Operation Manual – Routing ProtocolH3C S7500 Series Ethernet Switches Chapter 4 OSPF Configuration4-7When two routers synchronize their databases, they use database description (DD)packets to describe their own LSDBs, which contain the digest of each LSA. Thedigest refers to the HEAD of an LSA which uniquely identifies the LSA. This reducesthe size of traffic transmitted between the routers because the HEAD of an LSA onlyoccupies a small portion of the LSA. With the HEAD, the peer router can judgewhether it has the LSA or not.z LSR packetAfter exchanging DD packets, the two routers know which LSAs of the peer router arelacking in the local LSDB. They then send link state request (LSR) packets to the peerrequesting for the lacking LSAs. These LSR packets contain the digest of the neededLSAs.z LSU packetLink state update (LSU) packets are used to transmit the needed LSAs to the peerrouter. An LSU packet is a collection of multiple LSAs (complete LSAs, not LSAdigest).z LSAck packetLink state acknowledgment (LSAck) packets are used to acknowledge received LSUpackets. An LSAck contains the HEAD(s) of LSA(s) to be acknowledged (one LSAckpacket can acknowledge multiple LSAs).4.1.6 LSA TypesI. Five basic LSA typesAs described in the preceding sections, LSAs are the primary source for OSPF tocalculate and maintain routes. RFC 2328 defines five types of LSAs:z Router-LSA: Type-1 LSAs, generated by every router to describe the router's linkstates and costs, and advertised only in the area where the router resides.z Network-LSA: Type-2 LSAs, generated by the DRs of broadcast or NBMAnetworks to describe the link states of the current network segment and areadvertised only in the area where the DRs reside.z Summary-LSA: Type-3 and Type-4 LSAs, generated by ABRs and advertised inthe areas associated with the LSAs. Each Summary-LSA describes a route to adestination in another area of the AS (also called inter-area route). Type-3Summary-LSAs are for routes to networks (that is, their destinations aresegments), while Type-4 Summary-LSAs are for routes to ASBRs.z AS-external-LSA: Type-5 LSA, also called ASE LSA, generated by ASBRs todescribe the routes to other ASs and advertised to the whole AS (excluding stubareas). The default AS route can also be described by AS-external-LSAs.II. Type-7 LSAsIn RFC 1587 (OSPF NSSA Option), Type-7 LSA, a new LSA type, is added.
