User Guide
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responsive to other RSTP bridge's link status. The port does not need to wait for the topology to become
stable. Edge port and P2P port are introduced to the protocol for faster transition. The explanation of an
Edge port and a P2P port is shown below:
Edge Port
The edge port is a configurable designation port that is directly connected to a segment where a loop
cannot be created. Usually it would be a port connected directly to a single workstation. Ports that are
designated as edge ports transition to a forwarding state immediately without going through the listening
and learning states. An edge port loses its status if it receives a BPDU packet, immediately becoming a
normal spanning tree port.
P2P Port
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
collapses 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 the 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: