Revision as of 06:31, 1 May 2009

Link to CCNPv5 semester 3 chapter 3

Standards

There are two different EtherChannl standards

• PAgP - Port Aggregation Protocol
  • Cisco proprietary protocol.
• LACP - Link Aggregation Protocol
  • IEEE standard. 802.3ad

PAgP

PAgP packets are sent between Fast EtherChannel-capable ports to negotiate the forming of a channel. When PAgP identifies matched Ethernet links, it groups the links into an EtherChannel. The EtherChannel is then added to the spanning tree as a single bridge port.

PAgP manages EtherChannel. PAgP packets are sent every 30 seconds using multicast group MAC address 01-00-0C-CC-CC-CC with protocol value 0x0104. PAgP checks for configuration consistency and manages link additions and failures between two switches. It ensures that when an EtherChannel is created that all ports have the same type of configuration, because it is mandatory that all ports have the same speed, duplex setting, and VLAN information. Any port modification after the creation of the channel will also change all the other channel ports.

The last component of EtherChannel is the creation of the logical port. The logical port, or Agport, is composed of all the ports that make up the EtherChannel. The Agport’s functionality and behavior are no different than any other port. For instance, the spanning tree algorithm treats Agport as a single port.

LACP

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 PAgP with Cisco EtherChannel. Because LACP is an IEEE standard, it can be used to facilitate EtherChannels in mixed-switch environments.

Interfaces can be set in any of several modes to control EtherChannel formation.

The following parameters are used in configuring LACP:

• System priority
  • Each switch running LACP must have a system priority, which can be specified automatically or through the 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, which 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’s physical characteristics, such as data rate, duplex capability, and point-to-point or shared medium
    • The configuration constraints that you establish

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.

EtherChannel configuration modes