Operation Manual – Routing Protocol
H3C S5600 Series Ethernet Switches
Chapter 4 OSPF Configuration
4-4
IV. LSA Types
1)
Five basic LSA types
As described in the preceding sections, LSAs are the primary source for OSPF to
calculate and maintain routes. RFC 2328 defines five types of LSAs:
z
Router-LSA: Type-1 LSAs, generated by every router to describe the router's link
states and costs, and advertised only in the originating area.
z
Network-LSA: Type-2 LSAs, generated by the DRs on a broadcast or NBMA
network to describe the link states of the current network segment, and are
advertised only in the originating area.
z
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.
z
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 and NSSA areas). The default AS route can also be described by
AS-external-LSAs.
2) Type-7
LSAs
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:
z
Type-7 LSAs are generated and advertised in an NSSA, where Type-5 LSAs will
not be generated or advertised.
z
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).
V. Neighbor and Adjacency
In OSPF,
neighbor
and
adjacency
are two different concepts.
Neighbor: Two routers that have interfaces to a common network. Neighbor
relationships are maintained by, and usually dynamically discovered by, OSPF's hello
packets. When a router starts, it sends a hello packet through the OSPF interface, and
the router that receives the hello packet checks parameters carried in the packet. If
parameters of the two routers match, they become neighbors.
Adjacency: A relationship formed between selected neighboring routers for the
purpose of exchanging routing information. Not every pair of neighboring routers
become adjacent, which depends on network types. Only by synchronizing the LSDB
via exchanging DD packets and LSAs can two routers become adjacent.
