Designated routers (DRs)
In an OSPF network having two or more routers, one router is elected to serve as the DR and another router to
act as the BDR. All other routers in the area forward their routing information to the DR and BDR, and the DR
forwards this information to all of the routers in the network. This minimizes the amount of repetitive information
that is forwarded on the network by eliminating the need for each individual router in the area to forward its routing
information to all other routers in the network. If the area includes multiple networks, each network elects its own
DR and BDR.
In an OSPF network with no DR and no BDR, the neighboring router with the highest priority is elected the DR,
and the router with the next highest priority is elected the BDR. If the DR goes off-line, the BDR automatically
becomes the DR, and the router with the next highest priority then becomes the new BDR. If multiple routing
switches on the same OSPF network are declaring themselves DRs, both priority and router ID are used to select
the DR and BDRs.
Priority is configurable by using the
vlan vid ip ospf priority 0-255
command at the interface level.
You can use this parameter to help bias one router as the DR. For more information, see
on page 213. If two neighbors share the same priority, the router with the highest router ID is designated
the DR. The router with the next highest router ID is designated the BDR.
For example, in
Figure 41: Example of DRs in an OSPF area
on page 250, the DR and BDR for 10.10.10.0
network in area 5 are determined as follows:
Router A
Priority: 0
Cannot become a DR or BDR
Router B
Priority: 1
DR for the 10.10.10.0 network
Router C
Priority: 2
BDR for the 10.10.10.0 network
Router D
Priority: 3
Cannot become a DR or BDR
Router E
Priority: 4
Becomes the new BDR if router B becomes unavailable and router C
becomes the new DR
Figure 41: Example of DRs in an OSPF area
To learn the router priority on an interface, use the
show ip ospf interface
command and check the Pri
setting under OSPF interface configuration.
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Aruba 3810 / 5400R Multicast and Routing Guide for ArubaOS-
Switch 16.08
