Dell Force10 S4820T S4820T Configuration Guide - Page 691

Area Border Router (ABR) Within an AS, an Area Border (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 Area Border Router (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. Generally 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 shown in Figure 33-2. Designated and Backup Designated Routers OSPF elects a Designated Router and a Backup Designated router. Among other things, the designated router is responsible for generating LSAs for the entire multiaccess network. Designated routers allow a reduction in network traffic and in the size of the topological database. • The Designated Router (DR) maintains a complete topology table of the network and sends the 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, it sends it to the Designated Router (DR) and Backup Designated Router (BDR). The DR sends the update out to all other routers in the area. • The Backup Designated Router (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. These router designations are not the same ad the router IDs discussed earlier. The Designated and Backup Designated Routers are configurable in FTOS. If no DR or BDR is defined in FTOS, the system assigns them. OSPF looks at the priority of the routers on the segment to determine which routers are the DR and BDR. The router with the highest priority is elected the DR. If there is a tie, then 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 is cannot become the DR or BDR. Open Shortest Path First (OSPFv2) | 691