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BFD Overview
This implementation of BFD can be associated with tracking of next hops with the BGP, OSPF, VRRP
and other static route protocols.
Benefits of Using BFD For Failure Detection
It is more advantageous to implement BFD rather than reduce timer mechanisms for routing protocols due
to the following reasons:
•
BFD can detect failures in milliseconds without having to fine-tune routing protocol Hello timers.
•
BFD is not tied to any particular routing protocol. As a result, BFD provides a generic and consistent
failure detection mechanism for OSPF, BFP, VRRP Remote Tracking, and static routes.
•
BFD is less CPU-intensive than reduced timer mechanisms for routing protocols.
How the BFD Protocol Works
A BFD session must be explicitly configured between two adjacent systems. Once BFD has been enabled
on the interfaces and at the appropriate Layer 3 routing protocol level, a BFD session is created for the
adjacent systems and BFD timers are negotiated between these systems.
The BFD protocol does not have a neighbor discovery mechanism to detect neighboring systems;
protocols that BFD services notify BFD of devices to which it needs to establish sessions. For example, an
OSPF implementation can request BFD to establish a session with a neighbor discovered using the OSPF
Hello protocol.
Once a session is established, BFD peers - neighboring systems sharing a BFD interface - begin sending
BFD control packets to each other over the bidirectional forwarding path. The packets are transmitted
periodically at the negotiated rate. The BFD control packets function in a similar manner to that of an IGP
Hello protocol, except at a more accelerated rate.
Each time a BFD control packet is successfully received through a BFD interface, the detect-timer for that
session is reset to zero. As long as the BFD peer systems receive the control packets from each other
within the negotiated time interval [(Detect Time Multiplier) * (Required Minimum Rx Interval)], the
BFD session remains up. Any routing protocol that associates the BFD maintains its
adjacencies and continues its periodic transmission of BFD control packets at the negotiated rate.
In case a system stops receiving the packets within the predetermined time frame, some component in the
bidirectional path to that particular system is assumed to have failed, and the BFD system simply informs
its client protocol that a failure has occurred. It does this by sending rapid failure detection notices to
respective registered routing protocols in the local router to initiate the router table recalculation process in
order to accelerate routing convergence and network uptime.
In order to agree with its peers about how rapidly failure detection takes place, each system estimates the
rate at which it can send and receive BFD control packets. This design also enables fast systems on shared
medium with a slow system to detect failures more rapidly between fast systems while allowing the slow
system to participate to the best of its ability.
page 16-8
OmniSwitch AOS Release 7 Network Configuration Guide
Configuring BFD
March 2011
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