BGP

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Overview

Border Gateway Protocol, or BGP, dynamically controls routing traffic between networks, or Autonomous Systems. Routers setup BGP peering sessions with each other and share information about what networks connect to them directly, as well as what routes they learn from other peers. This means that if two BGP peers lose their session, the network knows this and can send traffic on an alternate path to its destination. Much of the internet uses BGP to ensure traffic flows quickly and efficiently to its destination rather than relying on human intervention to manage routes.

Terminology

There are a number of terms unusual to those who do not work with dynamic routing and BGP. The intent of these terms is not to confuse anyone, but rather to more specifically define the topic of discussion.
  • Autonomous System (AS) - A network or group of networks under the control of a single entity which is physically seen as contiguous. So, a bank that had T1 links from it's main office to all branches would be one AS, as all networks are directly connected. A bank which had all branches connect to the internet would have one AS for each branch.
  • Autonomous System Number (ASN)- A unique Identification number assigned by ARIN. The numbers 64512 through 65534 are reserved for private use. Systems connecting to the internet should have an ASN between 1 and 64511.
  • multihomed - Connecting your network, or AS, to multiple other networks. When the other networks are ISPs, then BGP can provide some measure of redundancy in the event that any one link fails.
  • path padding - Name given to the technique of adding your ASN to the route multiple times before passing it along. Doing this will cause a peers to avoid using that path to send traffic to your AS.
  • router id - Conventionally, this is simply the IP address of an interface on the BGP peer. It is used to identify peers within an AS.
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