Understanding Frame Distribution - Cisco WS-C4003 - Catalyst 4000 Chassis Switch Software Configuration Manual

Chapter 6 Configuring Fast EtherChannel and Gigabit EtherChannel
There are four user-configurable channel modes: on, off, auto, and desirable. PAgP packets are
exchanged only between ports in auto and desirable mode. Ports configured in on or off mode do not
exchange PAgP packets. The auto and desirable modes can be modified with the silent and non-silent
keywords.
Table 6-1
Mode
on
off
auto
desirable
Both the auto and desirable modes allow ports to negotiate with connected ports to determine if they
can form a channel, based on criteria such as port speed, trunking state, native VLAN, and so on.
Ports can form an EtherChannel when they are in different channel modes as long as the modes are
compatible. For example:
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•
•
•
•

Understanding Frame Distribution

EtherChannel distributes frames across the links in a channel based on the low-order bits of the source
and destination Media Access Control (MAC) addresses of each frame. The frame distribution method
is not configurable.
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Table 6-1 describes each mode.
Channel Modes
Description
Forces the port to channel without negotiation. PAgP packets are not exchanged.
The port is channeling regardless of how the peer port is configured. If the peer port
is in on mode, a channel is formed. In any other mode, the peer port is placed in the
errdisable state due to a channel misconfiguration.
Prevents the port from channeling. PAgP packets are not exchanged. The port is not
channeling regardless of how the peer port is configured. No channel is formed.
Places a port into a passive negotiating state, in which the port responds to PAgP
packets it receives but does not initiate PAgP packet negotiation. A channel is
formed only with another port group in desirable mode. (Default)
Places a port into an active negotiating state, in which the port initiates negotiations
with other ports by sending PAgP packets. A channel is formed with another port
group in either desirable or auto mode.
Use the silent keyword when you are connecting to a "silent partner" (a device that
is not generating BPDUs or other traffic). An example of a silent partner is a traffic
generator that is not transmitting packets. Use this keyword with the auto or
desirable mode. If you do not specify silent or non-silent, silent is assumed.
Use the non-silent keyword when you are connecting to a device that will transmit
BPDUs or other traffic. Use this keyword with the auto or desirable mode.
A port in desirable mode can form an EtherChannel successfully with another port that is in
desirable or auto mode.
A port in auto mode can form an EtherChannel with another port in desirable mode.
A port in auto mode cannot form an EtherChannel with another port that is also in auto mode, since
neither port will initiate negotiation.
A port in on mode can form a channel only with a port in on mode, because ports in on mode do not
exchange PAgP packets.
A port in off mode will not form a channel with any port.
Software Configuration Guide—Catalyst 4000 Family, Catalyst 2948G, Catalyst 2980G, Releases 6.3 and 6.4
Understanding How EtherChannel Works
6-3