Dell Force10 MXL Blade MXL 10/40GbE Switch IO Module Configuration Guide - Page 361
Backbone Router (BR), Area Border Router (ABR), Autonomous System Border Router (ASBR)
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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. Open Shortest Path First (OSPFv2) | 359
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