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3Com Switch 8800 Configuration Guide Chapter 19 OSPF Configuration
19-3
19.1.4 LSA Type
I. Five basic LSA types
As mentioned previously, OSPF calculates and maintains routing information from
LSAs. RFC2328 defines five LSA types as follows:
z Router-LSAs: Type-1. Each router generates Router-LSAs, which describe the
link state and cost of the local router. Router-LSAs are broadcast within the area
where the router is located.
z Network-LSAs: Type-2. DRs on the broadcast network and NBMA network
generate 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 related
to the LSA. Each Summary-LSA describes a route (inter-area route) to a certain
destination in other areas of this AS. Type-3 Summary-LSAs describe the routes
to networks (the destination is network). Type-4 Summary-LSAs describe the
routes 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 transmitted
to the whole AS (except Stub areas). AS-external-LSAs can also describe the
default route of an AS.
II. Type-7 LSA
RFC1587 (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 an
ABR, the ABR can convert part routing information of Type-7 LSAs into Type-5
LSAs and releases the information. Type-7 LSAs cannot be directly released to
other areas or backbone areas.
19.1.5 Basic Concepts Related to OSPF
I. Router ID
To run OSPF, a router must have a router ID. If no ID is configured, the system will
automatically pick an IP address from the IP addresses of the current interfaces as the
Router ID. The following introduces how to choose a router ID. If loopback interface
addresses exist, the system chooses the Loopback address with the greatest IP
address value as the router ID. If no Loopback interface configured, then the address of
the physical interface with the greatest IP address value will be the router ID.