How to configure cross-stack EtherChannel on Cisco Catalyst 3750 switches

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Core issue

The EtherChannel on multiple switches in the Cisco Catalyst 3750 stack is called cross-stack EtherChannel.

If a link within an EtherChannel fails, traffic previously carried over that failed link changes to the remaining links
within the EtherChannel. A trap is sent for a failure, which identifies the switch, the EtherChannel, and the failed link.
Inbound broadcast and multicast packets on one link in an EtherChannel are blocked, and cannot return on any other
link of the EtherChannel.

 

Resolution

These are the guidelines in order to configure cross-stack EtherChannel:

 

  • Do not configure an EtherChannel in both the Port Aggregation Control Protocol (PAgP) and Link Aggregation Control Protocol (LACP) modes. EtherChannel groups that run PAgP and LACP can co-exist on the same switch or on different switches in the stack, but not in a cross-stack configuration. Individual EtherChannel groups can run either PAgP or LACP, but cannot interoperate.
  • For cross-stack EtherChannel configurations, disable PAgP and LACP on all ports targeted for the EtherChannel with the channel-group channel-group-number mode oninterface configuration command. Before the addition of a stack member port to an existing EtherChannel, manually disable PAgP and LACP on all the ports that are members of the channel group, and then manually configure the cross-stack EtherChannel. PAgP and LACP are not supported on cross-stack EtherChannels.
  • In order to configure cross-stack EtherChannel with LACP, the minimum Cisco IOS Software release required is Cisco IOS Software Release 12.2(25)SEC and later.
  • If cross-stack EtherChannel is configured and the switch stack partitions, loops and forwarding misbehaviors can occur.

This is a sample configuration of cross-stack EtherChannel. This example assigns two ports on stack member 2 and one port on stack member 3 as static-access ports in VLAN 10 to channel 5 with the PAgP and LACP modes disabled (on):
Switch#configure terminal
Switch(config)#interface range gigabitethernet2/0/3 -4
Switch(config-if-range)#switchport mode access
Switch(config-if-range)# switchport access vlan 10
Switch(config-if-range)#channel-group 5 mode on
Switch(config-if-range)#exit

 

Switch(config)#interface gigabitethernet3/0/3
Switch(config-if)#switchport mode access
Switch(config-if)#switchport access vlan 10
Switch(config-if)#channel-group 5 mode on
Switch(config-if)#exit

 

Refer to the Cross-Stack EtherChannel Without PAgP or LACP and Cross-Stack EtherChannel with LACP sections of Cross-Stack EtherChannel on a Catalyst 3750 Switch Configuration Example for configuration examples.

Refer to the EtherChannel Configuration Guidelines section of Configuring EtherChannels and Link-State Tracking for more information.

Note: The gigabit Ethernet port can be made as trunk port before you assign it to EtherChannel. Also, the port can be made as a Layer 3 port if you issue the no switchport command.

The port channel cannot span chassis which means that in order for that EtherChannel to work, they need to be going to the same location and you are not able to split up the EtherChannel like you can do with a serial frame relay link.  The option is to create two EtherChannels, one to each location.

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