Ok, parece estar funcionando! Eu mudei a configuração para usar o LACP em vez do Etherchannel. Aqui está o que eu fiz para configurar as portas em cada switch:
SwitchA>
SwitchA> enable
SwitchA#
SwitchA# configure terminal
SwitchA(config)# interface range Gi0/21 - 24
SwitchA(config-if-range)# channel-group 1 mode active
SwitchA(config-if-range)# exit
SwitchA(config)# exit
Verificando o status da porta e mostrando
SwitchA# show interfaces status
Port Name Status Vlan Duplex Speed Type
Gi0/1 notconnect 1 auto auto 10/100/1000BaseTX
...
Gi0/21 connected trunk a-full a-1000 10/100/1000BaseTX
Gi0/22 connected trunk a-full a-1000 10/100/1000BaseTX
Gi0/23 connected trunk a-full a-1000 10/100/1000BaseTX
Gi0/24 connected trunk a-full a-1000 10/100/1000BaseTX
Po1 connected trunk a-full a-1000
SwitchA#
SwitchA# show etherchannel
Channel-group listing:
----------------------
Group: 1
----------
Group state = L2
Ports: 4 Maxports = 16
Port-channels: 1 Max Port-channels: 16
Protocol: LACP
Minimum Links: 0
SwitchA#