Basic

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.

EtherChannel can be implemented by grouping multiple physical ports into one or more logical EtherChannel links.

Note: Interface types cannot be mixed; for example, Fast Ethernet and Gigabit Ethernet cannot be mixed within a single EtherChannel.

Advantages:

  • 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. Depending on the hardware platform, one or more load-balancing methods can be implemented. These methods include source MAC to destination MAC load balancing, or source IP to destination IP load balancing, across the physical links.
  • 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, nor lose connectivity as long as at least one physical link is up; therefore a spanning tree recalculation is not required.

Limitations:

  • Currently each EtherChannel can consist of up to 8 compatibly-configured Ethernet ports. The Cisco IOS switch can currently support six EtherChannels.
  • Traffic cannot be sent to two different switches through the same EtherChannel link.

 

Requirement:

  • 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.
  • 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.

 

Protocols

 

PAgP( port aggregation protocols)

 

Cisco-proprietary protocol that aids in the automatic creation of EtherChannel links.

When an EtherChannel link is configured using PAgP, PAgP packets are sent between EtherChannel-capable ports to negotiate the forming of a channel.

  • When PAgP identifies matched Ethernet links, it groups the links into an EtherChannel.
    PAgP packets are sent every 30 seconds. PAgP checks for configuration consistency and manages link additions and failures between two switches. It ensures that when an EtherChannel is created, all ports have the same type of configuration.
  • The EtherChannel is then added to the spanning tree as a single port.

Note: In EtherChannel, it is mandatory that all ports have the same speed, duplex setting, and VLAN information.

Modes for PAgP.

  • 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

modes nego

 

  • If all modes are disabled by using the no command, or if no mode is configured, then the EtherChannel is disabled.
  • *The on mode manually places the interface in an EtherChannel, without any negotiation. It works only if the other side is also set to on. If the other side is set to negotiate parameters through PAgP, no EtherChannel forms, because the side that is set to on mode does not negotiate.
  • If one side is configure to be in auto mode, it is placed in a passive state, waiting for the other side to initiate the EtherChannel negotiation. If the other side is also set to auto, the negotiation never starts and the EtherChannel does not form.

 

LACP

 

LACP is part of an IEEE specification (802.3ad) that allows several physical ports to be bundled to form a single logical channel. LACP allows a switch to negotiate an automatic bundle by sending LACP packets to the peer.

Note: LACP was originally defined as IEEE 802.3ad. However, LACP is now defined in the newer IEEE 802.1AX standard for local and metropolitan area networks.
Can be used to facilitate EtherChannels in multivendor environments. On Cisco devices, both protocols are supported.

Modes for LACP.

  • 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.

LACP modes

 

lacp max-bundle

To define the maximum number of bundled Link Aggregation Control Protocol (LACP) ports allowed in a port channel, use the lacp max-bundle command in interface configuration mode. To return to the default settings, use the no form of this command.

lacp max-bundle max-bundles

no lacp max-bundle

 

Syntax Description
 

max-bundles

Maximum number of bundled ports allowed in the port channel. Valid values are from 1 to 8.

Note On the Cisco 10000 series router, the valid values are from 1 to 4.

The default settings:

 

•Maximum of eight bundled ports per port channel.

•Maximum of eight bundled ports and eight hot-standby ports per port channel; this setting applies if the port channel on both sides of the LACP bundle are configured the same.

 

Configuration

 

Requirement:

  • EtherChannel support – All Ethernet interfaces on all modules must support EtherChannel with no requirement that interfaces be physically contiguous, or on the same module.
  • Speed and duplex – Configure all interfaces in an EtherChannel to operate at the same speed and in the same duplex mode, as shown in the figure.
  • VLAN match – All interfaces in the EtherChannel bundle must be assigned to the same VLAN, or be configured as a trunk (also shown in the figure).
  • Range of VLAN – 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 set to auto or desirable mode.

After the port channel interface is configured,

  • Any configuration that is applied to the port channel interface also affects individual interfaces.
  • However, configurations that are applied to the individual interfaces do not affect the port channel interface.
    Therefore, making configurati`on changes to an interface that is part of an EtherChannel link may cause interface compatibility issues.

Procedure:

Step 1. Specify the interfaces that compose the EtherChannel group using the 

s1(config)#interface range interface

global configuration mode command. The range keyword allows you to select several interfaces and configure them all together.
A good practice is to start by shutting down those interfaces, so that any incomplete configuration does not create activity on the link.

Step 2. Create the port channel interface with the 

s1(config-if-range)# channel-group identifier 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.

Note: EtherChannel is disabled by default.

Step 3. select the channel and configure the channel

interface port-channel identifier

Remove: 

no interface port-channel identifier
Verification
  1. show interface port-channel command displays the general status of the port channel interface ( not apply on PT)
  2. show etherchannel summary command to simply display one line of information per port channel
    The group is a Layer 2 EtherChannel and that it is in use, as indicated by the letters SU next to the port channel number.
  3. show etherchannel port-channel command to display information about a specific port channel interface
  4. show interfaces etherchannel provide information about the role of the interface in the EtherChannel

 

Troubleshooting
  • Assign all ports in the EtherChannel to the same VLAN, or configure them as trunks. Ports with different native VLANs cannot form an EtherChannel.
  • When configuring an EtherChannel from trunk ports, verify that the trunking mode is the same on all the trunks. Inconsistent trunk modes on EtherChannel ports can cause EtherChannel not to function and ports to be shut down (errdisable state).
  • An EtherChannel supports the same allowed range of VLANs on all the ports. If the allowed range of VLANs is not the same, the ports do not form an EtherChannel even when PAgP is set to the auto or desirable mode.
  • The dynamic negotiation options for PAgP and LACP must be compatibly configured on both ends of the EtherChannel.

Note: It is easy to confuse PAgP or LACP with DTP, because they both are protocols used to automate behavior on trunk links. PAgP and LACP are used for link aggregation (EtherChannel). DTP is used for automating the creation of trunk links. When an EtherChannel trunk is configured, typically EtherChannel (PAgP or LACP) is configured first and then DTP.

troubleshooting EtherChannel1 troubleshooting EtherChannel2

Note: EtherChannel and spanning tree must interoperate. For this reason, the order in which EtherChannel-related commands are entered is important, which is why you see interface Port-Channel 1 removed and then re-added with the channel-group command, as opposed to directly changed. If one tries to change the configuration directly, spanning tree errors cause the associated ports to go into blocking or errdisabled state.