Connectivity Guide

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 trac and in the size of the topological database.
Designated router Maintains a complete topology table of the network and sends updates to the other routers via multicast. All
routers in an area form a slave/master relationship with the DR. Every time a router sends an update, the router
sends it to the DR and BDR. The DR sends the update to all other routers in the area.
Backup designated
router
Router that takes over if the DR fails.
Each router exchanges information with the DR and BDR. The DR and BDR relay information to other routers. On broadcast network
segments, the number of OSPF packets reduces by the DR sending OSPF updates to a multicast IP address that all OSPF routers on the
network segment are listening on.
DRs and BDRs are congurable. If you do not dene the DR or BDR, OS10 assigns them per the protocol. To determine which routers are
the DR and BDR, OSPF looks at the priority of the routers on the segment. The default router priority is 1. The router with the highest
priority is elected DR. If there is a tie, the router with the higher router ID takes precedence. After the DR is elected, the BDR is elected the
same way. A router with a router priority set to zero cannot become a DR or BDR.
Link-state advertisements
A link-state advertisement (LSA) communicates the router’s routing topology to all other routers in the network.
Type 1—Router LSA
Router lists links to other routers or networks in the same area. Type 1 LSAs ood across their own area only. The
link-state ID of the Type 1 LSA is the originating router ID.
Type 2—Network
LSA
DR in an area lists which routers are joined within the area. Type 2 LSAs ood across their own area only. The link-
state ID of the Type 2 LSA is the IP interface address of the DR.
Type 3—Summary
LSA (OSPFv2),
Inter-Area Prex
LSA (OSPFv3)
ABR takes information it has learned on one of its attached areas and summarizes it before sending it out on other
areas it connects to. The link-state ID of the Type 3 LSA is the destination network's IP address.
Type 4—AS Border
Router Summary
LSA (OSPFv2),
In some cases, Type 5 External LSAs ood to areas where the detailed next-hop information may not be available
because it may be using a dierent routing protocol. The ABR oods the information for the router, the ASBR
where the Type 5 originated. The link-state ID for Type 4 LSAs is the router ID of the described ASBR.
432 Layer 3