Cisco Systems A9014CFD manual Router a Router B Result, Passive mode Active mode

Chapter 9 Configuring EtherChannels

Understanding How EtherChannels Work

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.

Table 9-2provides a summary of these combinations.

Table 9-2	LACP EtherChannel Modes
Router A		Router B	Result
passive mode		passive mode	No EtherChannel group is created.
passive mode		active mode	EtherChannel group is created.
active mode		passive mode	EtherChannel group is created.
active mode		active mode	EtherChannel group is created.

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 command line interface (CLI) (see the “Configuring the LACP System Priority and System ID” section on page 9-6). 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 9-5). 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.

Note Port priority is only effective when it is configured on a device with an LACP system priority higher than the peer.

•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

Cisco ASR 901 Series Aggregation Services Router Software Configuration Guide

OL-23826-09 9-3