Multiple Stp; Figure 46 Stp/Rstp Network Example - ZyXEL Communications ZyXEL Dimension ES-4124 User Manual

Chapter 11 Spanning Tree Protocol

11.1.5 Multiple STP

Multiple Spanning Tree Protocol (IEEE 802.1s) is backward compatible with STP/RSTP and
addresses the limitations of existing spanning tree protocols (STP and RSTP) in networks to
include the following features:
• One Common and Internal Spanning Tree (CIST) that represents the entire network's
connectivity.
• Grouping of multiple bridges (or switching devices) into regions that appear as one single
bridge on the network.
• A VLAN can be mapped to a specific Multiple Spanning Tree Instance (MSTI). MSTI
allows multiple VLANs to use the same spanning tree.
• Load-balancing is possible as traffic from different VLANs can use distinct paths in a
region.
11.1.5.1 MSTP Network Example
The following figure shows a network example where two VLANs are configured on the two
switches. If the switches are using STP or RSTP, the link for VLAN 2 will be blocked as STP
and RSTP allow only one link in the network and block the redundant link.

Figure 46 STP/RSTP Network Example

VLAN 1
With MSTP, VLANs 1 and 2 are mapped to different spanning trees in the network. Thus
traffic from the two VLANs travel on different paths. The following figure shows the network
example using MSTP.
112
A
VLAN 2
B
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