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Cisco ME 3400 EthernetAccess Switch SoftwareConfiguration Guide
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Chapter31 Configuring EtherChannels Configuring EtherChannels
Configuring the PAgP Learn Method and Priority
Network devices are classified as PAgP physical learners or aggregate-port learners. A device is a
physical learner if it learns addresses by physical ports and directs transmissi ons ba sed o n that
knowledge. A device is an aggregate-port learner if it learns addresses by aggregate (logical) ports. The
learn method must be configured the same at both ends of the link.
Note PAgP is available only on NNIs.
When a device and its partner are both aggregate-port learners, they learn the address on the logical
port-channel. The device sends packets to the source by using any of the ports in the EtherChannel. With
aggregate-port learning, it is not important on which physical port the pack et arri ves.
PAgP cannot automatically detect when the partner de vice is a ph ysical learner an d when the local de vice
is an aggregate-port learner. Therefore, you must manually set the lea rn ing m e thod on the loc al device
to learn addresses by physical ports. You also must set the load-distribution method to source-based
distribution, so that any given source MAC address is always sent on the same physical port.
You also can configure a single port within the group for all transmissions and use other ports for hot
standby. The unused ports in the group can be swapped into operation in just a few seconds if the selected
single port loses hardware-signal detection. You can configure which port is always selected for packet
transmission by changing its priority with the pagp port-priority interface configuration command. The
higher the priority, the more likely that the port will be selected.
Note The switch supports address learning only on aggregate ports even though the physical-port keyword is
provided in the CLI. The pagp learn-method command and the pagp port-priority command have no
effect on the switch hardware, but they are required for PAgP interoperability with devices that only
support address learning by physical ports.
When the link partner to the switch is a physical learner, we recommend that you configure the Cisco
ME switch as a physical-port learner by using the pagp learn-method physical-port interface
configuration command. Set the load-distribution method based on the source MAC address by us ing the
port-channel load-balance src-mac global configuration command. The switch then sends packets to
the physical learner switch using the same port in the Ethe rCh anne l fr om whi ch i t lea rned the so urce
address. Use the pagp learn-method command only in this situation.