3Com Router 3000 Ethernet FamilyConfiguration Guide Chapter 4 OSPF Configuration3Com Corporation4-2z A router uses the SPF algorithm to calculate the shortest path tree with itself as theroot. The tree shows the routes to the nodes in the autonomous system. Theexternal routing information is leaf node. A router, which advertises the routes,also tags them and records the additional information of the autonomous system.Obviously, the routing tables obtained by different routers are different.Furthermore, to enable individual routers to broadcast information (such as availableinterface information and reachable neighbor information) of their local statuses to thewhole AS, any two routers in the environment should establish adjacency betweenthem. In this case, the route changes on any router will result in multiple transmissions,which are unnecessary and waste the precious bandwidth resources. To solve thisproblem, designated Router" (DR) is defined in the OSPF. Thus, all the routers onlysend information to the DR for broadcasting the network link states in the network.Thereby, the number of adjacent router relations on the multi-access network isreduced.OSPF supports interface-based packet authentication to guarantee the security ofroute calculation. Also, it transmits and receives packets by IP multicast.4.1.3 Basic Concepts Related to OSPFI. Router IDEvery OSPF router must have a Router ID. You may assign one to an OSPF routermanually. This Router ID is preferably the address of a loopback interface, because aloopback interface is always up until it is manually shut down. If no Router ID isspecified, the system automatically selects one for the router as follows:z If IP addresses of loopback interfaces are available, select the last configured one.z If not, select the first configured physical interface that is up.II. DR and BDRz DR (Designated Router)In the broadcast network or NBMA network, in order for each router to broadcast itslocal state information to the whole AS (Autonomous System), multiple neighboringrelationships should be created between routers. This, however, makes it possible forroute variation of any router to result in repeated transmission, which wastes thevaluable bandwidth resource. To solve the problem, OSPF defines the "DesignatedRouter" (DR). All the routers only need to transmit information to the DR forbroadcasting the network link states. Neighboring relationship is not establishedbetween two routers other than DRs (called as DR Others), nor do the DR Othersexchange any routing information.It is not manually specified which router will be the DR of the local network segment, butcommonly selected by all the routers in the network segment.z BDR (Backup Designated Router)
