ZyXEL Communications 4728F User Manual

Chapter 13 Spanning Tree Protocol

XGS-4526/4528F/4728F User’s Guide

Note: Each port can belong to one STP tree only. 

Multiple Spanning Tree Protocol (IEEE 802.1s) is backwards 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. 

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 57   STP/RSTP Network Example