Etherchannel And Switch Stacks - Cisco Catalyst 2960 Software Configuration Manual

Understanding EtherChannels
Figure 38-5

EtherChannel and Switch Stacks

If a stack member that has ports participating in an EtherChannel fails or leaves the stack, the stack
master 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
stack master 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.
With PAgP, if the stack master fails or leaves the stack, a new stack master is elected. A spanning-tree
reconvergence is not triggered unless there is a change in the EtherChannel bandwidth. The new stack
master synchronizes the configuration of the stack members to that of the stack master. The PAgP
configuration is not affected after a stack master change unless the EtherChannel has ports residing on
the old stack master.
With LACP, the system-id uses the stack MAC address from the stack master, and if the stack master
changes, the LACP system-id can change. If the LACP system-id changes, the entire EtherChannel will
flap, and there will be an STP reconvergence. Use the stack-mac persistent timer command to control
whether or not the stack MAC address changes during a master failover.
For more information about switch stacks, see
Catalyst 2960 and 2960-S Switches Software Configuration Guide, Release 15.0(1)SE
38-10
Load Distribution and Forwarding Methods
Switch with
source-based
forwarding enabled
EtherChannel
Cisco router
with destination-based
forwarding enabled
Chapter 38 Configuring EtherChannels and Link-State Tracking
Chapter 7, "Managing Switch Stacks."
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