The BGP PIC functionality is achieved by an extra functionality in the BGP, RIB, Cisco Express Forwarding, and MPLS.
BGP PIC affects prefixes under IPv4 and VPNv4 address families. For those prefixes, BGP calculates an extra second best path,
along with the primary best path. (The second best path is called the backup or alternate path.) BGP installs the best and
backup or alternate paths for the affected prefixes into the BGP RIB. The backup or alternate path provides a fast reroute
mechanism to counter a singular network failure. BGP also includes the alternate or backup path in its application programming
interface (API) to the IP RIB.
For BGP PIC, RIB installs an alternate path per route if one is available. If the RIB selects a BGP route containing a backup
or alternate path, it installs the backup or alternate path with the best path. The RIB also includes the alternate path in
its API with the FIB.
With BGP PIC, Cisco Express Forwarding stores an alternate path per prefix. When the primary path goes down, Cisco Express
Forwarding searches for the backup or alternate path in a prefix-independent manner. Cisco Express Forwarding also listens
to BFD events to rapidly detect local failures.
MPLS Forwarding is similar to Cisco Express Forwarding in that it stores alternate paths and switches to an alternate path
if the primary path goes down.
When the BGP PIC feature is enabled, BGP calculates a backup or alternate path per prefix and installs it into BGP RIB, IP
RIB, and FIB. This improves convergence after a network failure. There are two types of network failures that the BGP PIC
Core node or link failure (internal Border Gateway Protocol [iBGP] node failure): If a PE node or link fails, then the failure
is detected through IGP convergence. IGP conveys the failure through the RIB to the FIB.
Local link or immediate neighbor node failure (external Border Gateway Protocol [eBGP] node or link failure): To detect a
local link failure or eBGP single-hop peer node failure in less than a second, you must enable BFD. Cisco Express Forwarding
looks for BFD events to detect a failure of an eBGP single-hop peer.
Convergence in the Data Plane
Upon detecting a failure, Cisco Express Forwarding detects the alternate next hop for all prefixes that are affected by the
failure. The data plane convergence is achieved in subseconds depending on whether the BGP PIC implementation exists in the
software or hardware.
Convergence in the Control Plane
Upon detecting a failure, BGP learns about the failure through IGP convergence or BFD events and sends withdrawn messages
for the prefixes, recalculating the best and backup or alternate paths, and advertising the next best path across the network.