3Com 8807 Configuration Manual page 305

Basic Concepts Related
to OSPF
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.
Network-LSAs: Type-2. DRs on the broadcast 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.
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).
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.
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:
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.
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.
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.
DR and BDR
Designated Router (DR)
■
In multi-access networks, if any two routers establish adjacencies, the same LSA
will be transmitted repeatedly, wasting bandwidth resources. To solve this
problem, the OSPF protocol regulates that a DR must be elected in a multi-access
network and only the DR (and the BDR) can establish adjacencies with other
routers in this network. Two non-DR routers or non-BDR routers cannot establish
adjacencies and exchange routing information.
You cannot specify the DR in the segment. Instead, DR is elected by all the routers
in the segment.
OSPF Overview 305