3Com Switch 8800 Configuration Guide Chapter 19 OSPF Configuration19-2z A router uses the SPF algorithm to calculate the shortest path tree with itself as theroot, which shows the routes to the nodes in the autonomous system. The externalrouting information is the leave node. A router, which advertises the routes, alsotags 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 their local state information tothe entire AS, any two routers in the environment should establish adjacency betweenthem. In this case, however, the changes that any router takes will result in multipletransmissions, which are not only unnecessary but also waste the precious bandwidthresources. To solve this problem, “Designated Router” (DR) is defined in the OSPF.Thus, all the routers only send information to the DR for broadcasting the network linkstates in the network. Thereby, the number of router adjacent relations on themulti-access network is reduced.OSPF supports interface-based packet authentication to guarantee the security ofroute calculation. Also, it transmits and receives packets by IP multicast (224.0.0.5 and224.0.0.6).19.1.3 OSPF PacketsOSPF uses five types of packets:z Hello Packet:It is the commonest packet, which is periodically sent by a router to its neighbor. Itcontains the values of some timers, DR, BDR and the known neighbor.z Database Description (DD) Packet:When two routers synchronize their databases, they use the DD packets to describetheir own LSDBs, including the digest of each LSA. The digest refers to the HEAD ofLSA, which uniquely identifies the LSA. This reduces the traffic size transmittedbetween the routers, since the HEAD of a LSA only occupies a small portion of theoverall LSA traffic. With the HEAD, the peer router can judge whether it already has hadthe LSA.z Link State Request (LSR) Packet:After exchanging the DD packets, the two routers know which LSAs of the peer routersare lacked in the local LSDBs. In this case, they will send LSR packets requesting forthe needed LSAs to the peers. The packets contain the digests of the needed LSAs.z Link State Update (LSU) Packet:The packet is used to transmit the needed LSAs to the peer router. It contains acollection of multiple LSAs (complete contents).z Link State Acknowledgment (LSAck) PacketThe packet is used for acknowledging the received LSU packets. It contains theHEAD(s) of LSA(s) requiring acknowledgement.
