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of message loops in switched networks. A spanning tree uses a spanning tree algorithm
(STA) to automatically sense whether a switch has more than one way to communicate
with a node. It will then select the best path (primary), and block the other path(s). It will
also keep track of the blocked path(s) in case the primary path fails. Spanning Tree
Protocol (STP) introduced a standard method to accomplish this. It is specified in IEEE
802.1D-1998. Later, Rapid Spanning Tree Protocol (RSTP) was adopted and represents
the evolution of STP, providing much faster spanning tree convergence after a topology
change. This is specified in IEEE 802.1w. In 2004, 802.1w is included into 802.1D-2004
version. This switch supports both RSTP and STP (all switches that support RSTP are also
backward compatible with switches that support only STP).
Bridge Configuration
Priority (0-61440): RSTP uses bridge ID to determine the root bridge, the bridge with the
highest bridge ID becomes the root bridge. The bridge ID is composed of bridge priority
and bridge MAC address. So that the bridge with the highest priority becomes the highest
bridge ID. If all the bridge ID has the same priority, the bridge with the lowest MAC address
will then become the root bridge.
Note: The bridge priority value must be in multiples of 4096. A device with a lower number
has a higher bridge priority. Ex: 4096 is higher than 32768.
Note: The Web GUI allows user select the priority number directly. This is the convinent of
the GUI design. When you configure the value through the CLI or SNMP, you may need to
type the value directly. Please follow the n x 4096 ruls for the Bridge Priority.
Max Age (6-40): Enter a value from 6 to 40 seconds here. This value represents the time
that a bridge will wait without receiving Spanning Tree Protocol configuration messages
before attempting to reconfigure.
If the managed Switch is not the root bridge, and if it has not received a hello message
from the root bridge in an amount of time equal to Max Age, then the Managed Switch will
reconfigure itself as a root bridge. Once two or more devices on the network are
recognized as a root bridge, the devices will renegotiate to set up a new spanning tree
topology.
The MAX Age value affects the maximum volume of the RSTP loop. In the RSTP BPDU
packet, there is one field, message age which start from 0, add 1 after passed one hop in
the RSTP loop. When the message age is larger than MAX Age, the BPDU would be
ignored and the lower switches are separated to different RSTP domain. The switches in
other RSTP domain can’t be managed through upper switch.
Since different RSTP aware switches may have their own mechanism to calculate the
message age. So that this is most possibly occurred when interoperate different vendors’
RSTP aware switches together. The maximum volume of the RSTP domain is 23, configure
the MAX Age lower than 23 is recommended.
Hello Time (1-10): Enter a value from 1 to 10 seconds here. This is a periodic timer that
drives the switch to send out BPDU (Bridge Protocol Data Unit) packet to check current
STP status.
The root bridge of the spanning tree topology periodically sends out a “hello” message to
other devices on the network to check if the topology is “healthy”. The “hello time” is the
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