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OSI Layer 3: Routed and Routing Protocols

Different LSA types represent the type of route that is being advertised,

and assist in restricting the number and type of routes that are accepted by
a given area. As is shown in the table, an LSA of Type 5 is sent only by the
Autonomous System Border Router. This brings you to the point where you
need to define the different router types that belong to OSPF areas.

T A B L E 2 . 1 1

OSPF LSA Types

LSA Type

Description of LSA

(1) Router link entry

This LSA is broadcast only within its defined
area. The LSA contains all of the default link-
state information.

(2) Network entry

This LSA is multicast to all area routers by the
DR. This update contains network-specific in-
formation.

(3/4) Summary entry

Type 3 LSAs contain route information for in-
ternal networks and are sent to backbone
routers. Type 4 LSAs contain information
about Autonomous System Border Routers
(ASBRs). Summary information is multicast
by the Area Border Router (ABR) and the infor-
mation reaches all backbone routers.

(5) Autonomous System
entry

As the name indicates, these advertisements
originate from the ASBR. These packets con-
tain information about external networks.

(7) Not So Stubby Area

Not So Stubby Area (NSSA) permits Type 7
AS external routes to be imported inside the
NSSA area by redistribution.

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