Backbone Router (Br); Area Border Router (Abr); Autonomous System Border Router (Asbr); Internal Router (Ir) - Dell Force10 MXL Blade Configuration Manual

Backbone Router (BR)

A backbone router (BR) is part of the OSPF backbone, Area 0. This includes all ABRs. It can also include
any routers that connect only to the backbone and another ABR, but are only part of Area 0, such as
Router I in Figure 20-2.

Area Border Router (ABR)

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, so it may keep multiple copies of the link state
database. An ABR takes information it has learned on one of its attached areas and can summarize it before
sending it out on other areas it is connected to. An ABR can connect to many areas in an AS, and is
considered a member of each area it connects to.

Autonomous System Border Router (ASBR)

The autonomous system border area router (ASBR) connects to more than one AS and exchanges
information with the routers in other ASs. In general, the ASBR connects to a non-interior gate protocol
(IGP) such as BGP or uses static routes.

Internal Router (IR)

The internal router (IR) has adjacencies with ONLY routers in the same area, as Router E, M, and I are in
Figure 20-2.

Designated and Backup Designated Routers

OSPF elects a designated router (DR) and a backup designated router (BDR). Among other things, the
designated router is responsible for generating LSAs for the entire multi-access network. Designated
routers allow a reduction in network traffic and in the size of the topological database.
• The DR maintains a complete topology table of the network and sends the updates to the other routers
using multicast. All routers in an area form a slave/master relationship with the DR. Every time a
router sends an update, it sends it to the DR and BDR. The DR sends the update out to all other routers
in the area.
• The BDR is the router that takes over if the DR fails.
Each router exchanges information with the DR and BDR. The DR and BDR relay the information to the
other routers. On broadcast network segments, the number of OSPF packets is further reduced by the DR
and BDR sending such OSPF updates to a multicast IP address that all OSPF routers on the network
segment are listening on.
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