Border Gateway Protocol (BGP) is the standardized routing protocol that makes the Internet work. It enables data routing between different networks, known as autonomous systems (AS), and is essential for determining how packets travel across the web. By exchanging routing information with neighboring systems, BGP ensures that data takes the most efficient and reliable path to its destination.
Overview
BGP is classified as a path vector protocol and is the primary protocol used to route data across the Internet. Each autonomous system has a unique identifying number called an Autonomous System Number (ASN). Through BGP peering relationships, routers in different ASes share information about which IP address ranges they can reach. BGP then selects the best path based on multiple attributes, including path length, policy rules, and routing metrics.
When routers come online, they establish connections with peers and exchange route information. This data is stored in a Routing Information Base (RIB), which helps routers make informed forwarding decisions.
Internal vs. external BGP
There are two types of BGP sessions: external BGP (eBGP) and internal BGP (iBGP):
- eBGP is used between routers in different autonomous systems. It helps networks communicate over the Internet.
- iBGP is used within a single autonomous system. It distributes routing information learned from eBGP peers to other routers in the same AS.
While eBGP peers typically require direct connections, iBGP peers use logical connections and often rely on loopback interfaces for fault tolerance. A fully meshed topology is typically required for iBGP to prevent routing loops.
Common use cases
BGP is widely used wherever networks need to exchange routing information. Common scenarios include:
- Internet exchange points (IXPs)
- Multi-homed networks connected to multiple ISPs
- Global content delivery and cloud connectivity
- Large-scale enterprise or service provider networks
Even within a single organization, BGP is used to manage complex routing across distributed environments. Organizations can take advantage of Zenlayer’s full BGP support to build reliable, policy-driven interconnections at the edge and across cloud regions.
Routing decisions and convergence
BGP is policy-based and allows administrators to define routing policies based on attributes like AS path, origin type, and multi-exit discriminator (MED). It selects the best route not just based on shortest path but also on business or performance rules set by the network operator.
Unlike some dynamic routing protocols, BGP is not fast to converge. When routes change, BGP takes longer to update its routing tables. This behavior can help maintain stability but may introduce delays in reacting to changes in the network.
Security concerns and hijacking
Despite its critical role, BGP has known vulnerabilities. The protocol relies on trusting route announcements from peers, which opens the door to BGP hijacking. In these attacks, incorrect routing information is advertised—either by mistake or maliciously—redirecting traffic through unintended paths.
Although secure BGP variants have been proposed, widespread adoption has proven difficult. Many solutions are incompatible with existing BGP infrastructure, requiring global coordination to implement.
Key takeaways
BGP is the backbone of global Internet routing. It allows networks to share routing information and select efficient paths for data transmission. Despite some security limitations, it remains the most widely used exterior gateway protocol.
- Fundamental protocol: BGP enables routing between independent networks across the Internet.
- Flexible routing: It supports both internal (iBGP) and external (eBGP) peering relationships.
- Policy-driven control: Network administrators can influence routing decisions using custom rules.
- Security gaps: BGP's trust-based design makes it vulnerable to misconfigurations and hijacking.
- Slow convergence: Stability is prioritized, but responsiveness to network changes can lag.
While BGP has been instrumental in shaping the global Internet, its limitations, particularly around convergence and security, highlight the importance of ongoing innovation in routing technologies. As the digital landscape becomes more dynamic and distributed, understanding how BGP works, where it excels, and where it falls short is essential for anyone involved in managing or securing modern networks.