The HSRP BFD Peering
feature introduces Bidirectional Forwarding Detection (BFD) in the Hot Standby
Router Protocol (HSRP) group member health monitoring system. HSRP supports BFD
as a part of the HSRP group member health monitoring system. Without BFD, HSRP
runs as a process in a multiprocess system and cannot be guaranteed to be
scheduled in time to service large numbers of groups with hello and hold
timers, in milliseconds. BFD runs as a pseudopreemptive process and can
therefore be guaranteed to run when required. Only one BFD session between two
devices can provide early failover notification for multiple HSRP groups.
This feature is
enabled by default. The HSRP standby device learns the real IP address of the
HSRP active device from the HSRP hello messages. The standby device registers
as a BFD client and asks to be notified if the active device becomes
unavailable. When BFD determines that the connections between standby and
active devices has failed, it will notify HSRP on the standby device which will
immediately take over as the active device.
BFD provides a
low-overhead, short-duration method of detecting failures in the forwarding
path between two adjacent devices, including the interfaces, data links, and
forwarding planes. BFD is a detection protocol that you enable at the interface
and routing protocol levels. Cisco supports the BFD asynchronous mode, which
depends on the sending of BFD control packets between two systems to activate
and maintain BFD neighbor sessions between devices. Therefore, to create a BFD
session, you must configure BFD on both systems (or BFD peers). When BFD is
enabled on the interfaces and at the device level for HSRP, a BFD session is
created, BFD timers are negotiated, and the BFD peers will begin to send BFD
control packets to each other at the negotiated interval.
BFD provides fast BFD
peer failure detection times independently of all media types, encapsulations,
topologies, and routing protocols such as, Border Gateway Protocol (BGP),
Enhanced Interior Gateway Routing Protocol (EIGRP), Hot Standby Router Protocol
(HSRP), Intermediate System To Intermediate System (IS-IS), and Open Shortest
Path First (OSPF). By sending rapid failure detection notices to the routing
protocols in the local device to initiate the routing table recalculation
process, BFD contributes to greatly reduce overall network convergence time.
The figure below shows a simple network with two devices running HSRP and BFD.
Figure 1. HSRP BFD
For more information
about BFD, see the
IP Routing: BFD