Link-State Routing

Link-State Routing
Remember how with a distance-vector routing protocol, the router knew only the direction in
which to send the packet and the distance to get there? Link-state routing is different in that
each router knows the exact topology of the network. This in turn limits the number of bad
routing decisions that can be made. Link-state routing can accomplish this because every router
in the routing domain or area has a similar view of the network, placing itself at the root of a
hierarchical tree. Each router in the network will report on the state of each directly connected
link. Each router then plays a part in propagating this learned information until all routers in
the network have it. Each router that receives this information will take a snapshot of it.
It’s important to realize that the other routers do not make any change to the updates received.
This in turn ensures that all routers in the process have the same relative view of the network,
allowing each router to make its own routing decisions based upon the same information.
Another key difference of link-state routing is that each router does not send its entire routing
table. The only information that is sent are the changes that have occurred or a message stating
that nothing has changed after a given period of time has passed. This is known as a link-state
advertisement (LSA). An LSA is generated for each link on a router. Each LSA includes an identifier
for the link, the state of the link, and a metric for the link. With the use of LSAs, link-state
protocols cut down on the amount of bandwidth utilized. The disadvantage of a link-state routing
protocol is that it is more complex to configure than a distance-vector routing protocol.
16 Chapter 1  Routing Principles
The link-state routing protocols that are covered in this book are as follows:
 Open Shortest Path First (OSPF)
 Integrated Intermediate System to Intermediate System (Integrated IS-IS)
Keep in mind these are not the only link-state routing protocols. These are the ones that are
covered by the BSCI exam, though.
Because we will cover link-state routing in more detail in Chapter 4, Chapter 5, “OSPF Operation
in a Single Area,” and Chapter 6, “Interconnecting OSPF Areas,” we will give you only
a brief introduction to the operation of link-state routing here.
The basic functionality of link-state routing is broken down into the following steps:
1. The first thing each router does, as it becomes active, is form an adjacency with its directly
connected neighbors.
2. After forming adjacencies, the router then sends out link-state advertisements (LSAs) to
each of its neighbors. After receiving and copying the information from the LSA, the router
forwards—or floods—the LSA to each of its neighbors.
3. All of the routers then store the LSAs in their own database. This means that all routers
have the same view of the network topology.
4. Each router then uses the Dijkstra algorithm to compute its best route to a destination.
As stated previously, this is a brief introduction to link-state routing. Link-state routing will be
covered in greater detail later in this book. Table 1.2 compares the link-state routing protocols covered
in this study guide. Remember that EIGRP is considered a hybrid protocol, meaning that it contains
traits of both distance-vector and link-state routing protocols. Also remember that if you are
forced to consider EIGRP to be one or the other only, consider it a distance-vector routing protocol.