Switch Features 75
Bridge Protocol Data Unit (BPDU) Guard
Spanning Tree BPDU Guard is used to disable the port in case a new device
tries to enter the already existing topology of STP. Thus devices, which were
originally not a part of STP, are not allowed to influence the STP topology.
BPDU Filtering
When spanning tree is disabled on a port, the BPDU Filtering feature allows
BPDU packets received on that port to be dropped. Additionally, the BPDU
Filtering feature prevents a port in Port Fast mode from sending and receiving
BPDUs. A port in Port Fast mode is automatically placed in the forwarding
state when the link is up to increase convergence time.
Link Aggregation Features
For information about configuring link aggregation (port-channel) features,
see "Configuring Link Aggregation" on page 791.
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.
Per IEEE 802.1AX, only links with the same operational characteristics, such
as speed and duplex setting, may be aggregated. PowerConnect 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. Dissimilar ports will not become active in the LAG if their
operational settings do not match those of the first member of the LAG.
PowerConnect switches also support setting the MTU on a LAG. When a link
becomes active in a LAG, its MTU is dynamically changed to the LAG MTU.
When the link leaves the LAG, its MTU reverts to the link setting.
Link Aggregate Control Protocol (LACP)
Link Aggregate Control Protocol (LACP) uses peer exchanges across links to
determine, on an ongoing basis, the aggregation capability of various links,
and continuously provides the maximum level of aggregation capability
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