Border Gateway Protocol (BGP) is the routing protocol that effectively runs the internet. It’s an “exterior gateway protocol” designed to exchange routing and reachability information between different Autonomous Systems (AS). An Autonomous System is essentially a large, independent network or group of networks under a single administrative control (like an Internet Service Provider, a large corporation, or a university).

BGP, being a path-vector protocol, makes routing decisions based on paths, network policies, and rule-sets configured by network administrators, rather than simply metrics like hop count.

Within BGP, there are two main types of peering relationships: iBGP (internal BGP) and eBGP (external BGP). These two types serve distinct purposes in how routing information is exchanged.

iBGP (Internal Border Gateway Protocol)

• Purpose: iBGP is used to exchange routing information within the same Autonomous System (AS). Its primary goal is to ensure that all BGP-speaking routers within an AS have a consistent view of external routes (routes learned from other ASes). This internal consistency is crucial for efficient and loop-free routing within the AS.

• Peering: iBGP peers are routers that belong to the same Autonomous System. They don’t necessarily have to be directly connected; as long as there’s IP connectivity between them (often provided by an Interior Gateway Protocol like OSPF or EIGRP), an iBGP session can be established.

• Loop Prevention: To prevent routing loops within the AS, iBGP has a crucial rule: routes learned from one iBGP peer are not advertised to another iBGP peer. This is known as the iBGP split-horizon rule. To overcome this limitation and ensure all iBGP routers receive all external routes, a “full mesh” topology is traditionally required, where every iBGP router establishes a peer relationship with every other iBGP router. For larger ASes, this can become unmanageable, so alternative solutions like Route Reflectors or BGP Confederations are used to reduce the number of iBGP peerings.

• Next Hop: When an iBGP router advertises a route, it typically preserves the original next-hop address received from the eBGP peer. This means the internal routers need to be able to reach that next-hop via an IGP.

• Administrative Distance (AD): iBGP routes have a default administrative distance of 200. This makes them less preferred than eBGP routes (AD 20) if the same route is learned from both.

eBGP (External Border Gateway Protocol)

• Purpose: eBGP is used to exchange routing information between different Autonomous Systems. It’s the protocol that connects networks across the internet, allowing them to exchange information about reachable destinations and decide on how to forward traffic to external networks.

• Peering: eBGP peers are routers that belong to different Autonomous Systems. They are typically directly connected, although multi-hop eBGP can be configured.

• Loop Prevention: eBGP prevents loops using the AS_PATH attribute. When an eBGP router advertises a route to an external peer, it adds its own AS number to the AS_PATH list. If a router receives a route where its own AS number is already present in the AS_PATH, it discards the route, preventing a loop.

• Next Hop: When an eBGP router advertises a route to an eBGP peer, it typically changes the next-hop address to its own IP address.

• Administrative Distance (AD): eBGP routes have a default administrative distance of 20. This is a very low value, making eBGP routes highly preferred over most other routing protocols, which is essential for external routing.