Reference Guide

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Link Aggregation Control Protocol (LACP)
Link aggregation control protocol (LACP) is supported on the S4820T platform.
Introduction to Dynamic LAGs and LACP
A link aggregation group (LAG), referred to as a port channel by FTOS, can provide both load-sharing and port
redundancy across line cards. You can enable LAGs as static or dynamic.
The benefits and constraints are basically the same, as described in Port Channel Interfaces in the Interfaces chapter.
The unique benefit of a dynamic LAG is that its ports can toggle between participating in the LAG or acting as dedicated
ports, whereas ports in a static LAG must be removed from the LAG in order to act alone.
The Dell Networking operating system (FTOS) uses LACP to create dynamic LAGs. LACP provides a standardized means
of exchanging information between two systems (also called Partner Systems) and automatically establishes the LAG
between the systems. LACP permits the exchange of messages on a link to allow their LACP instances to:
• Reach an agreement on the identity of the LAG to which the link belongs.
• Move the link to that LAG.
• Enable the transmission and reception functions in an orderly manner.
The FTOS implementation of LACP is based on the standards specified in the IEEE 802.3: “Carrier sense multiple access
with collision detection (CSMA/CD) access method and physical layer specifications.”
LACP functions by constantly exchanging custom MAC protocol data units (PDUs) across local area network (LAN)
Ethernet links. The protocol packets are only exchanged between ports that are configured as LACP capable.
Important Points to Remember
• LACP allows you to add members to a port channel (LAG) as long as it has no static members. Conversely, if the
LAG already contains a statically defined member (the channel-member command), the port-channel
mode
command is not permitted.
• A static LAG cannot be created if a dynamic LAG using the selected number exists.
• No dual membership in static and dynamic LAGs:
– If a physical interface is a part of a static LAG, the port-channel-protocol lacp command is
rejected on that interface.
– If a physical interface is a part of a dynamic LAG, it cannot be added as a member of a static LAG. The
channel-member gigabitethernet x/y command is rejected in the static LAG interface for
that physical interface.
• A dynamic LAG can be created with any type of configuration.
• There is a difference between the shutdown and no interface port-channel commands:
– The shutdown command on LAG “xyz” disables the LAG and retains the user commands. However,
the system does not allow the channel number “xyz” to be statically created.
– The no interface port-channel channel-number command deletes the specified LAG,
including a dynamically created LAG. This command removes all LACP-specific commands on the
member interfaces. The interfaces are restored to a state that is ready to be configured.
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