Cisco Catalyst 2000 Configuration Handbook page 85

64 Cisco LAN Switching Configuration Handbook
b. Set the STP port cost per VLAN:
(Optional) Use PAgP on an EtherChannel:
3.
Tip When you make configuration changes to add or remove ports from an
EtherChannel, be aware of the effects this has on the STP. This is especially important in a
live production network where you might cause an interruption of service.
STP operates on an EtherChannel as if it were a normal switch port. After ports have been
assigned to an EtherChannel, STP moves through its various states to guarantee a loop-free
topology. Switch ports within the EtherChannel administrative group can be enabled and
disabled without triggering an STP topology change. As a result, the other links in the
EtherChannel remain in the STP "forwarding" state.
If you attempt to add a new port into an active EtherChannel administrative group, howev-
er, an STP topology change is triggered. The same result occurs if you change the adminis-
trative group number on an active EtherChannel. You have now reconfigured the logical
link, so STP moves the EtherChannel (and all its ports) back through the "listening" and
"learning" states. This interrupts traffic on the EtherChannel for up to 50 seconds.
a. Assign interfaces to the EtherChannel:
interface [mod[/port]]
spanning-tree cost cost
Use the set spantree channelvlancost command to enable the port cost per
VLAN to be configured for the EtherChannel with channel-id. The STP port
cost is set to cost for all VLANs that will be carried over the EtherChannel. Then
you should adjust the port cost for specific VLANs by using the set spantree
portvlancost command. Refer to section "7-1: STP Operation," in Chapter 7 for
more cost information.
interface [mod[/port]]
channel-group {channel group number} mode {active | auto | desirable | on |
passive}
One or more ports, given by mod/port, are assigned as an EtherChannel. A spe-
cific administrative group number admin-group can be given if desired. If this is
omitted, the switch automatically assigns these ports a new unique group num-
ber. If you do specify a group number and that number is already in use, the new
EtherChannel receives the group number, and the ports that were previously
assigned are moved to a different unique group number.
Ports are assigned to an EtherChannel group at the same time as the PAgP mode
is set. This is done in Step 3b.