Information Technology Reference
In-Depth Information
Advantages of a Link-State Routing Protocol Exercise
Explain briefly why each of the following is an advantage of link-state routing protocols over distance
vector routing protocols.
Instructor note:
The following answers are direct quotes from the text. The student should be encouraged to
answer these question in his or her own words.
Builds a Topological Map
Link-state routing protocols create a topological map, or SPF tree, of the network topology. Distance
vector routing protocols do not have a topological map of the network. Routers implementing a dis-
tance vector routing protocol only have a list of networks, which includes the cost (distance) and next-
hop routers (direction) to those networks. Because link-state routing protocols exchange link states,
the SPF algorithm can build an SPF tree of the network. Using the SPF tree, each router can inde-
pendently determine the shortest path to every network.
Fast Convergence
There are several reasons why a link-state routing protocol converges faster than a distance vector
routing protocol. When receiving a link-state packet (LSP), link-state routing protocols immediately
flood the LSP out all interfaces except for the interface from which the LSP was received. A router
using a distance vector routing protocol needs to process each routing update and update its routing
table before flooding them out other interfaces, even with triggered updates. Another reason is the lack
of a hold-down timer, which is a distance vector routing protocol feature designed to give the network
time to converge. Link-state routing protocols do not use a hold-down timer because any changes in
the topology are flooded immediately within the entire routing domain using LSPs.
Event-Driven Updates
After the initial flooding of LSPs, link-state routing protocols only send out an LSP when there is a
change in the topology. The LSP contains only the information regarding the affected link. Unlike
some distance vector routing protocols, link-state routing protocols do not send periodic updates.
Hierarchical Design
Link-state routing protocols such as OSPF and IS-IS use the concept of areas. Multiple areas create a
hierarchical design to networks, allowing better route aggregation (summarization) and the isolation of
routing issues within an area. Multiarea OSPF and IS-IS are discussed further in CCNP.
Requirements of a Link-State Routing Protocol Exercise
Explain briefly why each of the following requirements is important when a network administrator is
considering implementing a link-state routing protocol.
Instructor note:
The following answers are direct quotes from the text. The student should be encouraged to
answer these questions in his or her own words.
Memory Requirements
Link-state routing protocols typically require more memory, more CPU processing, and at times more
bandwidth than distance vector routing protocols. The memory requirements are because of the use of
link-state databases and the creation of the SPF tree.
