What is BGP (Border Gateway Protocol) ?

What is BGP (Border Gateway Protocol) ?

1. What is BGP ? 

BGP is used to exchange routing information between multiple autonomous systems (AS) on the Internet. An autonomous system is a group of networks that share a common administrative area. BGP is used to route traffic across multiple independent systems and is the protocol used by Internet Service Providers (ISPs) to communicate routing information with one another.

2. Why we need BGP ? 

• Scalability: BGP can handle the massive number of routes present on the Internet efficiently. It supports thousands of prefixes, making it suitable for global routing, which smaller protocols cannot manage reliably.
• Policy-Based Routing: BGP allows network administrators to define routing policies based on business agreements or preferences, such as selecting preferred paths for cost, security, or performance reasons. This control is essential for ISPs and large organizations managing multiple Internet connections.
• Loop Prevention: Since BGP operates across different autonomous systems, it is equipped with mechanisms to prevent routing loops, using attributes such as AS_PATH. This ensures data does not circulate endlessly between networks.
• Redundancy and Resilience: BGP enables multiple paths between networks. In case one link or path fails, BGP can reroute traffic through alternative paths, enhancing network reliability and fault tolerance.
• Interoperability Between Networks: Different networks might have different internal routing protocols. BGP provides a standardized method to exchange routing information between these heterogeneous networks, ensuring connectivity across the Internet.

3. When to use BGP ?

Interconnecting with Multiple ISPs (Multihoming)

Scenario: Your network connects to more than one Internet Service Provider (ISP) for redundancy or load balancing.

• Static routes or internal gateway protocols (IGPs) like OSPF/EIGRP do not scale or dynamically adjust to upstream failures.
• BGP allows advertisement of your IP prefixes to multiple providers while controlling the incoming and outgoing traffic paths.
• Supports traffic policies, failover, and granular path selection using attributes like Local Preference, Weight, MED.

Managing Public IP Space

Scenario: Your organization owns portable public IP addresses (not assigned by a specific ISP) and wants the addresses routed properly on the Internet.

• Advertise your network prefixes to one or more ISPs.
• Control which provider announces your prefixes externally, ensuring correct routing and resilience.

Large Enterprise or Global Networks

Scenario: Organizations with multiple geographically dispersed offices interconnected over WAN or MPLS.

• Distribution of network routes consistently across all locations.
• Traffic engineering policies tailored to business or operational requirements.
• Interoperability with provider MPLS networks using route redistribution between IGPs and BGP.

Internet Exchange Points and Peering

Scenario: Companies or ISPs participating in peering arrangements at Internet Exchange Points (IXPs).

• Facilitates route advertisement and policy enforcement.
• Ensures incoming traffic flows according to commercial agreements or performance needs.

Policy-Based Routing Control

Unlike IGPs, which purely optimize for shortest path / lowest cost, BGP supports policy-based routing, allowing operators to:

• Prefer one neighbor over another based on cost, business agreements, or path stability.
• Avoid certain paths or enforce multi-exit routing decisions.

4. When not to use BGP ?

• Small networks with a single ISP connection and where only default routes required.
• No public IP space or multihoming needs.
• In such cases, static routing or internal IGPs (like OSPF or EIGRP) are sufficient.

5. Types Of BGP ?

External BGP (eBGP):

• Runs between different autonomous systems.
• Used to connect to ISPs, between carriers, or across public networks.

Internal BGP (iBGP):

• Runs within a single AS.
• Typically used in large networks for internal route distribution.
• Ensures consistent reachability across internal routers and supports advanced traffic engineering.