Routing Protocols

Routing Protocols
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Types of Routing Protocols
-> There are two types of routing protocols:
1. Distance vector (RIP, IGRP)
2. Link state (OSPF, IS-IS)
• Cisco has created its own routing protocol – EIGRP. EIGRP is
considered to be an advanced distance vector protocol, although
some materials erroneously state that EIGRP is a hybrid routing
protocol, a combination of distance vector and link state.
• All of the routing protocols mentioned above are interior routing
protocols (IGP), which means that they are used to exchange
routing information within one autonomous system. BGP (Border
Gateway Protocol) is an example of an exterior routing protocol
(EGP) which is used to exchange routing information between
autonomous systems on the Internet.

Distance Vector Protocols
• As the name implies, distance vector routing protocols use
distance to determine the best path to a remote network.
• The distance is usually the number of hops (routers) to the
destination network.
• Distance vector protocols send complete routing table to each
neighbor (a neighbor is directly connected router that runs
the same routing protocol).
• They usually use some version of Bellman-Ford algorithm to
calculate the best routes.
• Compared with link state routing protocols, distance vector
protocols are simpler to configure and require little
management, but are susceptible to routing loops and
converge slower than link state routing protocols.
• Distance vector protocols also use more bandwidth because
they send complete routing table, while link state protocols
sends specific updates only when topology changes occur.
• RIP and EIGRP are examples of distance vector routing
protocols.

Link State Protocols
• Link state routing protocols are the second type of routing
protocols.
• They have the same basic purpose as distance vector protocols, to
find a best path to a destination, but use different methods to do
so.
• Unlike distance vector protocols, link state protocols don't advertise
the entire routing table.
• Instead, they advertise information about a network topology
(directly connected links, neighboring routers...), so that in the end
all routers running a link state protocol have the same topology
database.
• Link state routing protocols converge much faster than distance
vector routing protocols, support classless routing, send updates
using multicast addresses and use triggered routing updates.
• They also require more router CPU and memory usage than
distance-vector routing protocols and can be harder to configure.

Each router running a link state routing
protocol creates three different tables:
1. neighbor table - the table of neighboring routers running
the same link state routing protocol
2. topology table - the table that stores the topology of the
entire network
3. routing table - the table that stores the best routes
• Shortest Path First algorithm is used to calculate the best
route. OSPF and IS-IS are examples of link state routing
protocols.

Difference between distance vector and link
state routing protocols
• The following table summarizes the difference:
Distance Vector Link State
Sends the entire routing table Sends only link state information
Slow convergence Fast convergence
Susceptible to routing loops Less susceptible to routing loops
Updates are sometimes sent using
broadcast
Always uses multicast for the routing
updates
Doesn’t know the network topology Know the entire network topology
Simple to configure Can be harder to configure
Examples: RIP, IGRP Examples: OSPF, IS-IS

Routing Protocols

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Routing Protocols