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What is the routing selection process of Border Gateway Protocol?

Created on 2025-04-01 18:10:57
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The routing selection process of Border Gateway Protocol (BGP) involves several steps to determine the best path for data to travel across the internet. BGP is a path-vector routing protocol that maintains a table of IP networks or 'prefixes' which designate network reachability among autonomous systems (AS).

Here’s a simplified explanation of the routing selection process:

  1. Neighbor Discovery: BGP routers first establish connections with their neighbors, which are other BGP routers in the same or different autonomous systems.

  2. Route Exchange: Once connected, routers exchange information about the networks they can reach. This information includes the network prefix and the path to reach that network, known as the AS path.

  3. Path Selection: When a router receives multiple routes to the same destination, it uses a set of attributes to determine the best path. The primary attributes considered are:

    • Local Preference: A value set by the network administrator to prefer certain routes over others.
    • AS Path Length: Shorter paths are generally preferred to avoid unnecessary hops.
    • Origin: Routes learned from an Interior Gateway Protocol (IGP) like OSPF or EIGRP are preferred over those learned from another BGP.
    • MED (Multi-Exit Discriminator): This attribute is used to influence the routing decision when there are multiple entry points into an AS.

  4. Best Path Selection: Based on these attributes, the router selects the best path and adds it to its routing table.

  5. Route Propagation: The router then propagates this best path to its BGP neighbors.

Example: Suppose a company has two internet service providers (ISPs). Router A in the company connects to ISP X and ISP Y. BGP is configured on Router A. When Router A learns about a network from both ISPs, it compares the paths based on the attributes mentioned above. If ISP X offers a shorter AS path and higher local preference, Router A selects the route through ISP X for that network.

In the context of cloud services, Tencent Cloud offers BGP Anycast services, which leverage BGP's capabilities to provide lower latency and higher availability for applications deployed across multiple regions. This ensures that traffic is routed to the nearest or best-performing endpoint automatically.