according to
the baud rate on the current interface.
Perform the following configuration in Interface View:
Table 226 Configuring the cost for sending packets on the Interface
Operation
Configure the cost for sending packets on Interface
Restore the default cost for packet transmission on the Interface
For the Switch 5500 the default cost for running OSPF protocol on the VLAN interface
is 10.
Setting 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 will be 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 the priorities greater than 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 will
be elected as the DR. 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 higher priority is elected
DR. If the priorities are the same, the router with the higher router ID is elected DR. If
the priority of a router is 0, it will not be elected as DR or BDR.
If 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
be incorrect. To speed up this DR replacement process, OSPF implements the BDR as
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 exchanged between them. When the DR fails, the BDR becomes the
DR. Since no re-election is needed and the adjacencies have already been established,
the process is very quick. But in this case, a new BDR should be elected. Although it
also takes a long time, it does not influence 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. A router can be a DR
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on one interface, and a BDR or DROther on another interface.
DR election is only required for the broadcast or NBMA interfaces. For P2P or
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P2MP interfaces, DR election is not required.
OSPF Configuration 241
Command
ospf cost value
undo ospf cost