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Chapter 6
Troubleshooting TCP/IP Connectivity
debug Commands
OSPF runs many processes to maintain all of its databases, routing updates,
and peering connections. Most of these processes use Link-State Advertise-
ments (LSAs) to share information.
Let's briefly explore the different types of Link-State Advertisements.
LSAs are the heart of OSPF's information exchange. Different types of LSAs
represent different types of route information. All of the LSA types are sum-
marized in Table 6.12.
Different LSA types represent the type of route that is being advertised,
and they assist in restricting the number and types of routes that are accepted
by a given area. As is shown in the table, an LSA of type 5 is only sent by the
Autonomous System Border Router.
T A B L E 6 . 1 2
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
information.
(3/4) Summary entry
Type 3 LSAs contain route information for
internal networks and are sent to backbone
routers. Type 4 LSAs contain information
about ASBRs. Summary information is multi-
cast by the ABR, and the information 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) NSSA autonomous
system entry
Not So Stubby Area (NSSA) permits Type 7
AS external routers to be imported inside the
NSSA area by redistribution.
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