Configuring The Pagp Learn Method And Priority - Cisco WS-C3560-48PS-S Software Configuration Manual

Chapter 29 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 transmissions based on 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.
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 packet arrives.
PAgP cannot automatically detect when the partner device is a physical learner and when the local
device is an aggregate-port learner. Therefore, you must manually set the learning method on the local
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.
The Catalyst 3560 switch supports address learning only on aggregate ports even though the
Note
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, such as the
Catalyst 1900 switch.
When the link partner to the Catalyst 3560 switch is a physical learner (such as a Catalyst 1900 series
switch), we recommend that you configure the Catalyst 3560 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 using the port-channel load-balance src-mac global
configuration command. The switch then sends packets to the Catalyst 1900 switch using the same port
in the EtherChannel from which it learned the source address. Use the pagp learn-method command
only in this situation.
Beginning in privileged EXEC mode, follow these steps to configure your switch as a PAgP
physical-port learner and to adjust the priority so that the same port in the bundle is selected for sending
packets. This procedure is optional.
Catalyst 3560 Switch Software Configuration Guide
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