EtherChannel

Link Aggregation

  • There are scenarios in which more bandwidth or redundancy between devices is needed than what can be provided by a single link. Multiple links could be connected between devices to increase bandwidth. However, Spanning Tree Protocol (STP), which is enabled on Layer 2 devices like Cisco switches by default, will block redundant links to prevent switching loops.
  • A link aggregation technology is needed that allows redundant links between devices that will not be blocked by STP. That technology is known as EtherChannel.
  • EtherChannel is a link aggregation technology that groups multiple physical Ethernet links together into one single logical link. It is used to provide fault-tolerance, load sharing, increased bandwidth, and redundancy between switches, routers, and servers.
  • EtherChannel technology makes it possible to combine the number of physical links between the switches to increase the overall speed of switch-to-switch communication.

EtherChannel

EtherChannel technology was originally developed by Cisco as a LAN switch-to-switch technique of grouping several Fast Ethernet or Gigabit Ethernet ports into one logical channel.

When an EtherChannel is configured, the resulting virtual interface is called a port channel. The physical interfaces are bundled together into a port channel interface, as shown in the figure.

Advantages of EtherChannel

EtherChannel technology has many advantages, including the following:

  • Most configuration tasks can be done on the EtherChannel interface instead of on each individual port, ensuring configuration consistency throughout the links.
  • EtherChannel relies on existing switch ports. There is no need to upgrade the link to a faster and more expensive connection to have more bandwidth.
  • Load balancing takes place between links that are part of the same EtherChannel.
  • EtherChannel creates an aggregation that is seen as one logical link. When several EtherChannel bundles exist between two switches, STP may block one of the bundles to prevent switching loops. When STP blocks one of the redundant links, it blocks the entire EtherChannel. This blocks all the ports belonging to that EtherChannel link. Where there is only one EtherChannel link, all physical links in the EtherChannel are active because STP sees only one (logical) link.
  • EtherChannel provides redundancy because the overall link is seen as one logical connection. Additionally, the loss of one physical link within the channel does not create a change in the topology.

Implementation Restrictions

EtherChannel has certain implementation restrictions, including the following:

  • Interface types cannot be mixed. For example, Fast Ethernet and Gigabit Ethernet cannot be mixed within a single EtherChannel.
  • Currently each EtherChannel can consist of up to eight compatibly-configured Ethernet ports. EtherChannel provides full-duplex bandwidth up to 800 Mbps (Fast EtherChannel) or 8 Gbps (Gigabit EtherChannel) between one switch and another switch or host.
  • The Cisco Catalyst 2960 Layer 2 switch currently supports up to six EtherChannels.
  • The individual EtherChannel group member port configuration must be consistent on both devices. If the physical ports of one side are configured as trunks, the physical ports of the other side must also be configured as trunks within the same native VLAN. Additionally, all ports in each EtherChannel link must be configured as Layer 2 ports.
  • Each EtherChannel has a logical port channel interface. A configuration applied to the port channel interface affects all physical interfaces that are assigned to that interface.

PAgP Operation

PAgP helps create the EtherChannel link by detecting the configuration of each side and ensuring that links are compatible so that the EtherChannel link can be enabled when needed. The modes for PAgP as follows:

  • On– This mode forces the interface to channel without PAgP. Interfaces configured in the on mode do not exchange PAgP packets.
  • PAgP 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.
  • PAgP auto– This PAgP mode places an interface in a passive negotiating state in which the interface responds to the PAgP packets that it receives but does not initiate PAgP negotiation.

LACP Operation

LACP provides the same negotiation benefits as PAgP. LACP helps create the EtherChannel link by detecting the configuration of each side and making sure that they are compatible so that the EtherChannel link can be enabled when needed. The modes for LACP are as follows:

  • On– This mode forces the interface to channel without LACP. Interfaces configured in the on mode do not exchange LACP packets.
  • LACP active– This LACP mode places a port in an active negotiating state. In this state, the port initiates negotiations with other ports by sending LACP packets.
  • LACP passive– This LACP mode places a port in a passive negotiating state. In this state, the port responds to the LACP packets that it receives but does not initiate LACP packet negotiation.

Configuration Guidelines

The following guidelines and restrictions are useful for configuring EtherChannel:

  • EtherChannel support– All Ethernet interfaces must support EtherChannel with no requirement that interfaces be physically contiguous.
  • Speed and duplex– Configure all interfaces in an EtherChannel to operate at the same speed and in the same duplex mode.
  • VLAN match– All interfaces in the EtherChannel bundle must be assigned to the same VLAN or be configured as a trunk (shown in the figure).
  • Range of VLANs– An EtherChannel supports the same allowed range of VLANs on all the interfaces in a trunking EtherChannel. If the allowed range of VLANs is not the same, the interfaces do not form an EtherChannel, even when they are set to autoor desirable mode.

LACP Configuration Example

Configuring EtherChannel with LACP requires the following three steps:

  • Step 1.Specify the interfaces that compose the EtherChannel group using the interface rangeinterface global configuration mode command. The range keyword allows you to select several interfaces and configure them all together.
  • Step 2.Create the port channel interface with the channel-groupidentifier mode active command in interface range configuration mode. The identifier specifies a channel group number. The mode active keywords identify this as an LACP EtherChannel configuration.
  • Step3.To change Layer 2 settings on the port channel interface, enter port channel interface configuration mode using the interface port-channelcommand, followed by the interface identifier. In the example, S1 is configured with an LACP EtherChannel. The port channel is configured as a trunk interface with the allowed VLANs specified.

Common Issues with EtherChannel Configurations

All interfaces within an EtherChannel must have the same configuration of speed and duplex mode, native and allowed VLANs on trunks, and access VLAN on access ports. Ensuring these configurations will significantly reduce network problems related to EtherChannel. Common EtherChannel issues include the following:

  • Assigned ports in the EtherChannel are not part of the same VLAN, or not configured as trunks. Ports with different native VLANs cannot form an EtherChannel.
  • Trunking was configured on some of the ports that make up the EtherChannel, but not all of them. It is not recommended that you configure trunking mode on individual ports that make up the EtherChannel. When configuring a trunk on an EtherChannel, verify the trunking mode on the EtherChannel.
  • If the allowed range of VLANs is not the same, the ports do not form an EtherChannel even when PAgP is set to the autoor desirablemode.
  • The dynamic negotiation options for PAgP and LACP are not compatibly configured on both ends of the EtherChannel.

Verify EtherChannel

  • The show interfaces port-channelcommand displays the general status of the port channel interface. 
  • The show etherchannel summarycommand displays one line of information per port channel.
  • The show etherchannel port-channelcommand displays information about a specific port channel interface.
  • The show interfaces etherchannelcommand can provide information about the role of a physical member interface of the EtherChannel.
Configure Etherchannel LAb
Troubleshoot Ethernchannel