Echo Failure Detection Examples - Cisco ASR 9000 Series Routing Configuration Manual

Implementing BFD
However, this standard echo failure detection does not address latency between transmission and receipt of
any specific echo packet, which can build beyond (I x M) over the course of the BFD session. In this case,
BFD will not declare a neighbor down as long as any echo packet continues to be received within the multiplier
window and resets the counter to zero. Beginning in Cisco IOS XR 4.0.1, you can configure BFD to measure
this latency for non-bundle interfaces. For more information, see

Echo Failure Detection Examples

This section provides examples of several scenarios of standard echo packet processing and failure detection
without configuration of latency detection for non-bundle interfaces. In these examples, consider an interval
of 50 ms and a multiplier of 3.
The same interval and multiplier counter scheme for echo failure detection is used for bundle interfaces,
Note
but the values are determined by the bfd address-family ipv4 multiplier and bfd address-family ipv4
minimum-interval commands, and use a window of (I x M x M) to detect absence of receipt of echo
packets.
Example 1
The following example shows an ideal case where each echo packet is returned before the next echo is
transmitted. In this case, the counter increments to 1 and is returned to 0 before the next echo is sent and no
echo failure occurs. As long as the roundtip delay for echo packets in the session is less than the minimum
interval, this scenario occurs:
Time (T): Echo#1 TX (count = 1)
T + 1 ms: Echo#1 RX (count = 0)
T + 50 ms: Echo#2 TX (count = 1)
T + 51 ms: Echo#2 RX (count = 0)
T + 100 ms: Echo#3 TX (count = 1)
T + 101 ms: Echo#3 RX (count = 0)
T + 150 ms: Echo#4 TX (count = 1)
T + 151 ms: Echo#4 RX (count = 0)
Example 2
The following example shows the absence in return of any echo packets. After the transmission of the fourth
echo packet, the counter exceeds the multiplier value of 3 and echo failure is detected. In this case, echo failure
detection occurs at the 150 ms (I x M) window:
Time (T): Echo#1 TX (count = 1)
T + 50 ms: Echo#2 TX (count = 2)
T + 100 ms: Echo#3 TX (count = 3)
T + 150 ms: Echo#4 TX (count = 4 -> echo failure
Example 3
The following example shows an example of how roundtrip latency can build beyond (I x M) for any particular
echo packet over the course of a BFD session using the standard echo failure detection, but latency between
return of echo packets overall in the session never exceeds the (I x M) window and the counter never exceeds
the multiplier, so the neighbor is not declared down.
Cisco ASR 9000 Series Aggregation Services Router Routing Configuration Guide, Release 5.3.x
BFD Packet Information
Example 3 and the Echo Packet Latency.
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