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Cisco ME 3400 EthernetAccess Switch SoftwareConfiguration Guide
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Chapter15 Configuring MSTP Understanding MSTP
maximum value. When a switch receives this BPDU, i t de crem ents th e rec ei v ed r emain ing hop co unt b y
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 rema in the sam e
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 singl e s pan ni ng-t ree regi on runni ng 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 designat ed switch of which is
either a single spanning-tree switch or a switch with a di fferent M ST configura tion.
Note On the Cisco ME switch, only NNIs are MST ports. UNIs do not participate in STP.
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 exc ept a bac kup po rt r ole.
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 tim e 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 boun da ry port wi th t he IST ro ot o r desig na ted po rt
role receives a topology change notice external to the MST cloud, the MSTP switch triggers a top ology
change in the IST instance and in all the MST instances active on that port.
Interoperability with IEEE 802.1D STP
A switch running MSTP supports a built-in protocol migration mechanism that enables it to interoperate
with legacy IEEE 802.1D switches. If this switch receives a legacy IEEE 802.1D configuration BPDU
(a BPDU with the protocol version set to 0), it sends only IEEE 802.1D BPDUs on that port. An MSTP
switch also can detect that a port is at the boundary of a region when it receiv es a legac y 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 IEEE
802.1D BPDUs because it cannot detect whether the legacy switch has been removed from the link
unless the legacy switch is the designated switch. Also, a switch might cont inue to ass ign a bou nda ry
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 .