Will you please describe about bgp protocol?

The Border Gateway Protocol routing information is usually exchanged between competing business entities -- Internet Service Providers (ISPs) -- in an open, hostile environment (public Internet). BGP is thus very security-focused (for example, all adjacent routers have to be configured manually), and decent BGP implementations provide a rich set of route filters to allow the ISPs to defend their networks and control what they advertise to their competitors.

In BGP terminology, an independent routing domain (which almost always means an ISP) is called an autonomous system.

BGP is always used as the routing protocol of choice between ISPs (external BGP) but also as the core routing protocol within large ISP networks (internal BGP).

All other routing protocols are concerned solely with finding the optimal path toward all known destinations. BGP cannot take this simplistic approach because the peering agreements between ISPs almost always result in complex routing policies. To help network operators implement these policies, BGP carries a large number of attributes with each IP prefix, for example:

AS path -- the complete path documenting which autonomous systems a packet would have to travel through to reach the destination.

Local preference -- the "internal cost" of a destination, used to ensure AS-wide consistency.

Multi-exit discriminator -- this attribute gives adjacent ISPs the ability to prefer one peering point over another.

Communities -- a set of generic tags that can be used to signal various administrative policies between BGP routers.

As the focus of BGP design and implementation was always on security and scalability, it's harder to configure than other routing protocols, more complex (more so when you start configuring various routing policies), and one of the slowest converging routing protocols.

The slow BGP convergence dictates a two-protocol design of an ISP network:

An internal routing protocol (most often, OSPF or IS-IS) is used to achieve fast convergence for internal routes (including IP addresses of BGP routers).

BGP is used to exchange Internet routes.