HP 3500YL - Manual (Page 220)

IP Routing Features
Configuring OSPF
Overview of OSPF
OSPF is a link-state routing protocol applied to routers grouped into OSPF
areas identified by the routing configuration on each routing switch. The
protocol uses link-state advertisements (LSAs) transmitted by each router to
update neighboring routers regarding its interfaces and the routes available
through those interfaces. Each routing switch in an area also maintains a Link
State Database (LSDB) that describes the area topology. (All routers in a given
OSPF area have identical LSDBs.) The routing switches used to connect areas
to each other flood summary link LSAs and external link LSAs to neighboring
OSPF areas to update them regarding available routes. Through this means,
each OSPF router determines the shortest path between itself and a desired
destination router in the same OSPF domain (Autonomous System). Routed
traffic in an OSPF AS is classified as one of the following:
■ intra-area traffic
■ inter-area traffic
■ external traffic
The switches covered in this guide support the following types of LSAs, which
are described in RFCs 2328 and 3101:
Table 5-5. OSPF LSA Types
LSA Type LSA Name Use
1 Router Link Describes the state of each interface on a router for a
given area. Not propagated to backbone area.
2 Network Link Describes the OSPF routers in a given network. Not
propagated to backbone area.
3 Summary Link Describes the route to networks in another OSPF area
of the same Autonomous System (AS). Propagated
through backbone area to other areas.
4 Autonomous System
(AS) Summary Link Describes the route to an ASBR in an OSPF Normal or
Backbone area of the same AS. Propagated through
backbone area to other areas.
5 AS External Link Describes the route to a destination in another AS
(external route). Originated by ASBR in normal or
backbone areas of an AS and propagates through
backbone area to other normal areas. For injection into
an NSSA, ABR converts type-5 LSAs to a type-7 LSA
advertising the default route (0.0.0.0/0).
7 AS External Link in an
NSSA Area Describes the route to a destination in another AS
(external route). Originated by ASBR in NSSA. ABR
converts type-7 LSAs to type-5 LSAs for injection into
the backbone area.
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