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A P2P port is also capable of rapid transition. P2P ports may be used to connect to other bridges. Under
RSTP/MSTP, all ports operating in full-duplex mode are considered to be P2P ports, unless manually overridden
through configuration. The three protocols are mutually compatible and no conflicts or network collapse will be
caused in spanning tree application.
MSTP Overview
MSTP divides a network into several MST regions. The CST is generated between these MST regions, and
multiple spanning trees can be generated in each MST region. Each spanning tree is called an instance. As well as
STP, MSTP uses BPDUs to generate spanning tree. The only difference is that the BPDU for MSTP carries the
MSTP configuration information on the switches. MSTP allows formation of MST regions that can run multiple
MST instances (MSTI). Multiple regions and other STP bridges are interconnected using one single common
spanning tree (CST). Unlike some proprietary per-VLAN spanning tree implementations, MSTP includes all of its
spanning tree information in a single BPDU format. Not only does this reduce the number of BPDUs required on a
LAN to communicate spanning tree information for each VLAN, but it also ensures backward compatibility with
RSTP. MSTP does this by encoding additional region information after the standard RSTP BPDU as well as a
number of MSTI messages (from 0 to 64 instances, although in practice many bridges support fewer). Each of these
MSTI configuration messages conveys the spanning tree information for each instance. Each instance can be
assigned a number of configured VLANs and frames (packets) assigned to these VLANs operate in this spanning
tree instance whenever they are inside the MST region. In order to avoid conveying their entire VLAN to spanning
tree mapping in each BPDU, bridges encode an MD5 digest of their VLAN to instance table in the MSTP BPDU.
This digest is then used by other MSTP bridges, along with other administratively configured values, to determine
if the neighboring bridge is in the same MST region as itself.
MSTP packets are as follow:
Octet 39-89 for MST Configuration Identifier
Global Setup
Click
Device Management > STP > Global Setup
to enter interface below:
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