To limit the amount of routing protocol traffic, the Hello protocol elects a DR and a BDR on each multiaccess network. Instead of neighboring routers forming adjacencies and swapping link-state information, which on a large network can mean significant routing protocol traffic, all routers on the network form adjacencies with the DR and the BDR only, and send link-state information to them. The DR redistributes this information to every other adjacent router.
If the BDR operates in backup mode, it receives link-state information from all routers on the network and listens for acknowledgements. If the DR fails, the BDR can transition quickly to the role of DR because its routing tables are up-to-date.
Routers in an OSPF network can have various roles depending on how you configure them. The following table describes the router types you can configure in an OSPF network.
|
Router type |
Description |
|---|---|
|
AS boundary router |
A router that attaches at the edge of an OSPF network is an ASBR. An ASBR generally has one or more interfaces that run an interdomain routing protocol such as Border Gateway Protocol. In addition, a router that distributes static routes or RIP routes into OSPF is an ASBR. The ASBR forwards external routes into the OSPF domain. In this way, routers inside the OSPF network learn about destinations outside their domain. |
|
Area border router |
A router that attaches to two or more areas inside an OSPF network is an ABR. ABRs play an important role in OSPF networks by condensing the amount of disseminated OSPF information. |
|
Internal router (IR) |
A router that has interfaces only within a single area inside an OSPF network is an IR. Unlike ABRs, IRs have topological information only about the area in which they reside. |
|
Designated router |
In a broadcast or NBMA network, the routers elect a single router as the DR for that network. A DR makes sure that all routers on the network synchronize and advertises the network to the rest of the AS. |
|
Backup designated router |
A BDR is elected in addition to the DR and, if the DR fails, can assume the DR role quickly. |