Increasing bandwidth is a constant battle faced by every IT department. What happens when a high capacity server is hitting the limits of a single Gigabit link. What happens when 10G is not an option, but 1G is not enough. Simply connecting another cable is not the answer. The result will be a total loss of the network or a nice workout for your spanning tree configuration. The answer comes from the 802.3ad specification, or Ling Aggregation, LAG for short.
LAG enables multiple Ethernet links to become a single logical interface much like the MLPPP configuration for TDM circuits. By combining multiple interfaces into a single logical group the effective bandwidth can be up to 8 times that of a single interface. All major switch vendors support the IEEE 802.3ad standard and it’s sub-protocols. Some vendors add thier own tweaks to make it better when running a single vendor network. In general it works the same, one port is typically the master by which all configuration is managed. Up to 7 additional links are added to the group. Each end of the group sees the links as one large pipe and based on algorithms internal to the Ethernet interface traffic is distributed across the links.
In some scenarios this could be used for redundancy. Although there may be better options depending on the vendor, LAG is an OK choice with LACP implemented for dynamic control of the links. Failover can take 30 to 40 seconds in some cases, but is typically much faster, and can be as little several milliseconds.
Configuration is quite simple. Lets look at the major vendors and the commands needed to implement. In each example port 24 will be the master and 21-23 will be added to the group.
Extreme Networks XOS – reference Concepts and Examples Guide pg 205
enable sharing 24 grouping 21,22,23 L2 lacp
Juniper EX-4200 JUNOS 9.2 – reference JUNOS 9.2 Docs
set aggregated-devices device-count 4
set ae0 aggregated-ether-options minimum-links 1
set ae0 aggregated-ether-options link-speed 4g
set xe-24/1/0 ether-options 802.ad ae0
set xe-23/1/0 ether-options 802.ad ae0
set xe-22/1/0 ether-options 802.ad ae0
set xe-21/1/0 ether-options 802.ad ae0
set ae0 unit 0 family inet address 220.127.116.11/25
[edit interfaces ae0 aggregated-ether-options]
set ae 0 aggregated-ether-options lacp active
set ae 0 aggregated-ether-options lacp periodic fast
Adtran AOS -reference AOS Command Reference