Configuring EtherChannel
Port Aggregation Protocol – PAgP and Link Aggregation Control Protocol – LACP

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.