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| Configuring the Switch
C
4
HAPTER
Configuring the Spanning Tree Algorithm
C
G
ONFIGURING
LOBAL
S
STA
ETTINGS FOR
An MST Region consists of a group of interconnected bridges that have the
same MST Configuration Identifiers (including the Region Name, Revision
Level and Configuration Digest – see
on page
122). An MST Region may contain multiple MSTP Instances. An
Internal Spanning Tree (IST) is used to connect all the MSTP switches
within an MST region. A Common Spanning Tree (CST) interconnects all
adjacent MST Regions, and acts as a virtual bridge node for
communications with STP or RSTP nodes in the global network.
Figure 43: Common Internal Spanning Tree, Common Spanning Tree,
Internal Spanning Tree
Region 1
CIST
Region 4
IST
Region 2
MSTP connects all bridges and LAN segments with a single Common and
Internal Spanning Tree (CIST). The CIST is formed as a result of the
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).
Use the STP Bridge Settings page to configure settings for STA which apply
globally to the switch.
P
ATH
Configuration, Spanning Tree, Bridge Settings
C
U
OMMAND
SAGE
Spanning Tree Protocol
◆
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.
– 118 –
"Configuring Multiple Spanning Trees"
CST
Region 3
Region 2
1
Region 1
Region 4
Region 3
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