ZyXEL Communications ES3500 Series Figure 59 MSTP Network Example, Figure 58 STP/RSTP Network Example

Chapter 13 Spanning Tree Protocol

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

A

VLAN 1 VLAN 2

B

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

A

VLAN 1 VLAN 2

B

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.

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ES3500 Series User’s Guide