5-15
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.
Note 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-compatible allows 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.
