Understanding OSPF Designated Routers

Large LANs that have many routers and therefore many OSPF adjacencies can produce heavy control-packet traffic as link-state advertisements (LSAs) are flooded across the network. To alleviate the potential traffic problem, OSPF uses designated routers. Rather than broadcasting LSAs to all their OSPF neighbors, the routers send their LSAs to the designated router, which processes the LSAs, generates responses, and multicasts topology updates to all OSPF routers.

In LANs, the election of the designated router takes place when the OSPF network is initially established. When the first OSPF links are active, the router with the highest router identifier (defined by the router-id configuration value or the loopback address) is elected designated router. The router with the second highest router identifier is elected the backup designated router. If the designated router fails or loses connectivity, the backup designated router assumes its role and a new backup designated router election takes place between all the routers in the OSPF network.

OSPF uses the router identifier to elect a designated router, unless you manually specify a priority value. At designated router election, the router priorities are evaluated first, and the router with the highest priority is elected designated router.

By default, routers have a priority of 128. A priority of 0 marks the router as ineligible to become the designated router. To configure a router so it is always the designated router, set its priority to 255.

Related Topics

• Junos OS Feature Support Reference for SRX Series and J Series Devices
• OSPF Overview
• Example: Configuring an OSPF Router Identifier
• Example: Controlling OSPF Designated Router Election