Dell PowerConnect FCS624S Configuration Guide - Page 966

29 Overview of OSPF In an OSPF point-to-point network, where a direct Layer 3 connection exists between a single pair of OSPF routers, there is no need for Designated and Backup Designated Routers, as is the case in OSPF multi-access networks. Without the need for Designated and Backup Designated routers, a point-to-point network establishes adjacency and converges faster. The neighboring routers become adjacent whenever they can communicate directly. In contrast, in broadcast and non-broadcast multi-access (NBMA) networks, the Designated Router and Backup Designated Router become adjacent to all other routers attached to the network. To configure an OSPF point-to-point link, refer to "Configuring an OSPF point-to-point link" on page 963. Designated routers in multi-access networks In a network that has multiple routers attached, OSPF elects one router to serve as the designated router (DR) and another router on the segment to act as the backup designated router (BDR). This arrangement minimizes the amount of repetitive information that is forwarded on the network by forwarding all messages to the designated router and backup designated routers responsible for forwarding the updates throughout the network. Designated router election in multi-access networks In a network with no designated router and no backup designated router, the neighboring router with the highest priority is elected as the DR, and the router with the next largest priority is elected as the BDR, as shown in Figure 139 FIGURE 139 Designated and backup router election Designated Backup Router priority 10 Router A priority 5 Router C Designated Router priority 20 Router B If the DR goes off-line, the BDR automatically becomes the DR. The router with the next highest priority becomes the new BDR. This process is shown in Figure 140. 924 PowerConnect B-Series FCX Configuration Guide 53-1002266-01