Q. Explain any one routing protocol used in internet? (8 Marks)

Distance vector and link state routing are both intra-domain routing protocols. They can

be used inside an Autonomous System (AS), but not between AS. These two protocols are not

suitable for inter-domain routing mostly because of scalability. Link state routing need a huge

amount of resources to calculate routing tables. It also creates heavy traffic because of flooding.

There is a need for a third routing protocol called path vector routing (PVR).

PVR proved to be useful for inter-domain routing. The principle of PVR is similar to that

of distance vector routing. In PVR, we assume that there is a node in each AS that acts on behalf

of the entire AS. It is called speaker node. The speaker node in a AS, creates a routing table and

advertise it to speaker node in the neighboring AS. The idea is the same as for distance vector

routing except that only speaker nodes in each as, can communicate with each other. A speaker

node, advertises the path, not the metric of the nodes

BGP is an inter domain routing protocol using PVR. It first appeared in 1989 and has gone

through four versions.

Types of AS : Internet is divided in to hierarchical domains called AS. For example, a large

corporation that manages its own network and has full control over it is an AS. We can divide as

in to three categories.

Stub AS

Multi homed AS

Transit AS

Stub AS : A stub AS has only one connection to another AS, the inter domain data traffic in a

stub AS can be either created or terminated in the AS. The hosts in the AS can send data traffic

to other AS’s. The hosts in the AS can receive data coming from host in other AS. Data traffic

can not pass through a stub AS. A stub AS is either a source or sink

Multi homed AS : A multi homed AS as more than one connection to other AS’s. But it is only

a source or sinks for data transfer. It can receive data traffic from more than one AS. It does not

allow data coming from one AS and going to another AS to pass through. Example of multi

homed AS is a large corporation that is connected to more than one regional or national AS.

Transit AS : it is a multi homed AS that allows transient traffic. Example is national and

international ISP’s

BGP sessions

The exchange of routing information between two routers using BGP takes places in a

session. A session is a connection that is established between two BGP routers only for the sake

of exchanging routing information to create a reliable environment BGP uses the services of

TCP. A session of the BGP level. There is a subtle difference between a connection in TCP made

for BGP and other application program. When a TCP connection is created for BGP, it can last

for a long time until something unused happens. For this reason, BGP sessions are sometimes

referred to as semi permanent connections.

External and internal BGP

BGP have two types of sessions

External BGP (E-BGP)

Internal BGP (I-BGP)

The E-BGP session is used to exchange in to between two speaker nodes belonging to two

different AS’s. The I-BGP session is used to exchange routing information between two routers

inside a AS.

The session established between AS1 and AS2 is an E-BGP session. The two speaker routers

exchanges information. They know about network in the internet. These two routers need to

collect information from other routers in the AS. This is dine using I-BGP sessions.

Abbreviations Used

AS - Autonomous System

PVR - path vector routing

E-BGP - External BGP

I-BGP - Internal BGP

Reference

1 Forouzan A Behrouz; Data communications and Networking; The McGraw Hill Publications;

2006