Multiple Instance Spanning-Tree Operation

802.1s Multiple Spanning Tree Protocol (MSTP)

■ Within an MST region, there is one logical forwarding topology per instance, and each instance comprises a unique set of VLANs. Where multiple paths exist between a pair of nodes using VLANs belonging to the same instance, all but one of those paths will be blocked for that instance. However, if there are different paths in different instances, all such paths are available for traffic. Separate forwarding paths exist through separate spanning tree instances.

■ A port can have different states (forwarding or blocking) for different instances (which represent different forwarding paths).

■ MSTP interprets a switch mesh as a single link.

■ A dynamic VLAN learned by GVRP will always be placed in the IST instance and cannot be moved to any configured MST instance.

 

Transitioning from STP or RSTP to MSTP

 

IEEE 802.1s MSTP includes RSTP functionality and is designed to be compat-

 

ible with both IEEE 802.1D and 802.1w spanning-tree protocols. Even if all the

 

other devices in your network are using STP, you can enable MSTP on the

 

switches covered by this guide. Also, using the default configuration values,

 

your switches will interoperate effectively with STP and RSTP devices. MSTP

 

automatically detects when the switch ports are connected to non-MSTP

 

devices in the spanning tree and communicates with those devices using

 

802.1D or 802.1w STP BPDU packets, as appropriate.

 

Because MSTP is so efficient at establishing the network path, ProCurve

 

highly recommends that you update all of your switches to support 802.1s/

 

MSTP. (For switches that do not support 802.1s/MSTP, ProCurve recommends

 

that you update to RSTP to benefit from the convergence times of less than

 

one second under optimal circumstances.) To make the best use of MSTP and

 

achieve the fastest possible convergence times, there are some changes that

 

you should make to the MSTP default configuration.

 

 

N o t e

Under some circumstances, it is possible for the rapid state transitions

 

employed by MSTP and RSTP to result in an increase in the rates of frame

 

duplication and misordering in the switched LAN. In order to allow MSTP and

 

RSTP switches to support applications and protocols that may be sensitive to

 

frame duplication and misordering, setting the Force Protocol Version param-

 

eter to STP-compatibleallows MSTP and RSTP to operate with the rapid

 

transitions disabled. The value of this parameter applies to all ports on the

 

switch. See information on force version on page 5-17.

 

 

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