ZyXEL Communications ES3500-24 User Manual page 127

13.1.5.1 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 58 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.
Figure 59 MSTP Network Example
VLAN 1
13.1.5.2 MST Region
An MST region is a logical grouping of multiple network devices that appears as a single device to
the rest of the network. Each MSTP-enabled device can only belong to one MST region. When
BPDUs enter an MST region, external path cost (of paths outside this region) is increased by one.
Internal path cost (of paths within this region) is increased by one when BPDUs traverse the region.
ES3500 Series User's Guide
A
VLAN 2
B
A
VLAN 2
B
Chapter 13 Spanning Tree Protocol
127