Link Aggregation on the Switch
ExtremeWare XOS 11.3 Concepts Guide
119
Link Aggregation on the Switch
The link aggregation (also known as load sharing) feature allows you to increase bandwidth and
availability by using a group of ports to carry traffic in parallel between switches. Load sharing, link
aggregation, and trunking are terms that have been used interchangeably in Extreme Networks
documentation to refer to the same feature, which allows multiple physical ports to be aggregated into
one logical port, or link aggregation group (LAG). Refer to IEEE 802.3ad for more information on this
feature. The advantages to link aggregation include an increase in bandwidth and link redundancy.
NOTE
All ports in a LAG must be running at the same speed and duplex setting. Each port can belong to only one LAG.
Load sharing allows the switch to use multiple ports as a single logical port, or LAG. For example,
VLANs see the LAG as a single logical port. And, although you can only reference the master port of a
LAG to a Spanning Tree Domain (STPD), all the ports of the LAG actually belong to the specified STPD.
Most load-sharing algorithms guarantee packet sequencing between clients.
Link aggregation, or load sharing, is disabled by default.
If a port in a load-sharing group (or LAG) fails, traffic is redistributed to the remaining ports in the
LAG. If the failed port becomes active again, traffic is redistributed to include that port.
NOTE
Load sharing must be enabled on both ends of the link, or a network loop may result.
Link aggregation is most useful when:
â—Ź The egress bandwidth of traffic exceeds the capacity of a single link.
â—Ź Multiple links are used for network resiliency.
In both situations, the aggregation of separate physical links into a single logical link multiplies total
link bandwidth in addition to providing resiliency against individual link failures.
In modular switches, ExtremeWare XOS supports LAGs across multiple modules, so resiliency is also
provided against individual module failures.
The software supports control protocols across the LAGs, both static and dynamic. If you add the
protocols (for example, EAPS, ESRP, and so forth) to the port and then create a LAG on that port, you
may experience a slight interruption in the protocol operation. In order to seamlessly add or delete
bandwidth when running control protocols, Extreme Networks recommends that you create a LAG
consisting of only one port. Then add your protocols to that port and add other ports as needed.
VMAN ports can belong to LAGs. If any port in the LAG is enabled for VMAN, all ports in the group
are automatically enabled to handle jumbo size frames on the BlackDiamond 10K switch; you must
enable jumbo frames on the BlackDiamond 8800 family of switches and the Summit X450 switch. Also,
VMAN is automatically enabled on all ports of the untagged LAG.
You can run the Link Layer Discovery Protocol (LLDP) on ports in a LAG.
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