Border Gateway Protocol (BGP) (original) (raw)

Last Updated : 10 Oct, 2025

Border Gateway Protocol (BGP) is an Exterior Gateway Protocol (EGP) used to connect autonomous systems (AS) in any topology. Each AS must have at least one BGP-enabled router connected to another AS’s BGP router. Its primary function is to exchange network reachability information between BGP systems. It is a path-vector routing protocol (unlike distance-vector or link-state protocols).

Note: Border Gateway Protocol constructs an autonomous systems graph based on the information exchanged between BGP routers.

Characteristics of Border Gateway Protocol (BGP)

• **Inter-Autonomous System Configuration: The main role of BGP is to provide communication between two autonomous systems.
• BGP supports the Next-Hop Paradigm.
• Coordination among multiple BGP speakers within the AS (Autonomous System).
• **Path Information: BGP advertisements also include path information, along with the reachable destination and next destination pair.
• **Policy Support: BGP can implement policies that can be configured by the administrator.
• Runs Over TCP.
• BGP conserves network Bandwidth.
• BGP supports CIDR.
• BGP also supports Security.

**Example: A router running BGP can be configured to distinguish between the routes that are known within the AS and that which are known from outside the AS.

Functionality of Border Gateway Protocol (BGP)

BGP peers perform 3 functions, which are given below:

• The first function consists of initial peer acquisition and authentication. Both the peers established a TCP connection and perform message exchange that guarantees both sides have agreed to communicate.
• The second function mainly focuses on sending negative or positive reach-ability information.
• The third function verifies that the peers and the network connection between them are functioning correctly.

Importance of Border Gateway Protocol(BGP)

• **Security: BGP is highly secure because it authenticates messages between routers using preconfigured passwords through which unauthorized traffic is filtered out.
• **Scalability: BGP is more scalable because it manages a vast number of routes and networks present on the internet.
• **Supports Multihoming: BGP allows multihoming means an organization can connect to multiple networks simultaneously.
• **Calculate the Best Path: As we know data packets is traveled across the internet from source to destination every system in between the source and destination has to decide where the data packet should go next
• **TCP/IP Model: BGP is based on the TCP/IP model and it is used to control the network layer by using transport layer protocol.

Types of Border Gateway Protocol

1. External BGP

It is used to interchange routing information between the routers in different autonomous systems, it is also known as eBGP(External Border Gateway Protocol). The below image shows how eBGP interchange routing information.

External BGP

2. Internal BGP

It is used to interchange routing information between the routers in the same autonomous system, it is also known as iBGP(Internal Border Gateway Protocol). Internal routers also ensure consistency among routers for sharing routing information. The below image shows how iBGP interchange routing information.

Internal BGP

Elements of BGP

Some elements of BGP are assigned to each path and these elements help routers to select a path from multiple paths. Here below are some elements of BGP:

• **Weight: Weight is defined as a Cisco-specific attribute that tells a router which path is preferred. The weight having a higher value is preferred.
• **Originate: This tells how a router choose routes and adds to BGP itself.
• **Local Preference: Local Preference is an element used to select the outbound routing path. Greater local preference is preferred.
• **Autonomous System Path: This element tells the router to select a path having a shorter length.
• **Next Hop: To reach the destination the next hop elements specify the IP address that should be used as the next hop.

BGP Route Information Management Functions

• **Route Storage: Each BGP stores information about how to reach other networks.
• **Route Update: In this task, Special techniques are used to determine when and how to use the information received from peers to properly update the routes.
• **Route Selection: Each BGP uses the information in its route databases to select good routes to each network on the internet network.
• **Route advertisement: Each BGP speaker regularly tells their peer what is known about various networks and methods to reach them.

Difference Between BGP and OSPF