918 CHAPTER 58: OSPF C ONFIGURATION■ Multicast: Supports packet multicasting on some types of links.Basic Concepts Autonomous SystemA set of routers using the same routing protocol to exchange routing informationconstitute an Autonomous System (AS).OSPF route computationOSPF route computation is described as follows:■ Based on the network topology around itself, each router generates Link StateAdvertisements (LSA) and sends them to other routers in update packets.■ Each OSPF router collects LSAs from other routers to compose a LSDB (LinkState Database). An LSA describes the network topology around a router, sothe LSDB describes the entire network topology of the AS.■ Each router transforms the LSDB to a weighted directed graph, which actuallyreflects the topology architecture of the entire network. All the routers havethe same graph.■ Each router uses the SPF algorithm to compute a Shortest Path Tree that showsthe routes to the nodes in the autonomous system. The router itself is the rootof the tree.Router IDTo run OSPF, a router must have a Router ID, which is a 32-bit unsigned integer,the unique identifier of the router in the AS.You may assign a Router ID to an OSPF router manually. If no Router ID isspecified, the system automatically selects one for the router as follows:■ If the loopback interfaces are configured, select the highest IP address amongthem.■ If no loopback interface is configured, select the highest IP address amongaddresses of active interfaces on the router.OSPF packetsOSPF uses five types of packets:■ Hello Packet: Periodically sent to find and maintain neighbors, containing thevalues of some timers, information about DR, BDR and known neighbors.■ DD packet (Database Description Packet): Describes the digest of each LSA inthe LSDB, exchanged between two routers for data synchronization.■ LSR (Link State Request) Packet: Requests needed LSAs from the peer. Afterexchanging the DD packets, the two routers know which LSAs of the neighborrouters are missing from the local LSDBs. In this case, they send LSR packets,requesting the missing LSAs. The packets contain the digests of the missingLSAs.■ LSU (Link State Update) Packet: Transmits the needed LSAs to the peer router.■ LSAck (Link State Acknowledgment) Packet: Acknowledges received LSUpackets. It contains the Headers of LSAs requiring acknowledgement (a packetcan acknowledge multiple LSAs).
