Designated routers

In an OSPFv3 network having two or more routers, one router is elected to serve as the designated router (DR) and another router to act as the backup designated router (BDR). All other routers in the area forward their routing information to the DR and BDR, and the DR forwards this information to all 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 OSPFv3 network with no DR and no BDR, the neighboring router with the highest priority is elected as the DR, and the router with the next highest priority is elected as 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 OSPFv3 network are declaring themselves as DRs, both priority and router ID are used to select the designated router and backup designated routers.

Priority is configurable by using the vlan vid ipv6 ospfv3 priority 0 - 255 command at the interface level. You can use this parameter to help bias one router as the DR. If two neighbors share the same priority, the router with the highest router ID is designated as the DR. The router with the next highest router ID is designated as the BDR.

For example, in the following figure, the DR and BDR for the 2001:db8:0:5::/64 network in area 5 are determined as follows:

  • Router A Priority: 0 Cannot become a DR or BDR.

  • Router B Priority: 3 DR for the 2001:db8:0:5::/64 network.

  • Router C Priority: 2 BDR for the 2001:db8:0:5::/64 network.

  • Router D Priority: 0 Cannot become a DR or BDR.

  • Router E Priority: 1 Becomes the new BDR if router B becomes unavailable and router C becomes the new DR.

Figure 23: Example of designated routers in an OSPFv3 area

To verify the router priority on an interface, use the show ipv6 ospf3 interface command and check the Pri field.

NOTE:

Once a DR is elected, the DR and BDR status do not change if a higher-priority router joins the network, unless the DR or BDR goes down.

By default, the router ID is typically the lowest-numbered IPv4 loopback address or the lowest-numbered (user-configured) loopback interface configured on the device.

If multiple networks exist in the same OSPFv3 area, the recommended approach is to ensure that each network uses a different router as its DR. Otherwise, if a router is a DR for more than one network, latency in the router could increase because of the increased traffic load resulting from multiple DR assignments.

When only one router on an OSPFv3 network claims the DR role despite neighboring routers with higher priorities or router IDs, this router remains the DR. This is also true for BDRs.

The DR and BDR election process is performed when one of the following events occurs:

  • An interface is in a waiting state and the wait time expires

  • An interface is in a waiting state and a hello packet is received that addresses the BDR

  • A change in the neighbor state occurs, such as:

    • A neighbor state transitions from 2 or higher

    • Communication to a neighbor is lost

    • A neighbor declares itself to be the DR or BDR for the first time