MultiLink Trunking (MLT) with Link Aggregation Control Protocol (LACP) manages ports and port memberships to form a link aggregation group (LAG). Use Link Aggregation Control Protocol to gather one or more links to form a LAG, which a Media Access Control (MAC) client treats as a single link. Link Aggregation Control Protocol can dynamically add or remove LAG ports, depending on availability and state.
The IEEE 802.3ad standard comprises service interfaces, the LACP, the Marker Protocol, link aggregation selection logic, a parser or multiplexer, frame distribution, and frame collection functions.
The following illustration shows the major functions of IEEE 802.3ad defined as multiple link aggregation.
The link aggregation sublayer comprises the following functions:
frame distribution
This block takes frames submitted by the MAC client and sends them for transmission on the appropriate port based on a frame distribution algorithm employed by the Frame Distributor.
Frame distribution also includes an optional Marker Generator/Receiver used for the Marker Protocol. The switch only implements the Marker Receiver function. For more information about the frame distribution algorithm, see MLT traffic distribution algorithm.
frame collection
This block passes frames received from the various ports to the MAC client. Frame collection also includes a Marker Responder used for the Marker Protocol.
aggregator parser or multiplexers
During transmission operations, these blocks pass frame transmission requests from the Distributor, Marker Generator, and Marker Responder to the appropriate port.
During receive operations, these blocks distinguish among Marker Request, Marker Response, MAC Client Protocol Data Units (PDU), and pass the blocks to the appropriate entity (Marker Responder, Marker Receiver, and Collector, respectively).
aggregator
The combination of frame distribution and collection, and aggregator parser or multiplexers.
aggregation control
This block configures and controls link aggregation. It incorporates LACP for the automatic communication of aggregation capabilities between systems and automatic configuration of link aggregation.
control parser/multiplexers
During transmission operations, these blocks pass frame transmission requests from the aggregator and Control entities to the appropriate port.
During receive operations, these blocks distinguish Link Aggregation Control Protocol Data Units (LACPDUs) from other frames. The blocks pass, passing the LACPDUs to the appropriate sublayer entity and all other frames to the aggregator.
Use link aggregation to group ports together to form a link group to another device. Link groups increase aggregate throughput between devices and provide link redundancy.
Link aggregation employs the following principles and concepts:
A MAC client communicates with a set of ports through an aggregator, which presents a standard IEEE 802.3 service interface to the MAC client. The aggregator binds to one or more ports within a system.
The aggregator distributes frame transmissions from the MAC client to various ports, collects received frames from the ports, and transparently passes the frames to the MAC client.
A system can contain multiple aggregators serving multiple MAC clients. A port binds to a single aggregator at a time. A MAC client is served by a single aggregator at a time.
The Link Aggregation Control function binds ports to aggregators within a system. The control function aggregates links, binds the system ports to an appropriate aggregator, and monitors conditions to determine if a change in aggregation is needed. Network managers can manually provide link aggregation control by manipulating the link aggregation state variables (for example, keys). You can also use LACP to automatically determine, configure, bind, and monitor link aggregation.
LACP uses peer exchanges across links to continually determine the aggregation capability of the links and provide the maximum level of aggregation capability between a pair of systems.
Frame ordering is maintained for certain sequences of frame exchanges between MAC Clients. The distributor ensures that all frames of a conversation pass to a single port. The collector passes frames to the MAC client in the order they are received from the port. The collector can select frames received from the aggregated ports. Because the frames are not ordered on a single link, this guarantees that frame ordering is maintained for all conversations.
Conversations move among ports within an aggregation for load balancing and for maintaining availability if a link fails.
Each port is assigned a unique, globally administered MAC address.
After entities initiate frame exchanges within the link aggregation sublayer, the source address is the MAC address. An example of an entity that initiates frame exchanges is LACP and Marker Protocol exchanges.
Each aggregator is assigned a unique, globally administered MAC address that is used from the perspective of the MAC client, both as a source address for transmitted frames and as the destination address for received frames. You can use one of the port MAC addresses in the associated LAG as the MAC address of the aggregator.