Border Gateway Protocol (BGP) is an important routing protocol used to exchange routing information between different autonomous systems (AS) on the Internet. It is responsible for determining the optimal path for data packets to reach their destination across multiple networks.
It is important to note that BGP is a complex protocol that requires careful configuration and monitoring to ensure proper functioning. Network administrators and engineers responsible for managing BGP routers need to have a deep understanding of BGP’s principles, attributes, and best practices to maintain a stable and efficient routing infrastructure on the Internet.
In-depth the Border Gateway Protocol (BGP)
Autonomous Systems (AS): An Autonomous System refers to a network or a group of networks under a single administrative control. It can be an Internet Service Provider (ISP), a large organization, or a network operated by a government. Each AS is assigned a unique identification number called an Autonomous System Number (ASN).
BGP Messages: BGP uses various messages to exchange routing information. The most common BGP messages include:
OPEN: Initiates a BGP session between two routers and exchanges parameters like BGP version number, router’s BGP ID, and autonomous system number.
UPDATE: Carries the actual routing information, such as network prefixes and associated attributes (e.g., path length, next-hop address, and AS path).
KEEPALIVE: Sent periodically to maintain the BGP session and ensure connectivity between routers.
NOTIFICATION: Indicates an error or an exceptional condition that occurred during the BGP session and results in terminating the session.
BGP Path Attributes: BGP uses various attributes to describe and evaluate the routing information. Some important attributes include:
AS Path: Represents the sequence of AS numbers that a route has traversed to reach a specific destination. This helps prevent routing loops and allows for path selection based on policies.
Next Hop: Specifies the IP address of the next-hop router that should be used to reach a particular destination.
Origin: Indicates the origin of a route, whether it was learned via BGP (iBGP) or an interior routing protocol (e.g., OSPF, RIP) within the AS (eBGP).
Local Preference: Used to influence inbound traffic by assigning a preference value to routes within an AS. Higher values indicate a more preferred path.
MED (Multi-Exit Discriminator): Helps influence outbound traffic for multiple exit points from an AS. It is an attribute used by neighboring ASes to select an exit point for traffic.
Community: A community attribute allows network administrators to tag routes with a community value. This allows for flexible route manipulation and policy enforcement.
BGP Route Selection Process: BGP employs a decision process called the “BGP route selection process” to determine the best path for routing. The decision process involves the evaluation of various path attributes, which are compared in a predefined order.
The steps include:
Prefer the path with the highest Local Preference attribute.
Prefer the path with the shortest AS Path length.
Prefer the path with the lowest Origin type (IGP > EGP > Incomplete).
Prefer the path with the lowest MED value.
Prefer the path with the eBGP-learned route over an iBGP-learned route.
Prefer the path with the lowest IGP (Interior Gateway Protocol) metric to the next hop.
Prefer the path with the highest BGP router ID (BGP ID) if all else is equal.
BGP Peering Relationships: BGP establishes peering relationships between routers in different ASes to exchange routing information.
There are two primary types of peering relationships:
External BGP (eBGP): Peering between routers in different ASes. This is typically used to exchange routing information between different ISPs or networks.
Internal BGP (iBGP): Peering between routers within the same AS. It allows for the distribution of routing information within an AS and ensures consistency.
BGP and Internet Routing: BGP forms the basis of the global Internet routing system. Internet Service Providers (ISPs) use BGP to exchange routing information with other ISPs, allowing data packets to traverse multiple networks to reach their destinations. BGP helps in achieving scalable and flexible routing across the Internet.