BGP Prefix Independent Convergence


PIC improves BGP convergence during a failure by maintaining a backup path to a destination in a fast failover.


  • The main concept to BGP PIC is similar to EIGRP's feasible successor route, but converge at a predictable time without the dependence on number of prefixes, hence the name prefix-independent convergence. PIC performs that same with few prefixes to few hundred thousand prefixes.
  • Convergence time does not increase as the number of prefixes in the routing table increase, which means less packet loss during convergence.
  • Reduces packet loss by eliminating the time packets are dropped between the time link/node is detected as down and the time BGP converges onto a new path. With a 20Gbps of traffic and you can reduce the outage from 2 minutes to 100 ms which equates to max of max of 300GB of traffic.
  • Works for only these address families:
    • IPv4
    • IPv6
    • VPNv4
    • VPNv6
  • PIC works at four levels to improve the convergence by installing an alternate path. It effects the following area:
    • BGP - For each prefix a BGP router with PIC will calculate a secondary best path called alternate or backup.
    • RIB - Backup path is installed in the Routing Information Base.
    • FIB - Backup path is installed in the Forwarding Information Base (CEF and dCEF).
    • MPLS -

—unorganized notes——

  • Provides the capability to converge in ms instead of seconds or minutes.
  • This is accomplished by maintaining a backup path in the FIB.
  • PIC is in effect between the state of a BGP router detecting a link going down and the BGP convergence. It switches a MPLS path to the backup path that was already pre-calcualted.
  • BGP convergence could be seconds or minutes that would cause packet loss.
  • on IOS configured with "bgp additional-paths install" under the address family vpnv4.
  • By default RR will on reflect that best path, so have to use different RDs or RTs.

IGP Convergence

  • BGP PIC convergence relies heavily on IGP convergence. Make sure to tune OSPF of ISIS for fastest convergence for IOS. NX and XR already tuned.
  • Use BFD for link failures.
  • Change Carrier-delay to 0.
  • Minimize the IGP routing table size. Convergence in IGP is still linear.
  • Prefix Prioritization a feature that prioritizes which IGP prefixes are processes first in IGP convergence. Find out more Cisco IS-IS and OSPF prefix prioritisation - Beyond CCIE Blog and Implementing OSPF ASR 4.3 - Cisco

Main Concepts of BGP PIC

  • Convergence depends on how quickly next-hop convergence based on:
    • Fast failure detection like BFD.
    • Fast IGP convergence.
  • Tunneling - for certain scenarios on the edge, edge routers might have to tunnel/encapsulate traffic between edges.
  • Alternalte/Backup path. With Router Reflectors, this might require feature to send multiple prefixes to the iBGP neighbors. These include:
    • BGP Best-External.
    • Different RD.
    • BGP AddPath.
    • Shadow RR.

Failure Scenarios

PIC Core

  • Detection is with IGP, where the BGP NEXT-HOP IGP cost changes.
  • Detection of next-hop is optimized with BGP Next-Hop Address Tracking feature (not sure if that feature is still used as the default trigger delay is high on IOS).
  • Two possible failures:
    • Core Node Failure - Either core node or edge node fails. Core, new cost to next-hop. Edge, new next-hop.
    • Core Link Failure - New cost to next-hop.

PIC Edge

  • One possible failure:
    • Edge Link Failure - if PEs has a next-hop-self, a link failure to the CE will not trigger IGP next-hop cost change. In that case PE to PE encapsulation has to occur.
  • PIC can be configured under the VPNv4, VRF IPv4 or Global routing table.

Additional Resources

BGP PIC Edge for IP and MPLS-VPN - Cisco IOS Configuration Guide
Deploying BGP Fast Convergence BGP PIC - CiscoLive presentation.
BGP Convergence in much less than a second - Nanog 40 (PIC)


Add a New Comment
rating: 0+x
Unless otherwise stated, the content of this page is licensed under Creative Commons Attribution-ShareAlike 3.0 License