Routing and Routing Protocols

Dynamic Routing Overview

Dynamic Routing

• Dynamic routing allows the network to adjust to topology changes without administrative intervention.

• The routers exchange information to tell each other when a network link is no longer available. They also exchange information to

notify when the link is working

Routed and Routing Protocols

• Routing Protocol: Allow one router to

share information with other routers regarding the networks it knows about as well as its proximity to other routers.

The information received using a routing protocol is used to build and maintain a routing table.

• Routed Protocol: Used to direct user

traffic. Provide enough

information in its network layer address to allow a packet to be forwarded from one host to another

Routing Convergence

• The routing protocol learns all available routes, places the best routes into the routing table, and removes routes when they are no longer valid. Link failure, network modification

• Whenever the topology of a network changes the routers need to communicate that change.

• When all routers in a network are operating with the same knowledge, the network is said to have converged. The sooner the routers converge, the quicker the

routers will be able to make correct routing decisions.

Routing Protocol Classification

• Routing algorithms can be classified into one of two categories: Distance vector

• Determines the direction (vector) and distance to any link in the network

Link-state • Also called shortest path first,

recreates the exact topology of the entire network.

Distance Vector Features

• Pass periodic copies of the entire routing table from router to router

• Also known as Bellman-Ford algorithms

• Similar to the signs found at a highway intersection. A sign points towards a destination and

indicates the distance to the destination. Further down the highway, another sign

points toward the destination, but now the distance is shorter.

Distance Vector Routing• Each router begins by

identifying its own neighbors.

• The interface that leads to each directly connected network has a distance of 0.

• Router W learns about other networks based on information received from Router X

• Router W adds a distance vector number which increases the distance vector.

• Routers do not know the exact topology of an network as each router only sees its neighbor routers.

Link-State Features

• Also known as Dijkstra’s algorithm or as SPF (shortest path first) algorithms

• Maintain a database of topology information Which routers are connected to each

other Which routers connect to outside

networks Which routers are inside the network

More Link-State Features

• Link-state advertisements (LSAs) A small packet of routing information that is

sent between neighboring routers.• Topological database

A collection of information gathered from LSAs

The router’s map of the entire network• SPF algorithm

A calculation performed on the topological database resulting in the SPF tree, showing the best paths to destination networks

• Routing tables A list of the known paths and interfaces

Link-State Network Discovery

• LSAs are exchanged between directly connected routers with information about directly connected networks.

• These LSAs are accumulated on each router and a topological database is constructed.

• The SPF algorithm uses this database to calculate shortest path.• It then builds a tree, with itself as the root, consisting of all possible

paths to each network.• It sorts these paths Shortest Path First (SPF).• The router lists the best paths and ports to these destination networks

in the routing table.

Link-State Routing

• The router that first becomes aware of a topology change forwards the information to all neighboring routers

• Each router keeps track of its: Neighbor routers Neighbor’s name Link (interface) status Cost of the link to the neighbor

• Construct an LSA packet that lists its neighbor’s details• Send out this packet• When it receives an LSA packet it records it in its

database• Draws a topology map with the accumulated LSAs• Each time an LSA causes a change to the database, the

SPF algorithm is recalculated• The routing table is updated

Link-State Concerns

• CPU Usage: more processing requirements than distance-vector protocols

• RAM: holds info from various databases, holds the topology tree, and the routing table

• Bandwidth during the initial discovery process is heavy because LSA packets are sent to all neighboring routers

• After this initial flooding, bandwidth usage is minimal since LSAs are only sent when topology changes

Link-State vs. Distance Vector Routing Protocols

Distance Vector Link-State

Pros Easy to configure Once converged, sends only network topology changes

Simple operation Maintains a map of the entire network

Cons Transmits entire routing table in updates

Requires more memory and processing power