VLACP Flap Detect and Damping

When there is instability or packet loss in a connection, the Virtual Link Aggregation Control Protocol (VLACP) state of the port flaps, causing services such as IP multicast to stop and start rapidly. This behavior causes system-wide instability, including high CPU utilization. In such cases, VLACP Flap Detect and Damping provides link flap detection capability, by configuring a specific time interval and frequency to count the number of VLACP flaps. After the system detects excessive VLACP flaps, it disables the specific VLACP port until the root cause is resolved.

For example, VLACP Flap Detect and Damping detects 3 VLACP flaps within 60 seconds, be default. After the system detects the first VLACP flap, the flap timer starts to count the number of VLACP flaps occuring within 60 seconds. If the flap count reaches 3 before the timer ends, the system disables the specific VLACP port, and generates a Simple Network Management Protocol (SNMP) trap. For more information about SNMP traps, see Logs and Traps Fundamentals.

If you view the port state information, the port displays as operationally down and the reason provided is VLACP_FLAP. The following example shows a port that is disabled because of VLACP flapping; only the relevant sections of the show command output is included.

Switch:1#show interface gigabitethernet 1/1
==========================================================================================
                                      Port Interface
==========================================================================================
PORT                               LINK  PORT           PHYSICAL          STATUS
NUM      INDEX DESCRIPTION         TRAP  LOCK     MTU   ADDRESS           ADMIN  OPERATE
------------------------------------------------------------------------------------------
1/1      192   1000BaseTX          true false    1950  e4:5d:52:3c:64:00  up     down

==========================================================================================

                                   Port State
==========================================================================================
PORT NUM   ADMINSTATUS  PORTSTATE   REASON        DATE
------------------------------------------------------------------------------------------
1/1        up           down        VLACP_FLAP    09/19/19 02:22:58

VLACP Flap Detect and Damping Considerations

Use the following information when configuring VLACP Flap Detect and Damping.

• Do not enable VLACP Flap Detect and Damping on Link Aggregation Control Protocol (LACP) enabled ports.

  

  Note

  Link flap detection takes priority if you enable both VLACP Flap Detect and Damping and link flap detection on the same port.