Port Aggregation Protocol; Pagp Modes - Cisco Catalyst 3750 Software Configuration Manual

Understanding EtherChannels

Port Aggregation Protocol

The Port Aggregation Protocol (PAgP) is a Cisco-proprietary protocol that can be run only on Cisco
switches and on those switches licensed by vendors to support PAgP. PAgP facilitates the automatic
creation of EtherChannels by exchanging PAgP packets between Ethernet ports.
By using PAgP, the switch learns the identity of partners capable of supporting PAgP and the capabilities
of each port. It then dynamically groups similarly configured ports into a single logical link (channel or
aggregate port). Similarly configured ports are grouped based on hardware, administrative, and port
parameter constraints. For example, PAgP groups the ports with the same speed, duplex mode, native
VLAN, VLAN range, and trunking status and type. After grouping the links into an EtherChannel, PAgP
adds the group to the spanning tree as a single switch port.

PAgP Modes

Table 27-1
configuration command.
Table 27-1 EtherChannel PAgP Modes
Mode
auto
desirable Places a port into an active negotiating state, in which the port starts negotiations with other
on
Switch ports exchange PAgP packets only with partner ports configured in the auto or desirable modes.
Ports configured in the on mode do not exchange PAgP packets.
Both the auto and desirable modes enable ports to negotiate with partner ports to form an EtherChannel
based on criteria such as port speed and, for Layer 2 EtherChannels, trunking state and VLAN numbers.
Ports can form an EtherChannel when they are in different PAgP modes as long as the modes are
compatible. For example:
•
•
A port in the auto mode cannot form an EtherChannel with another port that is also in the auto mode
because neither port starts PAgP negotiation.
A port in the on mode that is added to a port channel is forced to have the same characteristics as the
already existing on mode ports in the channel.
Caution You should exercise care when setting the mode to on (manual configuration). All ports configured in
the on mode are bundled in the same group and are forced to have similar characteristics. If the group is
misconfigured, packet loss or spanning-tree loops might occur.
Catalyst 3750 Metro Switch Software Configuration Guide
27-4
shows the user-configurable EtherChannel PAgP modes for the channel-group interface
Description
Places a port into a passive negotiating state, in which the port responds to PAgP packets
it receives but does not start PAgP packet negotiation. This setting minimizes the
transmission of PAgP packets.
ports by sending PAgP packets.
Forces a port to channel without PAgP (or the Link Aggregation Control Protocol [LACP]).
With the on mode, a usable EtherChannel exists only when a port group in the on mode is
connected to another port group in the on mode.
A port in the desirable mode can form an EtherChannel with another port that is in the desirable or
auto mode.
A port in the auto mode can form an EtherChannel with another port in the desirable mode.
Chapter 27 Configuring EtherChannels
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