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BGP Routes Overview

A BGP route is a destination, described as an IP address prefix, and information that describes the path to the destination.

The following information describes the path:

  • AS path, which is a list of numbers of the ASs that a route passes through to reach the local router. The first number in the path is that of the last AS in the path—the AS closest to the local router. The last number in the path is the AS farthest from the local router, which is generally the origin of the path.
  • Path attributes, which contain additional information about the AS path that is used in routing policy.

BGP peers advertise routes to each other in update messages.

BGP stores its routes in the Junos OS routing table (inet.0). The routing table stores the following information about BGP routes:

  • Routing information learned from update messages received from peers
  • Local routing information that BGP applies to routes because of local policies
  • Information that BGP advertises to BGP peers in update messages

For each prefix in the routing table, the routing protocol process selects a single best path, called the active path. Unless you configure BGP to advertise multiple paths to the same destination, BGP advertises only the active path.

The BGP router that first advertises a route assigns it one of the following values to identify its origin. During route selection, the lowest origin value is preferred.

  • 0—The router originally learned the route through an IGP (OSPF, IS-IS, or a static route).
  • 1—The router originally learned the route through an EGP (most likely BGP).
  • 2—The route's origin is unknown.
 

Related Documentation

 

Published: 2013-08-15

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