The Border Gateway Protocol (BGP) was designed to perform edge routing, transmitting information about the entire network infrastructure between autonomous systems (AS). It is the protocol used between Internet service providers, but it can also be used between a company and an ISP. BGP was built for reliability, scalability, and control.
When we have established communication between neighbors using BGP, information is sent describing which networks are located behind the edge router. This information includes prefixes (IP class assigned to an AS), next-hop (next hop/path to reach the destination prefix), AS-Path (which ASes I must pass through to reach the destination prefix), and routing metrics.
We have two types of BGP connections: EBGP and IGBP. EBGP (External Border Gateway Protocol) is used when we establish communication between two different ASes, and IBGP (Internal Border Gateway Protocol) is used when we establish communication between two identical ASes.
Important Features of BGP
- The administrative distance (default distance when the prefix is installed in the routing table) for EBGP routes (routes learned through different ASes) is 20, and for IBGP routes (routes learned through the same ASes) it is 200.
- The classification for BGP neighbors is traditionally known as peer and must be manually configured by their administrators;
- The BGP protocol uses port 179 for route update exchange and periodic keepalives to assess connection integrity;
- BGP uses several attributes in its routing decision algorithm.
Like other routing protocols, BGP supports best-route manipulation through the addition of metric values. For example, when we receive two identical prefixes from two BGP neighbors, we can manipulate which path we prefer to reach the destination prefix using metrics. There are also metrics to worsen the path to our own prefix.
When configuring a peer with a neighboring Autonomous System, the connection can display 4 types of messages:
Open: Sends the message "open" to your neighbor and includes information such as AS number, router ID, and timeout.
Update: This message is used to transfer routing information between peers. It includes new routes and path attributes (metrics).
Keepalive: neighbors exchange messages every 60 seconds by default to keep the session active.
Notification: This message is sent when a problem occurs that causes a router to terminate the session.
Understanding BGP in a simple way
In general terms, the BGP protocol can be compared to a GPS system. For example, when we are about to leave home for a specific destination, we inform our GPS system where we are and where we want to go. With this information, the GPS system will evaluate the possible routes to that destination and determine which is the best one to display to the user.
BGP does essentially the same thing. When a user at home decides to access any website, they specify a destination to be reached, and their request travels across the internet along a route defined by the protocol until it reaches its destination.
And we're going to continue with this little lesson about BGP. Stay tuned.
“The name of a resource indicates what we seek, an address indicates where it is, and a route tells us how to get there”. – John F. Shoch
"The name of a resource indicates what we are looking for, an address indicates where it is, and a route tells us how to get there." – John F. Shoch
* Lucas Moreira – NIS Specialist
