40-21
Catalyst 3750-X and 3560-X Switch Software Configuration Guide
OL-21521-01
Chapter 40 Configuring EtherChannels and Link-State Tracking Configuring EtherChannels
If you configure more than eight links for an EtherChannel group, the software automatically decides
which of the hot-standby ports to make active based on the LACP priority. To every link between systems
that operate LACP, the software assigns a unique priority made up of these elements (in priority order):
• LACP system priority
• System ID (the switch MAC address)
• LACP port priority
• Port number
In priority comparisons, numerically lower values have higher priority. The priority decides which ports
should be put in standby mode when there is a hardware limitation that prevents all compatible ports
from aggregating.
Determining which ports are active and which are hot standby is a two-step procedure. First the system
with a numerically lower system priority and system-id is placed in charge of the decision. Next, that
system decides which ports are active and which are hot standby, based on its values for port priority and
port number. The port-priority and port-number values for the other system are not used.
You can change the default values of the LACP system priority and the LACP port priority to affect how
the software selects active and standby links. For more information, see the “Configuring the LACP
System Priority” section on page 40-21 and the “Configuring the LACP Port Priority” section on
page 40-22.
Configuring the LACP System Priority
You can configure the system priority for all the EtherChannels that are enabled for LACP by using the
lacp system-priority global configuration command. You cannot configure a system priority for each
LACP-configured channel. By changing this value from the default, you can affect how the software
selects active and standby links.
You can use the show etherchannel summary privileged EXEC command to see which ports are in the
hot-standby mode (denoted with an H port-state flag).
Beginning in privileged EXEC mode, follow these steps to configure the LACP system priority. This
procedure is optional.
To return the LACP system priority to the default value, use the no lacp system-priority global
configuration command.
Command Purpose
Step 1 configure terminal Enter global configuration mode.
Step 2 lacp system-priority priority Configure the LACP system priority.
For priority, the range is 1 to 65535. The default is 32768.
The lower the value, the higher the system priority.
Step 3 end Return to privileged EXEC mode.
Step 4 show running-config
or
show lacp sys-id
Verify your entries.
Step 5 copy running-config startup-config (Optional) Save your entries in the configuration file.