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Cisco IE 2000 Switch Software Configuration Guide
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Chapter 40 Configuring EtherChannels
Information About Configuring EtherChannels
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 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 pack et 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.
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 of the switch is a physical learner (such as a Catalyst 1900 series switch), we
recommend that you configure the 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. Only use the pagp learn-method command in this situation.
PAgP Interaction with Virtual Switches and Dual-Active Detection
A virtual switch can be two or more Catalyst 6500 core switches c onnected by virtual switch links
(VSLs) that carry control and data traffic between them. One of the switches is in active mode. The
others are in standby mode. For redundancy, remote switches, are connected to the virtual switch by
remote satellite links (RSLs).
If the VSL between two switches fails, one switch does not know the status of the other. Both switches
could change to the active mode, causing a dual-active situation in the network with duplicate
configurations (including duplicate IP addresses and bridge i dentifiers). The network might go down.
To prevent a dual-active situation, the core switches send PAgP protocol data units (PDUs) through the
RSLs to the remote switches. The PAgP PDUs identify the active switch, and the remote switches
forward the PDUs to core switches so that the core switches are in sync. If the active switch fails or
resets, the standby switch takes over as the active switch. If the VSL goes down, one core switch knows
the status of the other and does not change state.