Nokia Network Voyager for IPSO 4.0 Reference Guide 355Area Border RoutersRouters called Area Border Routers (ABR) have interfaces to multiple areas. ABRs compact thetopological information for an area and transmit it to the backbone area. Nokia supports theimplementation of ABR behavior as outlined in the Internet draft of the Internet EngineeringTask Force (IETF). The definition of an ABR in the OSPF specification as outlined in RFC 2026does not require a router with multiple attached areas to have a backbone connection. However,under this definition, any traffic destined for areas that are not connected to an ABR or that areoutside the OSPF domain is dropped. According to the Internet draft, a router is considered to bean ABR if it has more than one area actively attached and one of them is the backbone area. Anarea is considered actively attached if the router has at least one interface in that area that is notdown.Rather than redefine an ABR, the Nokia implementation includes in its routing calculationsummary LSAs from all actively attached areas if the ABR does not have an active backboneconnection, which means that the backbone is actively attached and includes at least one fullyadjacent neighbor. You do not need to configure this feature; it functions automatically undercertain topographies.OSPF uses the following types of routes: Intra-area—Have destinations within the same area. Interarea—Have destinations in other OSPF areas. Autonomous system external (ASE)—Have destinations external to the autonomoussystem (AS). These are the routes calculated from Type 5 LSAs. NSSA ASE Router—Have destinations external to AS. These are the routes calculatedfrom Type 7 LSAs.All routers on a link must agree on the configuration parameters of the link. All routers in anarea must agree on the configuration parameters of the area. A separate copy of the SPFalgorithm is run for each area. Misconfigurations prevent adjacencies from forming betweenneighbors, and routing black holes or loops can form.High Availability Support for OSPFVRRPIPSO supports the advertising of the virtual IP address of the VRRP virtual router. You canconfigure OSPF to advertise the virtual IP address rather than the actual IP address of theinterface.You must use monitored-circuit VRRP, not VRRP v2, when configuring virtual IP support forOSPF or any other dynamic routing protocol. If you enable this option, OSPF runs only on themaster of the virtual router; on a failover, OSPF stops running on the old master and then startsrunning on the new master. A traffic break might occur during the time it takes both the VRRPand OSPF protocols to learn the routes again. The larger the network, the more time it takesOSPF to synchronize its database and install routes again. For more information on enabling theadvertising of a virtual IP address when running OSPF, see “Configuring OSPF,” step 14f.
