Etherchannel And Switch Stacks - Cisco Catalyst 2960 series Configuration Manual

Information About EtherChannels
layer switches into a single logical entity with a single point of configuration, management, and troubleshooting.
The following illustration represents a sample network where an Instant Access system interacts with a switch
(Catalyst 2960-X Series Switches) that is connected via a port channel to stacked clients (Member 1 and
Member 2).
When the EtherChannel Load Deferral feature is configured and a new Instant Access client stack member
comes up, ports of this newly-joined stack member is bundled into the port channel. In the transition period,
the data path is not fully established on the distribution switch (Catalyst 6000 Series Switches), and traffic
originating from the access layer switch (Catalyst 2960-X Series Switches) reaches the non-established ports
and the traffic gets lost.
When load share deferral is enabled on a port channel, the assignment of a member port's load share is delayed
for a period that is configured globally by the port-channel load-defer command. During the deferral period,
the load share of a deferred member port is set to 0. In this state, the deferred port is capable of receiving data
and control traffic, and of sending control traffic, but the port is prevented from sending data traffic to the
virtual switching system (VSS). Upon expiration of the global deferral timer, the deferred member port exits
the deferral state and the port assumes its normal configured load share.
Load share deferral is applied only if at least one member port of the port channel is currently active with a
nonzero load share. If a port enabled for load share deferral is the first member bringing up the EtherChannel,
the deferral feature does not apply and the port will forward traffic immediately.
This feature is enabled on a per port-channel basis; however, the load deferral timer is configured globally
and not per port-channel. As a result, when a new port is bundled, the timer starts only if it is not already
running. If some other ports are already deferred then the new port will be deferred only for the remaining
amount of time.
The load deferral is stopped as soon as a member in one of the deferred port channels is unbundled. As a
result, all the ports that were deferred is assigned a group-mask in the event of an unbundling during the
deferral period.
When you try to enable this feature on a stack member switch, the following message is displayed:
Note
Load share deferral is supported only on stand-alone stack.

EtherChannel and Switch Stacks

If a stack member that has ports participating in an EtherChannel fails or leaves the stack, the active switch
removes the failed stack member switch ports from the EtherChannel. The remaining ports of the EtherChannel,
if any, continue to provide connectivity.
When a switch is added to an existing stack, the new switch receives the running configuration from the active
switch and updates itself with the EtherChannel-related stack configuration. The stack member also receives
the operational information (the list of ports that are up and are members of a channel).
When two stacks merge that have EtherChannels configured between them, self-looped ports result. Spanning
tree detects this condition and acts accordingly. Any PAgP or LACP configuration on a winning switch stack
is not affected, but the PAgP or LACP configuration on the losing switch stack is lost after the stack reboots.
For a mixed stack containing one or more Catalyst 2960-S switches, we recommend that you configure no
more than six EtherChannels on the stack.
Consolidated Platform Configuration Guide, Cisco IOS Release 15.2(4)E (Catalyst 2960-X Switches)
336