Information Technology Reference
In-Depth Information
On Switch B, we can turn on the channel—notice that it indicates that the ports are channeling fine, but
we know that Switch A is not configured correctly.
Switch-B (enable)
show port channel
No ports channelling
Switch-B (enable)
show port
Port Name Status Vlan Level Duplex Speed Type
----- ------------------ ---------- ---------- ------ ------ ----- ------------
2/1
connected 1
normal a-full a-100 10/100BaseTX
2/2
connected 1
normal a-full a-100 10/100BaseTX
2/3
connected 1
normal a-full a-100 10/100BaseTX
2/4
connected 1
normal a-full a-100 10/100BaseTX
Switch-B (enable)
set port channel 2/1-4 on
Port(s) 2/1-4 channel mode set to on.
Switch-B (enable)
2000 Jan 17 22:54:59 %PAGP-5-PORTFROMSTP:Port 2/1 left bridge port 2/1
2000 Jan 17 22:54:59 %PAGP-5-PORTFROMSTP:Port 2/2 left bridge port 2/2
2000 Jan 17 22:54:59 %PAGP-5-PORTFROMSTP:Port 2/3 left bridge port 2/3
2000 Jan 17 22:54:59 %PAGP-5-PORTFROMSTP:Port 2/4 left bridge port 2/4
2000 Jan 17 22:55:00 %PAGP-5-PORTTOSTP:Port 2/1 joined bridge port 2/1-4
2000 Jan 17 22:55:00 %PAGP-5-PORTTOSTP:Port 2/2 joined bridge port 2/1-4
2000 Jan 17 22:55:00 %PAGP-5-PORTTOSTP:Port 2/3 joined bridge port 2/1-4
2000 Jan 17 22:55:00 %PAGP-5-PORTTOSTP:Port 2/4 joined bridge port 2/1-4
Switch-B (enable)
show port channel
Port Status Channel Channel Neighbor Neighbor
mode status device port
----- ---------- --------- ----------- ------------------------- ----------
2/1 connected on channel WS-C5505 066507453(Sw 2/1
2/2 connected on channel WS-C5505 066507453(Sw 2/2
2/3 connected on channel WS-C5505 066507453(Sw 2/3
2/4 connected on channel WS-C5505 066507453(Sw 2/4
----- ---------- --------- ----------- ------------------------- ----------
This makes it clear that you must check both sides of the channel when manually configuring the channel
to make sure that both sides are up, not just one side. The previous output shows that Switch B is set for
a channel, but Switch A is not channeling because it has one port that is in the wrong vlan.
Waiting Too Long Before Configuring the Other Side
In our situation, Switch B has EtherChannel turned on, but Switch A does not have EtherChannel turned
on because it has a VLAN configuration error (ports 2/1-3 are in VLAN 1, and port 2/4 is in VLAN 2).
Here is what happens when one side of a EtherChannel is set to on while the other side is still in auto
mode: After a few minutes, Switch B shut down its ports because of a spanning-tree loop detection. This
is because Switch B ports 2/1-4 all act like one big port, while Switch A ports 2/1-4 are all totally
independent ports. A broadcast sent from Switch B to Switch A on port 2/1 will be sent back to Switch
