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Chapter 6
Configuring Fast EtherChannel and Gigabit EtherChannel
Understanding the PAgP
Use the information in the following sections if you are configuring EtherChannel using PAgP. If you
are using LACP, see the
PAgP Modes
The Port Aggregation Protocol (PAgP) facilitates the automatic creation of Fast EtherChannel and
Gigabit EtherChannel links by exchanging packets between channel-capable ports. The protocol learns
the capabilities of port groups dynamically and informs the neighboring ports.
After PAgP identifies correctly paired channel-capable links, it groups the ports into a channel. The
channel is then added to the spanning tree as a single bridge port. A given outbound broadcast or
multicast packet is transmitted out one port in the channel only, not out every port in the channel. In
addition, outbound broadcast and multicast packets that are transmitted on one port in a channel are
blocked from returning on any other port of the channel.
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 that are 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.
78-15486-01
"Understanding the LACP" section on page
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.
Catalyst 4500 Series, Catalyst 2948G, Catalyst 2980G Switches Software Configuration Guide
Understanding the PAgP
6-16.
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Release 8.1
6-5
- Quick Links:
- Configuration Guide
- Catalyst 2980G Switch
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