15-5
Catalyst 2950 and Catalyst 2955 Switch Software Configuration Guide
78-11380-10
Chapter 15 Configuring MSTP
Understanding MSTP
received remaining hop count by one and propagates this value as the remaining hop count in the BPDUs
it generates. When the count reaches zero, the switch discards the BPDU and ages the information held
for the port.
The message-age and maximum-age information in the RSTP portion of the BPDU remain the same
throughout the region, and the same values are propagated by the region’s designated ports at the
boundary.
Boundary Ports
A boundary port is a port that connects an MST region to a single spanning-tree region running RSTP,
to a single spanning-tree region running PVST+ or rapid PVST+, or to another MST region with a
different MST configuration. A boundary port also connects to a LAN, the designated switch of which
is either a single spanning-tree switch or a switch with a different MST configuration.
At the boundary, the roles of the MST ports do not matter, and their state is forced to be the same as the
IST port state (MST ports at the boundary are in the forwarding state only when the IST port is
forwarding). An IST port at the boundary can have any port role except a backup port role.
On a shared boundary link, the MST ports wait in the blocking state for the forward-delay time to expire
before transitioning to the learning state. The MST ports wait another forward-delay time before
transitioning to the forwarding state.
If the boundary port is on a point-to-point link and it is the IST root port, the MST ports transition to the
forwarding state as soon as the IST port transitions to the forwarding state.
If the IST port is a designated port on a point-to-point link and if the IST port transitions to the
forwarding state because of an agreement received from its peer port, the MST ports also immediately
transition to the forwarding state.
If a boundary port transitions to the forwarding state in an IST instance, it is forwarding in all MST
instances, and a topology change is triggered. If a boundary port with the IST root or designated port
role receives a topology change notice external to the MST cloud, the MSTP switch triggers a topology
change in the IST instance and in all the MST instances active on that port.
Interoperability with 802.1D STP
A switch running MSTP supports a built-in protocol migration mechanism that enables it to interoperate
with legacy 802.1D switches. If this switch receives a legacy 802.1D configuration BPDU (a BPDU with
the protocol version set to 0), it sends only 802.1D BPDUs on that port. An MSTP switch can also detect
that a port is at the boundary of a region when it receives a legacy BPDU, an MSTP BPDU (version 3)
associated with a different region, or an RSTP BPDU (version 2).
However, the switch does not automatically revert to the MSTP mode if it no longer receives 802.1D
BPDUs because it cannot determine whether the legacy switch has been removed from the link unless
the legacy switch is the designated switch. Also, a switch might continue to assign a boundary role to a
port when the switch to which this switch is connected has joined the region. To restart the protocol
migration process (force the renegotiation with neighboring switches), use the clear spanning-tree
detected-protocols privileged EXEC command.
If all the legacy switches on the link are RSTP switches, they can process MSTP BPDUs as if they are
RSTP BPDUs. Therefore, MSTP switches send either a version 0 configuration and TCN BPDUs or
version 3 MSTP BPDUs on a boundary port. A boundary port connects to a LAN, the designated switch
of which is either a single spanning-tree switch or a switch with a different MST configuration.
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