12-4 | Cisco Systems 7600 guide

Chapter 12 Configuring EtherChannels

Understanding How EtherChannels Work

Understanding IEEE 802.3ad LACP EtherChannel Configuration

LACP supports the automatic creation of EtherChannels by exchanging LACP packets between LAN ports. LACP packets are exchanged only between ports in passive and active modes.

The protocol learns the capabilities of LAN port groups dynamically and informs the other LAN ports. Once LACP identifies correctly matched Ethernet links, it facilitates grouping the links into an EtherChannel. The EtherChannel is then added to the spanning tree as a single bridge port.

Both the passive and active modes allow LACP to negotiate between LAN ports to determine if they can form an EtherChannel, based on criteria such as port speed and trunking state. Layer 2 EtherChannels also use VLAN numbers.

LAN ports can form an EtherChannel when they are in different LACP modes as long as the modes are compatible. For example:

•A LAN port in active mode can form an EtherChannel successfully with another LAN port that is in active mode.

•A LAN port in active mode can form an EtherChannel with another LAN port in passive mode.

•A LAN port in passive mode cannot form an EtherChannel with another LAN port that is also in passive mode, because neither port will initiate negotiation.

LACP uses the following parameters:

•LACP system priority—You must configure an LACP system priority on each router running LACP. The system priority can be configured automatically or through the CLI (see the “Configuring the LACP System Priority and System ID” section on page 12-10). LACP uses the system priority with the router MAC address to form the system ID and also during negotiation with other systems.

Note The LACP system ID is the combination of the LACP system priority value and the MAC address of the router.

•LACP port priority—You must configure an LACP port priority on each port configured to use LACP. The port priority can be configured automatically or through the CLI (see the “Configuring Channel Groups” section on page 12-8). LACP uses the port priority with the port number to form the port identifier. LACP uses the port priority to decide which ports should be put in standby mode when there is a hardware limitation that prevents all compatible ports from aggregating.

•LACP administrative key—LACP automatically configures an administrative key value equal to the channel group identification number on each port configured to use LACP. The administrative key defines the ability of a port to aggregate with other ports. A port’s ability to aggregate with other ports is determined by these factors:

–Port physical characteristics, such as data rate, duplex capability, and point-to-point or shared medium

–Configuration restrictions that you establish

On ports configured to use LACP, LACP tries to configure the maximum number of compatible ports in an EtherChannel, up to the maximum allowed by the hardware (eight ports). If LACP cannot aggregate all the ports that are compatible (for example, the remote system might have more restrictive hardware limitations), then all the ports that cannot be actively included in the channel are put in hot standby state and are used only if one of the channeled ports fails. You can configure an additional 8 standby ports (total of 16 ports associated with the EtherChannel).

Cisco 7600 Series Router Cisco IOS Software Configuration Guide, Release 12.2SX

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