87• Inter-area route• Type-1 external route• Type-2 external routeThe intra-area and inter-area routes describe the network topology of the AS, while externalroutes describe routes to destinations outside the AS.OSPF classifies external routes into two types: Type-1 and Type-2. A Type-1 external route is anIGP route, such as a RIP or static route, that has high credibility and whose cost is comparablewith the cost of an OSPF internal route. The cost from a router to the destination of the Type-1external route= the cost from the router to the corresponding ASBR + the cost from the ASBR tothe destination of the external route.A Type-2 external route is an EGP route. It has low credibility, so OSPF considers the cost fromthe ASBR to the destination of the Type-2 external route is much greater than the cost from theASBR to an OSPF internal router. Therefore, the cost from the internal router to the destination ofthe Type-2 external route= the cost from the ASBR to the destination of the Type-2 external route.If two routes to the same destination have the same cost, then take the cost from the router to theASBR into consideration.Classification of OSPF networksOSPF network typesOSPF classifies networks upon the link layer protocol into the following types:• Broadcast: When the link layer protocol is Ethernet or FDDI, OSPF considers the networktype broadcast by default. On broadcast networks, hello packets, LSU packets, and LSAckpackets are generally sent to multicast addresses (such as 224.0.0.5 (reserved for OSPFrouters) and 224.0.0.6 (reserved for OSPF DRs), while DD packets and LSR packets areunicast.• NBMA: Packets on these networks are sent to unicast addresses.• P2MP: By default, OSPF considers no link layer protocol as P2MP, which is a conversionfrom other network types such as NBMA. On P2MP networks, packets are sent to multicastaddresses (224.0.0.5).• P2P: On P2P networks, packets are sent to multicast addresses (224.0.0.5).NBMA network configuration principleTypical NBMA networks are ATM and Frame Relay networks.You must perform a special configuration on NBMA interfaces. These interfaces cannot broadcasthello packets for neighbor location; therefore, you must specify neighbors manually and configurewhether the neighbors have the DR election right.An NBMA network is fully meshed, which means any two routers in the NBMA network have adirect virtual link for communication. If direct connections are not available between somerouters, the type of interfaces associated should be configured as P2MP, or as P2P for interfaceswith only one neighbor.The differences between NBMA and P2MP networks are as follows:
