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Chapter 17
Configuring MSTP
Designating the Neighbor Type
A topology could contain both prestandard and IEEE 802.1s standard compliant devices. By default,
ports can automatically detect prestandard devices, but they can still receive both standard and
prestandard BPDUs. When there is a mismatch between a device and its neighbor, only the CIST runs
on the interface.
You can choose to set a port to send only prestandard BPDUs. The prestandard flag appears in all the
show commands, even if the port is in STP compatibility mode.
Beginning in privileged EXEC mode, follow these steps to override the default link-type setting. This
procedure is optional.
Command
Step 1
configure terminal
Step 2
interface interface-id
Step 3
spanning-tree mst pre-standard
Step 4
end
Step 5
show spanning-tree mst interface interface-id
Step 6
copy running-config startup-config
To return the port to its default setting, use the no spanning-tree mst prestandard interface
configuration command.
Restarting the Protocol Migration Process
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 receives a legacy BPDU, an MST
BPDU (Version 3) associated with a different region, or an RST 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. A switch also might continue to assign a boundary role
to a port when the switch to which it is connected has joined the region.
To restart the protocol migration process (force the renegotiation with neighboring switches) on the
switch, use the clear spanning-tree detected-protocols privileged EXEC command.
To restart the protocol migration process on a specific interface, use the clear spanning-tree
detected-protocols interface interface-id privileged EXEC command.
OL-8915-03
Purpose
Enter global configuration mode.
Specify an interface to configure, and enter interface
configuration mode. Valid interfaces include physical ports.
Specify that the port can send only prestandard BPDUs.
Return to privileged EXEC mode.
Verify your entries.
(Optional) Save your entries in the configuration file.
Cisco Catalyst Blade Switch 3020 for HP Software Configuration Guide
Configuring MSTP Features
17-25
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