Routing Algorithm / Border Gateway Protocol(bgp)

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