Spanning-Tree Operation
802.1s Multiple Spanning Tree Protocol (MSTP)
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 5300xl and 3400cl/6400cl 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, HP highly
recommends that you update all of your 5300xl switches to support 802.1s/
MSTP. (All 3400cl/6400cl switch software versions support 802.1s. Also, for
switches that do not support 802.1s/MSTP, HP 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
6-14.
As indicated above, one of the benefits of MSTP and RSTP is the implemen-
tation of a larger range of port path costs, which accommodates higher
network speeds. New default values have also been implemented for the path
costs associated with the different network speeds. This can create some
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