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Cisco ASR 901 Series Aggregation Services Router Software Configuration Guide
OL-23826-09
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 9-2 provides a summary of these combinations.
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
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.