BGPpäätösviestissä

BGPpäätösviestissä, often shortened to BGP decision process, refers to the sequence of steps a Border Gateway Protocol (BGP) router takes to select the best path for routing traffic when multiple paths to a destination exist. This process is fundamental to BGP's operation as an inter-domain routing protocol.

When a BGP router receives multiple Network Layer Reachability Information (NLRI) updates for the same destination prefix, it must choose the most preferred path to install in its routing table. The BGP decision process involves a series of attribute comparisons. It starts by checking if the next-hop is reachable. If it is, the router then evaluates various BGP path attributes, such as Weight, Local Preference, AS_PATH, Origin, MED (Multi-Exit Discriminator), and BGP Router ID, in a predefined order.

The highest value for attributes like Weight and Local Preference is preferred. Shorter AS_PATH lengths are favored, and the Origin attribute (IGP, EGP, or Incomplete) is also considered. The MED is used to influence inbound traffic from external BGP peers, with lower values generally preferred. Finally, if all other attributes are equal, the BGP Router ID is used as a tie-breaker. The path that survives this rigorous evaluation is declared the best path and is installed in the BGP routing table. This decision process ensures loop-free routing and allows network administrators to influence traffic flow through policy implementation.