Connectivity Guide
Each router has a unique ID, written in decimal A.B.C.D format. You do not have to associate the router ID with a valid IP address. To make
troubleshooting easier, ensure the router ID is identical to the router’s IP address.
Backbone router A backbone router (BR) is part of the OSPF Backbone, Area 0, and includes all ABRs. The BR includes routers
connected only to the backbone and another ABR, but are only part of Area 0—shown as Router I in the example.
Area border router Within an AS, an area border router (ABR) connects one or more areas to the backbone. The ABR keeps a copy of
the link-state database for every area it connects to. It may keep multiple copies of the link state database. An ABR
summarizes learned information from one of its attached areas before it is sent to other connected areas. An ABR
can connect to many areas in an AS and is considered a member of each area it connects to—shown as Router H
in the example.
Autonomous system
border router
The autonomous system border router (ASBR) connects to more than one AS and exchanges information with the
routers in other ASs. The ASBR connects to a non-IGP such as BGP or uses static routes—shown as Router N in
the example.
Internal router The internal router (IR) has adjacencies with ONLY routers in the same area—shown as Routers E, F, I, K, and M in
the example.
Designated and backup designated routers
OSPF elects a designated router (DR) and a backup designated router (BDR). The DR generates LSAs for the entire multiaccess network.
Designated routers allow a reduction in network trac and in the size of the topological database.
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Layer 3