User manual

C HAPTER 4 | Configuring the Switch
Configuring the Spanning Tree Algorithm
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running spanning tree algorithm between switches that support the STP,
RSTP, MSTP protocols.
Once you specify the VLANs to include in a Multiple Spanning Tree Instance
(MSTI), the protocol will automatically build an MSTI tree to maintain
connectivity among each of the VLANs. MSTP maintains contact with the
global network because each instance is treated as an RSTP node in the
Common Spanning Tree (CST).
CONFIGURING GLOBAL
SETTINGS FOR STA
Use the STP Bridge Settings page to configure settings for STA which apply
globally to the switch.
PATH
Basic/Advanced Configuration, Spanning Tree, Bridge Settings
COMMAND USAGE
◆ Spanning Tree Protocol
1
Uses RSTP for the internal state machine, but sends only 802.1D
BPDUs. This creates one spanning tree instance for the entire network.
If multiple VLANs are implemented on a network, the path between
specific VLAN members may be inadvertently disabled to prevent
network loops, thus isolating group members. When operating multiple
VLANs, we recommend selecting the MSTP option.
◆ 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.
◆ Multiple Spanning Tree Protocol
MSTP generates a unique spanning tree for each instance. This provides
multiple pathways across the network, thereby balancing the traffic
load, preventing wide-scale disruption when a bridge node in a single
instance fails, and allowing for faster convergence of a new topology for
the failed instance.
■
To allow multiple spanning trees to operate over the network, you
must configure a related set of bridges with the same MSTP
1. STP and RSTP BPDUs are transmitted as untagged frames, and will cross any VLAN
boundaries.