Higher Link Availability – No single link within the bundle will cause a failure of communications and resultant disruption to network traffic.
Increased Link Capacity – Multiple links forming the bundle provide aggregated bandwidth which is greater than that of a single link.
LACP conforms to the IEEE 802.3ad standard, while many Cisco devices also support the proprietary PAgP (Port Aggregation Protocol. Another term to describe both methods of port aggregation is Etherchannel, ports bundled together are said to form an Etherchannel. This technology is designed to be used for Switch to Switch connections, Server to Server connections and Switch to Server or Router connections.
Once an Etherchannel is formed using LACP, that trunk will perform in a similar manner to that as a single static trunk, albeit that it might contain 2.4 or even 8 physical static trunks.
This protocol can operate using 10/100Mbps Fast Ethernet ports or Gigabit Ethernet ports. In order to function as an aggregated trunk, LACP must be implemented and configured on both ends of a link between two switches.
On Cisco Ethernet switches, there are two options for automatically configuring and monitoring Etherchannel groups. The first is PAgP, and later Cisco adopted the standards-based LACP.
When the switch detects a number of LACP ports, the switchports are grouped together dynamically to form the Etherchannel, which are regarded as a single physical link by both versions of the Spanning-Tree protocol, both IEEE 802.1d and IEEE 802.1w.
LACP actually works by sending LACPDUs (Link Access Control Protocol Data Units down all links that have the protocol enabled. If a device is present on the other end of that link that also has LACP enabled then the two physical devices will detect each other and form a single logical link. LACP can be configured in one of two modes:
Active – where frames are always sent across the LACP configured links or
Passive – where the device with port in this mode will listen for LACPDUs from the active switch.
Up to 16 ports can be configured with a maximum of 8 ports used in an Etherchannel at any one time, and 4 ports when used with Gigabit Ethernet.
The operation of LACP is discussed on a number of training courses run by NSTUK Ltd, in some cases the training course will build on the theoretical description of LACP with practical demonstrations of its use.