Exterior Gateway Protocols (EGPs) are routing protocols used to exchange routing information between different autonomous systems (AS). An autonomous system is a collection of IP networks and routers under the control of a single organization that presents a common routing policy to the internet. While there have been a few EGPs developed over time, the most prominent one is the Border Gateway Protocol (BGP).

Here’s a brief overview:

Border Gateway Protocol (BGP):

  1. Functionality: BGP is the de facto standard for routing between ISPs and for connecting ISPs to their larger, enterprise customers. It’s also used in many large-scale internet data centers to manage routing within the data center.
  2. Path Vector Protocol: BGP is often classified as a path vector protocol, where each entry in the routing table contains the path of AS numbers that a route has traversed. This helps prevent routing loops and allows for policy decisions based on the path a route has taken.
  3. Routing Policy: BGP is highly policy-based, allowing administrators to dictate routing decisions based on various criteria like AS path, prefix length, next-hop, and many others.
  4. Scalability: BGP can handle thousands of routes, making it suitable for global internet routing.
  5. Loop Prevention: BGP uses the AS-Path attribute, a list of AS numbers that a route advertisement has traversed, to prevent routing loops.
  6. Prefix Aggregation: BGP can aggregate routes, reducing the size of routing tables.

Other Exterior Gateway Protocols:

While BGP is the primary EGP in use today, it’s worth mentioning the original Exterior Gateway Protocol (EGP), from which the category gets its name:

  1. Exterior Gateway Protocol (EGP): This is an obsolete protocol, defined in RFC 904, and was used to connect hosts on the ARPANET. EGP was replaced by BGP in modern internet routing.

In modern networking, when people refer to EGPs, they are typically referring to BGP, as it’s the predominant protocol in this category. The use of BGP is critical for the decentralized nature of the internet, allowing different networks, each managed by different organizations, to interoperate and route traffic between one another.