3Com Switch 7750 Configuration Guide Guide

OSPF Overview 293
■ Network-LSA: Type-2 LSAs, generated by the DRs of broadcast or NBMA
network to describe the link states of the current network segment and are
advertised only in the area where the DRs reside.
■ Summary-LSA: Type-3 and Type-4 LSAs, generated by ABRs and advertised in
the areas associated with the LSAs. Each Summary-LSA describes a route to a
destination in another area of the AS (also called inter-area route).Type-3
Summary-LSAs are for routes to networks (that is, their destinations are
segments), while Type-4 Summary-LSAs are for routes to ASBRs.
■ AS-external-LSA: Type-5 LSA, also called ASE LSA, generated by ASBRs to
describe the routes to other ASs and advertised to the whole AS (excluding
stub areas). The default AS route can also be described by AS-external-LSAs.
Type-7 LS As
In RFC 1587 (OSPF NSSA Option), Type-7 LSA, a new LSA type, is added.
As described in RFC 1587, Type-7 LSAs and Type-5 LSAs mainly differ in the
following two ways:
■ Type-7 LSAs are generated and advertised in an NSSA, where Type-5 LSAs will
not be generated or advertised.
■ Type-7 LSAs can only be advertised in an NSSA area. When Type-7 LSAs reach
an ABR, the ABR can convert part of the routing information carried in the
Type-7 LSAs into Type-5 LSAs and advertise the Type-5 LSAs. Type-7 LSAs are
not directly advertised to other areas (including the backbone area).
OSPF Features Switch 7750 Family supports the following OSPF features:
■ Stub area: Stub area is defined to reduce the cost for the routers in the area to
receive ASE routes.
■ NSSA area: NSSA area is defined to remove the limit on the topology in a stub
area.
■ OSPF multi-process: Multiple OSPF processes can be run on a router.
■ Sharing discovered routing information with other dynamic routing protocols:
At present, OSPF supports importing the routes of other dynamic routing
protocols (such as RIP), and static routes as OSPF external routes into the AS to
which the router belongs. In addition, OSPF supports advertising the routing
information it discovered to other routing protocols.
■ Authentication key: OSPF supports the authentication of the packets between
neighboring routers in the same area by using one of the two methods: plain
text authentication key and MD5 authentication key.
■ Flexible configuration of router interface parameters: For a router interface, you
can configure the following OSPF parameters: output cost, Hello interval,
interface transmission delay, route priority, dead time for a neighboring router,
and packet authentication mode and authentication key.
■ Virtual link: Virtual links can be configured.