Users Guide

Switch Feature Overview 97
Link Aggregation Features
For information about configuring link aggregation (port-channel) features,
see "Link Aggregation" on page 1045.
Link Aggregation
Up to eight ports can combine to form a single Link Aggregation Group
(LAG). This enables fault tolerance protection from physical link disruption,
higher bandwidth connections and improved bandwidth granularity. LAGs
are formed from similarly configured physical links; i.e., the speed, duplex,
auto-negotiation, PFC configuration, DCBX configuration, etc., must be
compatible on all member links.
Per IEEE 802.1AX, only links with the identical operational characteristics,
such as speed and duplex setting, may be aggregated. Dell EMC Networking
N-Series switches aggregate links only if they have the same operational speed
and duplex setting, as opposed to the configured speed and duplex setting.
This allows operators to aggregate links that use auto-negotiation to set values
for speed and duplex or to aggregate ports with SFP+ technology operating at
a lower speed, e.g., 1G. Dissimilar ports will not become active in the LAG if
their operational settings do not match those of the first member of the LAG.
In practice, some ports in a LAG may auto-negotiate a different operational
speed than other ports depending on the far-end settings and any link
impairments. Per the above, these ports will not become active members of
the LAG. On a reboot or on flapping the LAG links, a lower-speed port may
be the first port selected to be aggregated into the LAG. In this case, the
higher-speed ports are not aggregated. Use the lacp port-priority command to
select one or more primary links to lead the formation of the aggregation
group.
While it is a requirement of a port-channel that the link members operate at
the same duplex and speed settings, administrators should be aware that
copper ports have larger latencies than fiber ports. If fiber and copper ports
are aggregated together, packets sent over the fiber ports would arrive
significantly sooner at the destination than packets sent over the copper
ports. This can cause significant issues in the receiving host (e.g., a TCP
receiver) as it would be required to buffer a potentially large number of out-