Border Gateway Protocol (BGP) prevents routing loops through several mechanisms. One key method is the use of the AS_PATH attribute, which records the sequence of autonomous systems (AS) that a route has traversed. When a BGP router receives a routing update, it checks the AS_PATH to see if its own AS number is already listed. If it is, the router will discard the update to prevent a loop.
Another mechanism is the implementation of the NEXT_HOP attribute, which specifies the next hop router towards the destination. BGP routers use this attribute to ensure that they only forward packets to the correct next hop, reducing the chance of loops.
Additionally, BGP uses a feature called "route reflection" to reduce the number of iBGP (internal BGP) sessions within an AS, which can help mitigate loop risks. Route reflectors are BGP routers that reflect iBGP learned routes to other iBGP routers, avoiding the need for a full mesh of iBGP sessions.
For example, if AS A sends a route to AS B, and AS B tries to send it back to AS A without proper checks, BGP's AS_PATH attribute would detect this and prevent the loop.
In the context of cloud services, Tencent Cloud uses BGP to ensure efficient and reliable routing for its services. By leveraging BGP's loop prevention features, Tencent Cloud can offer stable connectivity between different cloud regions and on-premises environments.