1 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.

Echo Packet Latency

In Cisco IOS XR software releases prior to Cisco IOS XR 4.0.1, BFD only detects an absence of receipt of

echo packets, not a specific delay for TX/RX of a particular echo packet. In some cases, receipt of BFD echo

packets in general can be within their overall tolerances for failure detection and packet transmission, but a

longer delay might develop over a period of time for any particular roundtrip of an echo packet (See

3).

Beginning in Cisco IOS XR Release 4.0.1, you can configure the router to detect the actual latency between

transmitted and received echo packets on non-bundle interfaces and also take down the session when the

latency exceeds configured thresholds for that roundtrip latency. For more information, see the

BFD Session Teardown Based on Echo Latency

In addition, you can verify that the echo packet path is within specified latency tolerances before starting a

BFD session. With echo startup validation, an echo packet is periodically transmitted on the link while it is

down to verify successful transmission within the configured latency before allowing the BFD session to

change state. For more information, see the

and

Latency.

Priority Settings for BFD Packets

For all interfaces under over-subscription, the internal priority needs to be assigned to remote BFD Echo

packets, so that these BFD packets are not overwhelmed by other data packets. In addition, CoS values need

to be set appropriately, so that in the event of an intermediate switch, the reply back of remote BFD Echo

packets are protected from all other packets in the switch.

As configured CoS values in ethernet headers may not be retained in Echo messages, CoS values must be

explicitly configured in the appropriate egress QoS service policy. CoS values for BFD packets attached to a

traffic class can be set using the set cos command. For more information on configuring class-based

OL-30423-03

Configured

Multiplier

(bfd address-family

ipv4 multiplier)

Delaying BFD Session Startup Until Verification of Echo Path

Cisco ASR 9000 Series Aggregation Services Router Routing Configuration Guide, Release 5.1.x

Async Control

Echo Packet

Packet Failure

Interval

Detection Time

(Async Control

(ms)

Packet Failure

(Interval x

Detection Time)

Multiplier)

150

300

400

6000

45000

30000

Detection.

BFD Packet Information

Echo Packet

Failure Detection

Time

(Echo Interval x

Multiplier)

450

1200

800

18000

90000

Example

Configuring

183

Table of Contents

Echo Packet Latency; Priority Settings For Bfd Packets - Cisco ASR 9000 Series Configuration Manual