BGP Neighbors: A Quick Guide

If you're responsible for networking infrastructure, you've probably had to deal with BGP (Border Gateway Protocol).

It's one of the core protocols used on the internet and can be quite complex.

This post will provide a quick guide to understanding BGP neighbors. We'll cover what they are, how they are formed, and why they are important. Finally, we'll provide some tips on troubleshooting BGP neighbors. Stay tuned!

What Does "BGP Neighbor" Mean?

In the realm of computer networking, a BGP neighbor (often abbreviated as "NBR") refers to a router that shares a common BGP session with another router. In order for two routers to establish this connection, there must be an interface configured for BGP on both sides, and they must have matching BGP settings, such as an Autonomous System number. Once the two routers have established a BGP neighbor relationship, they exchange routing information and can communicate with each other as needed.

It's important to note that just because two routers can establish a BGP neighbor relationship, it does not necessarily mean that they will exchange routing information; this depends on whether or not their respective networks have been configured to do so. In summary, a BGP neighbor is simply another router with which one has established a successful BGP session.

Why Are BGP Neighbors Important?

In a computer network, BGP (Border Gateway Protocol) neighbors are crucial for routing data between different autonomous systems. Network traffic can become inconsistent or even fail altogether without properly functioning BGP neighbors. As networks grow larger and more complex, it becomes increasingly important to have reliable BGP neighbors to ensure smooth communication and data transfer.

In addition, establishing a strong BGP neighbor relationship can help prevent security breaches and enhance network resilience in the event of a failure or attack. While managing BGP neighbors may seem like a tedious task, the benefits they provide make the effort well worth it in the long run.

BGP Neighbor States:

In order for routers to exchange information and make routing decisions, they must establish a BGP session with their neighboring routers. This connection progresses through several "states," including Idle, Connect, Active, OpenSent, OpenConfirm, and Established.

Idle:

Idle is a BGP neighbor state, meaning the peering session is up and operational, but no routes are exchanged. It's essentially a "ready to go" state and usually happens when both routers have been powered on and initialized, but no routes have yet been exchanged. It is the first stage of the BGP finite state machine.

Connect:

When BGP is in this state, it initiates a TCP connection. If the 3-way TCP handshake completes successfully, the established BGP Session process will reset the ConnectRetryTimer and send an Open message to the neighbor. After that, it changes to the OpenSent State.

Active:

If the ConnectRetry timer expires before the connect stage is complete, a new TCP connection is attempted, and the state changes to Active after resetting the ConnectRetry timer. However, If any other input is received during this process, the state immediately becomes Idle.

OpenSent:

OpenSent is a BGP neighbor state in which the router has sent an OPEN message to the neighbor but has not yet received a KEEPALIVE message in response. The router will remain in this state until it either receives a KEEPALIVE message from the neighbor or times out waiting for one.

OpenConfirm:

OpenConfirm describes a state when BGP neighbors have exchanged messages, established that they can communicate with one another, and have agreed to exchange routes. The state is moved to the Established state.

Established:

Established is the state when a TCP session has been successfully established, and both endpoints can exchange messages. Once in this state, the routers will continue to exchange messages until one side decides to tear down the session.

Final Words

That’s a quick overview of some of the most important terms related to BGP neighbors.

We hope this was helpful! If you want to learn more or need help configuring your routers for optimal BGP performance, click here and watch CCIE Enterprise Infrastructure course that covers BGP topics.

Created by
Stanley Avery

I am a certified network engineer with over 10 years of experience in the field. I have a deep understanding of networking and IT security, and I am always looking for new challenges.

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