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Hi! Today, I configured an EtherChannel to see how it affects STP. For the setup, I prepared two switches (Switch1 & Swtich2) and connected them with 2 cables. Below shows how STP is configured on Switch1 before EtherChannel is ready on Switch2. Switch1(config)# Switch1(config)#interface range fastEthernet 0/1-2 Switch1(config-if-range)#channel-group 1 mode active //Swtich1はactiveに設定 Creating a port-channel interface Port-channel 1 Switch1(config-if-range)# Switch1(config-if-range)#channel-protocol LACP Switch1(config-if-range)#exit Switch1(config)#exit Switch1#show 00:11:48: %SYS-5-CONFIG_I: Configured from console by console % Type "show ?" for a list of subcommands Switch1#show etherchannel summary Flags: D - down P - in port-channel I - stand-alone s - suspended H - Hot-standby (LACP only) R - Layer3 S - Layer2 U - in use f - failed to allocate aggregator u - unsuitable for bundling w - waiting to be aggregated d - default port Number of channel-groups in use: 1 Number of aggregators: 1 Group Port-channel Protocol Ports ------+-------------+-----------+----------------------------------------------- 1 Po1(SD) LACP Fa0/1(I) Fa0/2(I) Switch1# Switch1#show spanning-tree VLAN0001 Spanning tree enabled protocol ieee Root ID Priority 32769 Address ec30.918e.c100 This bridge is the root Hello Time 2 sec Max Age 20 sec Forward Delay 15 sec Bridge ID Priority 32769 (priority 32768 sys-id-ext 1) Address ec30.918e.c100 Hello Time 2 sec Max Age 20 sec Forward Delay 15 sec Aging Time 300 Interface Role Sts Cost Prio.Nbr Type ---------------- ---- --- --------- -------- -------------------------------- Fa0/1 Desg FWD 19 128.1 P2p Fa0/2 Desg FWD 19 128.2 P2p As you can see from the above, since the EtherChannel is still not configured on Switch2, you can see that STP recognizes 2 ports as they are not yet bundled even though the 2 ports are registered on the channel-group 1. Below is how Switch2 is configured after setting up Switch. Switch1# Switch1#show etherchannel summary Flags: D - down P - in port-channel I - stand-alone s - suspended H - Hot-standby (LACP only) R - Layer3 S - Layer2 U - in use f - failed to allocate aggregator u - unsuitable for bundling w - waiting to be aggregated d - default port Number of channel-groups in use: 1 Number of aggregators: 1 Group Port-channel Protocol Ports ------+-------------+-----------+----------------------------------------------- 1 Po1(SU) LACP Fa0/1(P) Fa0/2(P) Switch1#show spanning-tree VLAN0001 Spanning tree enabled protocol ieee Root ID Priority 32769 Address ec30.918e.c100 This bridge is the root Hello Time 2 sec Max Age 20 sec Forward Delay 15 sec Bridge ID Priority 32769 (priority 32768 sys-id-ext 1) Address ec30.918e.c100 Hello Time 2 sec Max Age 20 sec Forward Delay 15 sec Aging Time 300 Interface Role Sts Cost Prio.Nbr Type ---------------- ---- --- --------- -------- -------------------------------- Po1 Desg FWD 12 128.56 P2p After the ports are bundled, you can see that there are only 1 port recognized by STP. Another observation you can make is that the cost of the bundle port has decreased from 19 to 12. I also tested how STP will behave if I plugged out one of the port used by the channel-group. Switch1#show etherchannel summary Flags: D - down P - in port-channel I - stand-alone s - suspended H - Hot-standby (LACP only) R - Layer3 S - Layer2 U - in use f - failed to allocate aggregator u - unsuitable for bundling w - waiting to be aggregated d - default port Number of channel-groups in use: 1 Number of aggregators: 1 Group Port-channel Protocol Ports ------+-------------+-----------+----------------------------------------------- 1 Po1(SU) LACP Fa0/1(P) Fa0/2(D) Switch1#show spanning-tree VLAN0001 Spanning tree enabled protocol ieee Root ID Priority 32769 Address ec30.918e.c100 This bridge is the root Hello Time 2 sec Max Age 20 sec Forward Delay 15 sec Bridge ID Priority 32769 (priority 32768 sys-id-ext 1) Address ec30.918e.c100 Hello Time 2 sec Max Age 20 sec Forward Delay 15 sec Aging Time 300 Interface Role Sts Cost Prio.Nbr Type ---------------- ---- --- --------- -------- -------------------------------- Po1 Desg FWD 19 128.56 P2p Comparing before and after plugging one of the port on Switch1 shows that the cost has decreased but it is still recognized as a port-channel.