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    Example: Advertising Multiple BGP Paths to a Destination

    Understanding the Advertisement of Multiple Paths to a Single Destination in BGP

    BGP peers advertise routes to each other in update messages. BGP stores its routes in the Junos OS routing table (inet.0). 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.

    Instead of advertising only the active path to a destination, you can configure BGP to advertise multiple paths to the destination. Within an autonomous system (AS), the availability of multiple exit points to reach a destination provides the following benefits:

    • Fault tolerance—Path diversity leads to reduction in restoration time after failure. For instance, a border after receiving multiple paths to the same destination can precompute a backup path and have it ready so that when the primary path becomes invalid, the border routing device can use the backup to quickly restore connectivity. Without a backup path, the restoration time depends on BGP reconvergence, which includes withdraw and advertisement messages in the network before a new best path can be learned.
    • Load balancing—The availability of multiple paths to reach the same destination enables load balancing of traffic, if the routing within the AS meets certain constraints.
    • Maintenance—The availability of alternate exit points allows for graceful maintenance operation of routers.

    The following limitations apply to advertising multiple routes in BGP:

    • Address families supported:
      • IPv4 unicast (family inet unicast)
      • IPv6 unicast (family inet6 unicast)
      • IPv4 labeled unicast (family inet labeled-unicast)
      • IPv6 labeled unicast (family inet6 labeled-unicast)
    • Internal BGP (IBGP) peers only. No support on external BGP (EBGP) peers.
    • Master instance only. No support for routing instances.
    • Graceful restart and nonstop active routing (NSR) are supported.
    • No BGP Monitoring Protocol (BMP) support.
    • No support for EBGP sessions between confederations.
    • Prefix policies enable you to filter routes on a router that is configured to advertise multiple paths to a destination. Prefix policies can only match prefixes. They cannot match route attributes, and they cannot change the attributes of routes.

    Modified: 2016-06-16