BGP Additional Path

Introduction

Additional Path is very similar in functionality of a Route Distinguisher in an MPLS network, where in certain topologies multiple prefixes are advertised to VPNs by Route Reflectors instead of one best path. Additional Path does the same thing but for all address-families not only VPNv4.

Add Path

  • Add-path is a negotiated BGP capability for each address family. It can be set for send or receive or both.
  • Each NLRI gets a Path ID which distinguishes each prefixes.

-—unorganized notes-——

  • Main function is to provide a backup route. This is very useful if using regular RR without MPLS.
  • Add-path feature will send best-path, n-best-path, n-1-best-path…etc.
  • Add-path is a BGP capability for a session.
  • Good for host potato routing, no need for full mesh, but could use RR.
  • IETF has 5 flavors of add-path:
  • Configured on IOS with "additional-paths send" under each address-family.
  • Configured on XR with "additional-paths receive" under each address-family.

  • BGP without the capability of advertising multiple paths, prevent:
    • Multipath load balancing working behind a router reflector. Router Reflector will only pick the best path even if it is using iBGP multipathing. RR could be the best path based on their cost to the next hop instead of the cost of route-reflector-clients, which is an issue for hot-potato routing.
    • Prevents fast data plane convergence. During convergence, the data plane will lose packets.

Options

  • Add All Paths - add paths are shared and advertised.
  • Add N Paths - send N best paths. Implementation could be send best, 2nd best, …nth best. The nice part about this feature is you can deterministically know how many additional path will be advertised for memory consumption of routers.
  • Add Group Best Paths -

Additional Resources

Add-Path Overivew - Nanog 48
Fast Connectivity Restoration Using BGP Add-path - draft-pmohapat-idr-fast-conn-restore-03
Analysis of paths selection modes for Add-Paths - draft-vvds-add-paths-analysis-00
BGP Add-Paths The Scaling/Performance Tradeoffs - Universit́e catholique de Louvain, Belgium
To Add-Path or not to Add-path - Nanog 48

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