Designating The Neighbor Type; Restarting The Protocol Migration Process - Cisco 3845 - Security Bundle Router Software Manual

Configuring MSTP Features

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 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 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 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.
Cisco ME 3800X and 3600X Switch Software Configuration Guide
14-26
Purpose
Enter global configuration mode.
Specify an interface to configure, and enter interface
configuration mode. Valid interfaces include physical
interfaces, VLANs, and port channels. The VLAN ID range is 1
to 4094. The port-channel range is 1 to 26.
Note If the interface is a VLAN, only ports with spanning tree
enabled in the VLAN will run spanning tree.
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.
Chapter 14 Configuring MSTP
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