Link aggregation
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Link aggregation, or IEEE 802.3ad, is a computer networking term which describes using multiple Ethernet network cables/ports in parallel to increase the link speed beyond the limits of any one single cable or port. Other terms for this include "Ethernet trunk", "NIC teaming", "port teaming", "port trunking", "EtherChannel" "NIC bonding", and "link aggregate group" (LAG). Most implementations now conform to clause 43 of IEEE 802.3 standard, informally referred to as "802.3ad".
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[edit] Trunking and the Network Backbone
Link aggregation is an inexpensive way to set up a high-speed backbone network that transfers much more data than any one single port or device can deliver. This allows several devices to communicate simultaneously at their full single-port speed while not allowing any one single device to monopolize all available backbone capacity.
Link aggregation also allows the network's backbone speed to grow incrementally as demands on the network increase without having to replace everything and buy all-new, more expensive hardware.
For most backbone installations, it is common to install more cabling or fiberoptic pairs than are initially necessary, even if there is no immediately projected need for the additional cabling. This is done because the cost of labor is much higher than that of the cable and avoids the need for a complete rewiring with more cable, if networking needs change. Link aggregation can allow the use of these extra cables to increase backbone speeds for little or no extra cost.
As of June 2006, 10-gigabit network equipment is becoming available but remains comparatively expensive. With trunking, a network backbone can be established with a speed of up to 8-gigabits while using the much more common 1-gigabit switches.
[edit] Trunk size and using ports efficiently
Trunking becomes an inefficient technology beyond a certain bandwidth depending on the total number of ports on the switch equipment. A 24-port gigabit switch with two 8-gigabit trunks is using sixteen of its available ports just for the two trunks, and leaves only eight of its 1-gigabit ports for other devices. This same configuration on a 48-port gigabit switch leaves 32 1-gigabit ports available, and so it is much more efficient (assuming of course that those ports are actually needed at the location the switch is sitting at).
When 40-50% of the switch ports are being utilized for backbone trunking, upgrading to a switch with either more ports or a higher base-operating speed may be a better option than simply adding more switches, especially if the old switch can be re-used elsewhere on a less performance critical part of the network.
[edit] Trunking of Network Interface Cards
Trunking is not just for the core switching equipment. Network interface cards (NICs) can also sometimes be trunked together to form network links beyond the speed of any one single NIC. For example, this allows a central file server to establish a 2-gigabit connection using two 1-gigabit NICs trunked together.
Note that establishing a trunk with NICs usually only works among certain NIC types, and all must usually be of the same brand. The trunk itself is typically established at the driver level, before the operating system gets involved. If trunking is desired for future expandability, it may be an idea to ensure that the first NIC will be able to trunk with others to be added later—on the other hand, it may be simpler to just buy new nics all at once for a trunking setup and re-use the old ones elsewhere.
[edit] Trunking of different types of cabling and speeds
Typically the ports used in a trunk should be all of the same type, such as all copper ports (CAT-5E/CAT-6), all multi-mode fiber ports (SX), or all single-mode fiber ports (LX).
The ports also need to operate at the same speed. It is possible to trunk 100-megabit ports together, but trunking a 100-megabit port and a gigabit port together will most likely not work, even though mixing port sizes within a trunk is technically supported in the 802.3ad standard.
[edit] Trunking support and cross-brand compatibility
A limitation on link aggregation is that an ethernet network must maintain the sequential order of ethernet packets. Different techniques have been used to maintain this order.
Most gigabit trunking is now based on clause 43 of the IEEE 802.3 standard added in March 2000 by the IEEE 802.3ad task force,[1]. Other proprietary trunking protocols existed before this standard was established. Some examples include Cisco Systems, Port Aggregation Protocol (PAgP), Adaptec's Duralink trunking, and Nortel MLT MultiLink Trunking. These custom trunking protocols typically only work for interconnecting equipment from the same manufacturer or product line.
Today, mostly all manufacturers keep the standard definition, so interoperability between devices is without problems, but still some issues can appear and testing is recommended.
Intel has released a package called Advanced Networking Services (ANS) to bind Intel Fast Ethernet and Gigabit cards.[2] for Linux. Also, newer Linux kernels support bonding between NICs of the same type.
