User manual

C HAPTER 8 | Spanning Tree Algorithm
Configuring Global Settings for STA
– 108 –
ES-2000 Series
Figure 56: STP Root Ports and Designated Ports
Once a stable network topology has been established, all bridges listen for
Hello BPDUs (Bridge Protocol Data Units) transmitted from the Root Bridge.
If a bridge does not get a Hello BPDU after a predefined interval (Maximum
Age), 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.
RSTP – RSTP is designed as a general replacement for the slower, legacy
STP. RSTP achieves much faster reconfiguration (i.e., around 1 to 3
seconds, compared to 30 seconds or more for STP) by reducing the
number of state changes before active ports start learning, predefining an
alternate route that can be used when a node or port fails, and retaining
the forwarding database for ports insensitive to changes in the tree
structure when reconfiguration occurs.
CONFIGURING GLOBAL SETTINGS FOR STA
Use the Spanning Tree > STA (Configure Global - Configure) page to
configure global settings for the spanning tree that apply to the entire
switch.
COMMAND USAGE
◆ Spanning Tree Protocol
1
Uses RSTP for the internal state machine, but sends only 802.1D
BPDUs.
◆ Rapid Spanning Tree Protocol
1
RSTP supports connections to either STP or RSTP nodes by monitoring
the incoming protocol messages and dynamically adjusting the type of
protocol messages the RSTP node transmits, as described below:
■
STP Mode – If the switch receives an 802.1D BPDU (i.e., STP BPDU)
after a port’s migration delay timer expires, the switch assumes it is
connected to an 802.1D bridge and starts using only 802.1D
BPDUs.
■
RSTP Mode – If RSTP is using 802.1D BPDUs on a port and receives
an RSTP BPDU after the migration delay expires, RSTP restarts the
migration delay timer and begins using RSTP BPDUs on that port.
x
Designated
Root
Designated
Port
Designated
Bridge
x x
x
Root
Port
x
1. STP and RSTP BPDUs are transmitted as untagged frames, and will cross any VLAN
boundaries.