What are the External Routing Protocols Used by Most ISPs?

Internet Service Providers (ISPs) play a crucial role in ensuring efficient and reliable communication over the internet. One of the key tasks for ISPs is to exchange routing information with other networks. While there have been various external routing protocols in the past, such as EGP, modern ISPs primarily rely on a single protocol: BGP (Border Gateway Protocol).

BGP: The Universal Routing Protocol

Long ago, there was EGP (Exterior Gateway Protocol), but it has been superseded by BGP for the better part of two decades. EGP, which was introduced in the mid-1970s, was the first exterior gateway protocol designed for exchanging routing information between different autonomous systems (ASes). However, EGP has been decommissioned and replaced by BGP due to its limited scalability and numerous security vulnerabilities.

In some unusual scenarios, ISPs might resort to using RIP (Routing Information Protocol) or other IGP (Interior Gateway Protocols) for inter-provider routing. For instance, RIP has been used in certain situations, but this is more of an exception rather than the norm. These instances often occur in small private networks or experimental setups.

BGP: The Preferred Choice

Currently, the vast majority of ISPs use BGP for their external routing needs. BGP is a path-vector protocol that exchanges routing information between autonomous systems. It is designed to be highly scalable and flexible, making it suitable for the complex and vast internet infrastructure. ISPs use BGP to exchange routes with their peers and transit providers. This protocol supports rich features such as policy-based routing, path multiplexing, and support for multiple address families (like IPv4 and IPv6) which are critical for modern network operations.

BGP is not only used for inter-ISP routing but is also employed within ISP backbones (sometimes referred to as Provider Edge routers, or PE) to exchange routes with other networks. However, within the core network of an ISP, other IGPs like OSPF, EIGRP, or IS-IS are typically used. These IGPs are designed to efficiently exchange routing information within a single AS and are optimized for high-speed, low-latency communication within the ISP's own infrastructure.

H3: Why BGP is the Go-To Protocol

Scalability and Flexibility: BGP is designed to handle the massive scale of the internet. It supports a wide range of routing policies, making it highly flexible and customizable. ISPs can use BGP to implement complex routing strategies, such as prefix-based routing, AS path filtering, and route aggregation.

Security: BGP has several security features, such as BGPsec, to prevent routes from being tampered with or injected maliciously. This is crucial for maintaining the integrity and reliability of the internet.

Reliability: BGP is built to be highly fault-tolerant. It supports mechanisms like BGP route hijacking protection, which can help mitigate security threats. Additionally, BGP uses best-path determination, allowing it to find the optimal path for routing traffic.

H3: How ISPs Use BGP Internally and Externally

While BGP is predominantly used for external routing, many ISPs also use it internally to some extent. ISP backbone routers often run BGP and/or IS-IS (Intermediate System to Intermediate System) to carry routes within the ISP backbone core network. BGP is particularly popular because of its ability to handle complex routing policies and its wide deployment in the internet.

For external routing, BGP is almost exclusively used. ISPs exchange routes with their customers and peers through BGP. This includes direct connections with other ISPs for transit and customer connections for accessing the internet.

H3: Other Internal Routing Protocols Used by ISPs

While BGP is the dominant external routing protocol, ISPs often use other IGPs for internal routing. OSPF (Open Shortest Path First), EIGRP (Enhanced Interior Gateway Routing Protocol), and IS-IS are commonly used within ISP backbones. These IGPs offer efficient routing within a single AS, enabling ISPs to handle large volumes of traffic within their networks.

OSPF: This is a link-state protocol that is widely used for intra-AS routing. It supports multiple areas, making it suitable for large networks. OSPF is known for its scalability and fault tolerance, making it a popular choice for ISPs.

EIGRP: A hybrid distance-vector/link-state protocol, EIGRP is known for its speed, reliability, and convergence. It is often used in large enterprise networks and by some ISPs for their backbone routing.

IS-IS: Another link-state protocol, IS-IS is used in large backbone networks. It supports multiple network layers (IP, IPv6, and CLNP) and is a good choice for ISPs that need to support multiple address families.

H2: Conclusion

The modern internet relies heavily on BGP for external routing, and almost all ISPs use this protocol to exchange routes with each other. However, for internal routing, ISPs may choose from a variety of IGPs, including OSPF, EIGRP, and IS-IS. BGP, with its scalability and security features, remains the preferred choice for the vast majority of ISPs for external routing needs.