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.
You can configure BFD to detect roundtrip latency on non-bundle interfaces using the
echo latency detect
command beginning.
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 + 150 ms: Echo#4 TX (count = 2)
T + 151 ms: Echo#3 RX (count = 0; ~50 ms roundtrip latency)
T + 200 ms: Echo#5 TX (count = 1)
T + 250 ms: Echo#6 TX (count = 2)
T + 251 ms: Echo#4 RX (count = 0; ~100 ms roundtrip latency)
T + 300 ms: Echo#7 TX (count = 1)
T + 350 ms: Echo#8 TX (count = 2)
T + 351 ms: Echo#5 RX (count = 0; ~150 ms roundtrip latency)
T + 451 ms: Echo#6 RX (count = 0; ~200 ms roundtrip latency; no failure detection)
T + 501 ms: Echo#7 RX (count = 0; ~200 ms roundtrip latency; no failure detection)
T + 551 ms: Echo#8 RX (count = 0; ~200 ms roundtrip latency; no failure detection)
Note
Looking at the delay between receipt of echo packets for the BFD session, observe that no latency is beyond
the (I x M) window:
Echo#1 RX – Echo#2 RX: 50 ms
Echo#2 RX – Echo#3 RX: 100ms
Echo#3 RX - Echo#4 RX: 100ms
Echo#4 RX - Echo#5 RX: 100ms
Echo#5 RX - Echo#6 RX: 100ms
Echo#6 RX - Echo#7 RX: 50ms
Echo#7 RX - Echo#8 RX: 50ms
Summary of Packet Intervals and Failure Detection Times for BFD on Bundle Interfaces
For BFD on bundle interfaces, with a session interval
I
and a multiplier
M
, these packet intervals and failure
detection times apply for BFD asynchronous mode (
Table 4: BFD Packet Intervals and Failure Detection
Time Examples on Bundle Interfaces
):
• Value of
I—
Minimum period between sending of BFD control packets.
• Value of
I
x
M
• BFD control packet failure detection time.
Routing Configuration Guide for Cisco NCS 6000 Series Routers, IOS XR Release 6.4.x
133
Implementing BFD
Summary of Packet Intervals and Failure Detection Times for BFD on Bundle Interfaces