The Port Aggregation Protocol (PAgP) aids in the automatic creation of Fast
EtherChannel links. PAgP packets are sent between Fast EtherChannel-capable
ports in order to negotiate the forming of a channel. When PAgP identifies
matched Ethernet links, PAgP groups the links into an EtherChannel. The
EtherChannel is then added to the spanning tree as a single bridge port.
Link Aggregation Control Protocol (LACP) is part of an IEEE specification
(802.3ad) that allows several physical ports to be bundled together to form a
single logical channel. LACP allows a switch to negotiate an automatic bundle
by sending LACP packets to the peer. It performs a similar function as Port
Aggregation Protocol (PAgP) with Cisco EtherChannel. Because LACP is an IEEE
standard, it can be used to facilitate EtherChannels in mixed switch
environments.
Interface Modes
Interfaces can be set in any of
several modes to control EtherChannel formation.
This mode enables
Etherchannel:
-
On – This is the mode that forces the interface to channel without
PAgP or LACP.
The next two modes enable PAGP:
-
Auto – This PAgP mode places an interface in a passive negotiating
state in which the interface responds to the PAgP packets it receives, but it
does not initiate PAgP negotiation.
-
Desirable – This PAgP mode places an interface in an active
negotiating state in which the interface initiates negotiations with other
interfaces by sending PAgP packets. Interfaces configured in the on mode do not
exchange PAgP packets. The default mode for PAgP is auto mode.
The EtherChannel modes that use LACP are as follows:
-
Passive – This LACP mode places a port into a passive negotiating
state. In this state, the port responds to the LACP packets that it receives,
but it does not initiate LACP packet negotiation (default).
-
Active – This LACP mode places a port into an active negotiating
state. In this state, the port initiates negotiations with other ports by
sending LACP packets.
LACP Parameters
The following parameters are used in
configuring LACP:
-
System priority – Each switch running LACP must have a system
priority. The system priority can be specified automatically or through the
command-line interface (CLI). The switch uses the MAC address and the system
priority to form the system ID.
-
Port priority – Each port in the switch must have a port priority.
The port priority can be specified automatically or through the CLI. The port
priority and the port number form the port identifier. The switch uses the port
priority to decide which ports to put in standby mode when a hardware
limitation prevents all compatible ports from aggregating.
-
Administrative key – Each port in the switch must have an
administrative key value, which can be specified automatically or through the
CLI. The administrative key defines the ability of a port to aggregate with
other ports, determined by the following:
- The port physical characteristics, such as data rate, duplex capability,
and point-to-point or shared medium
- The configuration constraints that you establish
When enabled, LACP attempts to configure the maximum number of
compatible ports in a channel. In some instances LACP is not able to aggregate
all the ports that are compatible; for example the remote system might have
more restrictive hardware limitations. When this occurs, all the ports that
cannot be actively included in the channel are put in hot standby state and
used only if one of the channeled ports fails.