Figure
3-26 shows the components of a link-state protocol.
Figure 3-26 Link-State Protocols
Understanding the operation of link-state routing protocols is critical to being able to enable,
verify, and troubleshoot their operation.
B C E
A
D
10.4.0.0
Link Up!
Link-State Packets
SPF
Algorithm
SPF Tree
Topological
Database
C A
B
D
Routing
Table
116 Chapter 3: Medium-Sized Routed Network Construction
Link-state-based routing algorithms??”also known as shortest path ?¬?rst (SPF) algorithms??”
maintain a complex database of topology information. Whereas the distance vector algorithm has
nonspeci?¬?c information about distant networks and no knowledge of distant routers, a link-state
routing algorithm maintains full knowledge of distant routers and how they interconnect.
Link-state routing uses link-state advertisements (LSA), a topological database, the SPF
algorithm, the resulting SPF tree, and, ?¬?nally, a routing table of paths and ports to each network.
Open Shortest Path First (OSPF) and Intermediate System-to-Intermediate System (IS-IS) are
classi?¬?ed as link-state routing protocols. RFC 2328 describes OSPF link-state concepts and
operations.
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