Dell N1100-ON, Reference Manual

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Layer 2 Switching Commands
618
Port Channel Commands
Dell EMC Networking N1100-ON/N1500/N2000/N2100-
ON/N3000/N3100-ON/N4000 Series Switches
A port channel is a set of one or more links that can be aggregated together to
form a bonded channel (Link Aggregation Group or LAG or port channel).
Individual conversations in a particular direction always travel over a single
link in the port channel, however, in aggregate, the bandwidth usage of all of
the links is fairly evenly distributed. Port channels have the advantage of
allowing incremental bandwidth to be added as needed (by adding additional
links) and supporting a form of fault tolerance (traffic on failed links is
redistributed among other links in the LAG). 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 same operational characteristics,
such as speed and duplex setting, may be aggregated. Dell EMC Networking
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 speeds, 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 link to lead the formation of the aggregation
group.
While it is a requirement of a LAG 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 may 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-of-order frames. Devices