3Com 3C16476BS-US - Baseline 2250 Plus Switch, User Manual

Page: 41 / 67

Using the System Tools 41
CAUTION: Do not interrupt power to the Switch
during the upgrade procedure. If you do, the soft-
ware may be corrupted and the Switch may not start
up properly afterwards.
Configuring the Spanning Tree
This administrative tool supports the configuration of
the Switch to forward, or block and discard 802.1D
spanning tree BPDU packets.
Spanning tree is a bridge-based system for providing
fault tolerance on networks and can be used to
detect and disable network loops. The spanning tree
ensures that the optimal path is maintained between
spanning tree-compliant networked devices by:
■
Disabling redundant paths when the main paths
are operational.
■
Enabling redundant paths if the main paths fail.
Spanning tree uses a distributed algorithm to select a
bridging device that serves as the root of the span-
ning tree network. The bridging device, known as the
Root Bridge, generates bridge protocol data units
(BPDUs) on all ports at a regular interval known as the
Hello Time. All other spanning tree-compliant devices
on the network have a designated Root Port. This is
the Port nearest the Root Bridge and it is used for
receiving the BPDUs initiated by the Root Bridge. If a
bridge does not get a Hello BPDU after a predeter-
mined interval, the bridge assumes that the link to the
Root Bridge is down. This bridge will then initiate
negotiations with other bridges to reconfigure the
network to reestablish a valid network topology.
After all the bridges on the network have determined
the configuration of their ports, each bridge only for-
wards traffic between the Root Port and the ports
that are the Designated Bridge Ports for each network
segment. All other ports are blocked, which means
that they are prevented from forwarding traffic.
Figure 27 Spanning Tree Screen
■
Forward – Spanning tree BPDU packets received on
a port are forwarded to all other ports
■
Block and Discard – Spanning tree BPDUs received
on a port are dropped. The Switch does not for-
ward BPDUs to other ports.
The Switch does not participate as a bridge node in
the spanning tree, it can only be configured to for-
ward or block spanning tree BPDUs. If the Switch is
connected to other bridging devices, such as
switches, that are part of the spanning tree network,
set the Switch to "Forward." If the Switch is con-
nected only to workstations, it can be set to "Block
and Discard."
802.1p Prioritization
The Switch has priority queuing enabled, which
means all packets that are received are examined to
see if they have been priority encoded. If a packet has