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- J Series
- Understanding BGP Path Selection
- Example: Advertising Multiple Paths in BGP
- M Series
- Understanding BGP Path Selection
- Example: Advertising Multiple Paths in BGP
- MX Series
- Understanding BGP Path Selection
- Example: Advertising Multiple Paths in BGP
- PTX Series
- Understanding BGP Path Selection
- Example: Advertising Multiple Paths in BGP
- QFX Series
- Understanding BGP Path Selection
- SRX Series
- Understanding BGP Path Selection
- Example: Advertising Multiple Paths in BGP
- T Series
- Understanding BGP Path Selection
- Example: Advertising Multiple Paths in BGP
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 router 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 router 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:
- IPv4 unicast (family inet unicast) routes only.
- Internal BGP (IBGP) peers only. No support on external BGP (EBGP) peers.
- Master instance only. No support for routing instances.
- No support for nonstop active routing (NSR).
- 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. However, prefix policies can only match routes. Prefix policies cannot change the attributes of routes.
Related Documentation
- J Series
- Understanding BGP Path Selection
- Example: Advertising Multiple Paths in BGP
- M Series
- Understanding BGP Path Selection
- Example: Advertising Multiple Paths in BGP
- MX Series
- Understanding BGP Path Selection
- Example: Advertising Multiple Paths in BGP
- PTX Series
- Understanding BGP Path Selection
- Example: Advertising Multiple Paths in BGP
- QFX Series
- Understanding BGP Path Selection
- SRX Series
- Understanding BGP Path Selection
- Example: Advertising Multiple Paths in BGP
- T Series
- Understanding BGP Path Selection
- Example: Advertising Multiple Paths in BGP
