3Com Switch 8800 Configuration Guide Chapter 19 OSPF Configuration19-319.1.4 LSA TypeI. Five basic LSA typesAs mentioned previously, OSPF calculates and maintains routing information fromLSAs. RFC2328 defines five LSA types as follows:z Router-LSAs: Type-1. Each router generates Router-LSAs, which describe thelink state and cost of the local router. Router-LSAs are broadcast within the areawhere the router is located.z Network-LSAs: Type-2. DRs on the broadcast network and NBMA networkgenerate Network-LSAs, which describe the link state of the local network.Network-LSAs are broadcast within the area where a DR is located.z Summary-LSAs: Include Type-3 and Type-4. Area border routers (ABRs)generate Summary-LSAs. Summary-LSAs are broadcast within the area relatedto the LSA. Each Summary-LSA describes a route (inter-area route) to a certaindestination in other areas of this AS. Type-3 Summary-LSAs describe the routesto networks (the destination is network). Type-4 Summary-LSAs describe theroutes to autonomous system border routers (ASBRs).z AS-external-LSAs: or ASE LSA, the Type-5. ASBRs generate AS-external-LSAs,which describe the routes to other ASs. AS-external-LSA packets are transmittedto the whole AS (except Stub areas). AS-external-LSAs can also describe thedefault route of an AS.II. Type-7 LSARFC1587 (OSPF NSSA Option) adds a new LSA type: Type-7 LSAs.According to RFC1587, Type-7 LSAs differ from Type-5 LSAs as follows:z Type-7 LSAs are generated and released within a Not-So-Stubby Area (NSSA).Type-5 LSAs cannot be generated or released within a NSSA.z Type-7 LSAs can only be released within an NSSA. When Type-7 LSAs reach anABR, the ABR can convert part routing information of Type-7 LSAs into Type-5LSAs and releases the information. Type-7 LSAs cannot be directly released toother areas or backbone areas.19.1.5 Basic Concepts Related to OSPFI. Router IDTo run OSPF, a router must have a router ID. If no ID is configured, the system willautomatically pick an IP address from the IP addresses of the current interfaces as theRouter ID. The following introduces how to choose a router ID. If loopback interfaceaddresses exist, the system chooses the Loopback address with the greatest IPaddress value as the router ID. If no Loopback interface configured, then the address ofthe physical interface with the greatest IP address value will be the router ID.
