Fixed Switch Configuration Guide 11-1
11
Configuring Link Aggregation
This chapter describes how to configure link aggregation on the fixed switch platforms.
Link Aggregation Overview
IEEE 802.3ad link aggregation provides a standardized means of grouping multiple parallel
Ethernet interfaces into a single logical Layer 2 link. The formed group of Ethernet interfaces is
referred to as a Link Aggregation Group (LAG). Dynamic LAG formation and activation is
provided by the Link Aggregation Control Protocol (LACP).
Each pair of LAG physical ports is made up of a local port on the device responsible for LACP
negotiation, referred to as the actor, and its directly linked remote port on the device participating
in the LACP negotiation, referred to as the partner. LAGs form automatically based upon a set of
criteria (see “How a LAG Forms” on page 11-3).
Only LAG members in the attached state carry user traffic. Once the LAG is formed, the system
ID, made up of a system priority and the device MAC address, determines which device will be in
charge of choosing the LAG port members that will be moved to the attached state. While port
speed is not a criteria for joining a LAG, the port speed must match for all ports that are placed in
the LACP attached state. Aggregatable ports not selected to carry traffic for this LAG are available
to the next LAG as long as LAG resources are not depleted. Should LAG resources become
depleted, aggregatable ports are placed in LACP standby state.
802.3ad LACP aggregations can be run between combinations of switches, routers, and edge
devices, such as a server, that support LACP.
Using Link Aggregation in a Network
The concept of grouping multiple ports into a single link is not a new idea. Cabletron's
SmartTrunk, Cisco's Inter Switch Link trunking, and Adaptec's Duralink are previous examples.
The problem with these older methods, from the network administrators point of view, is that
they are proprietary. Administrators who wanted to implement faster logical links faced major
For information about... Refer to page...
Link Aggregation Overview 11-1
Configuring Link Aggregation 11-9
Link Aggregation Configuration Example 11-11
Terms and Definitions 11-15
Note: Earlier (proprietary) implementations of port aggregation referred to groups of aggregated
ports as “trunks”.
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