ZyXEL Communications OLT2406, User Manual

Page: 471 / 766

Chapter 63 Spanning Tree Protocol
OLT2406 User’s Guide
471
63.2.4 Multiple STP
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.
63.2.4.1 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 3 will be blocked as STP and RSTP allow only one link
in the network and block the redundant link.
Figure 256 STP/RSTP Network Example
With MSTP, VLANs 1 and 3 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.
Learning All BPDUs are received and processed. Information frames are submitted to the learning process
but not forwarded.
Forwarding All BPDUs are received and processed. All information frames are received and forwarded.
Table 234 STP Port States
PORT STATE DESCRIPTION
VLAN 2 VLAN 1
A
B