Bfd Packet Intervals On Bundle Member Links; Control Packet Failure Detection In Asynchronous Mode; Echo Packet Failure Detection In Asynchronous Mode - Cisco ASR 9000 Series Routing Configuration Manual

BFD Packet Information
BFD sessions running over physical interfaces when echo mode is enabled send BFD control packets at a
slow rate of every two seconds. There is no need to duplicate control packet failure detection at a fast rate
because BFD echo packets are already being sent at fast rates and link failures will be detected when echo
packets are not received within the echo failure detection time.

BFD Packet Intervals on Bundle Member Links

On each bundle member interface, BFD asynchronous mode control packets run at user-configurable interval
and multiplier values, even when echo mode is running.
However, on a bundle member interface when echo mode is enabled, BFD asynchronous mode must continue
to run at a fast rate because one of the requirements of enabling BFD echo mode is that the bundle member
interface is available in BFD asynchronous mode.
The maximum echo packet interval for BFD on bundle member links is the minimum of either 30 seconds or
the asynchronous control packet failure detection time.
When echo mode is disabled, the behavior is the same as BFD over physical interfaces, where sessions
exchange BFD control packets at the configured rate.

Control Packet Failure Detection In Asynchronous Mode

Control packet failure in asynchronous mode without echo is detected using the values of the minimum interval
(bfd minimum-interval for non-bundle interfaces, and bfd address-family ipv4 minimum-interval for bundle
interfaces) and multiplier (bfd multiplier for non-bundle interfaces, and bfd address-family ipv4 multiplier
for bundle interfaces) commands.
For control packet failure detection, the local multiplier value is sent to the neighbor. A failure detection timer
is started based on (I x M), where I is the negotiated interval, and M is the multiplier provided by the remote
end.
Whenever a valid control packet is received from the neighbor, the failure detection timer is reset. If a valid
control packet is not received from the neighbor within the time period (I x M), then the failure detection timer
is triggered, and the neighbor is declared down.

Echo Packet Failure Detection In Asynchronous Mode

The standard echo failure detection scheme is done through a counter that is based on the value of the bfd
multiplier command on non-bundle interfaces, and the value of the bfd address-family ipv4 multiplier
command for bundle interfaces.
This counter is incremented each time the system sends an echo packet, and is reset to zero whenever any
echo packet is received, regardless of the order that the packet was sent in the echo packet stream.
Under ideal conditions, this means that BFD generally detects echo failures that exceed the period of time (I
x M) or (I x M x M) for bundle interfaces, where:
• I—Value of the minimum interval (bfd minimum-interval for non-bundle interfaces, and bfd
address-family ipv4 minimum-interval for bundle interfaces).
• M—Value of the multiplier (bfd multiplier for non-bundle interfaces, and bfd address-family ipv4
multiplier for bundle interfaces) commands.
So, if the system transmits one additional echo packet beyond the multiplier count without receipt of any echo
packets, echo failure is detected and the neighbor is declared down (See
Cisco ASR 9000 Series Aggregation Services Router Routing Configuration Guide, Release 5.3.x
236
Implementing BFD
Example 2).