Distance Vector vs. Link State vs. Hybrid Routing
By Stephen Bucaro
Routers direct traffic between networks or subnetworks, for this they operate at Layer 3, the network layer,
of the OSI Model. Routers don't track individual computers, but they contain a routing table and other
information which allows them identify the best path to take and transporting a packet to get to its destination.
Distance Vector routing uses RIP (Routing Information Protocol) or RIP2 routing protocol. Both of these
protocols use the hop count as their routing metric. RIP allows a maximum of 15 hops from source to a
destination. This prevents routing loops. Distance vector routers exchange routing table information with other
routers on a regular schedule.
Link State routing is an improvement over Distance Vector routing. Link State routing protocols such as
OSPF (Open Shortest Path First) and IS-IS (Intermediate System to Intermediate System) use statistics
like number of hops, distance, link speed, and traffic to calculate the cost of different routes through the
Internet. They use Dijkstra's algorithm to determine the lowest cost route. Link State routers exchange routing
table information only when their tables change.
Convergence is the state of when a group of routers have exchanged routing table information so that
they all have the same topological information about the network. The goal of any routing protocol is to achieve
convergence as quickly as possible and with the least amount of overhead.
Since Link State routing needs to exchange routing table information only when there is a change in the
network, it requires less transmission overhead to achieve convergence. However it requires more processing
and more powerful routers.
Hybrid routing has the advantages of both distance vector and link state protocols. It uses distance vector
protocol to reduce the processing power requirement, but exchanges routing table information only when their
tables change to reduce transmission overhead.
Today, many modern routing protocols that don't conform strictly to distance vector routing or link state
routing are referred to as Hybrid Routing Protocols, but they actually use and exchange a variety of network
statistics used to calculate the cost of different routes through the Internet.
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