EtherChannels

EtherChannel is a way of combining several physical links between switches into one logical connection. Normally, Spanning Tree blocks redundant links; EtherChannel gets around that and allows load balancing across those links. Load is balancing on the basis of such things as source or destination MAC address or IP address. The Etherchannel load-balancing method is configured at global configuration mode.

(config)#port-channel load-balance type

A logical interface—the Port Channel interface—is created. Configuration can be applied to both the logical and physical interfaces.

Some guidelines for EtherChannels are as follows:

• Interfaces in the channel do not have to be physically next to each other or on the same module.

• All ports must be the same speed and duplex.

• All ports in the bundle should be enabled.

• None of the bundle ports can be a SPAN port.

• Assign an IP address to the logical Port Channel interface, not the physical ones.

• Put all bundle ports in the same VLAN, or make them all trunks.

• If they are trunks, they must all carry the same VLANs and use the same trunking mode.

• Configuration you apply to the Port Channel interface affects the entire EtherChannel. Configuration you apply to a physical interface only affects that interface.

Configuring an EtherChannel

Basically, for a Layer 3 EtherChannel, you should configure the logical interface and then put the physical interfaces into the channel group:

(config)#interface port-channel number
(config-if)#no switchport
(config-if)#ip address address mask

Then, at each port that is part of the EtherChannel, use the following:

(config)#interface { number | range interface – interface}
(config-if)#channel-group number mode {auto | desirable | on}

Putting the IP address on the Port Channel interface creates a Layer 3 EtherChannel. Simply putting interfaces into a channel group creates a Layer 2 EtherChannel, and the logical interface is automatically created.

The Cisco proprietary Port Aggregation Protocol (PAgP) dynamically negotiates the formation of a channel. There are three PAgP modes:

• On—The port channels without using PAgP negotiation. The port on the other side must also be set to On.

• Auto—Responds to PAgP messages but does not initiate them. Port channels if the port on the other end is set to Desirable. This is the default mode.

• Desirable—Port actively negotiates channeling status with the interface on the other end of the link. Port channels if the other side is Auto or Desirable.

There is also a non-proprietary protocol called Link Aggregation Control Protocol (LACP), IEEE 802.3ad, which does the same thing. LACP has two modes:

• Active—Port actively negotiates channeling with the port on the other end of the link. A channel forms if the other side is Passive or Active.

• Passive—Responds to LACP messages but does not initiate them. A channel forms if the other end is set to Active.

If you want to use LACP, specify it under the interface and put the interface in either active or passive mode:

(config-if)#channel-protocol lacp


Verifying an EtherChannel

Some typical commands for verifying include:

• #show running-config interface number

• #show interfaces number etherchannel

• #show etherchannel number port-channel

• #show etherchannel summary