3Com Switch 4200G Configuration Guide

Root Bridge Configuration 123
■ RSTP mode: In this mode, the protocol packets sent out of the ports of the switch
are RSTP packets. If the switched network contains RSTP-enabled switches, you
can configure the current MSTP-enabled switch to operate in this mode by using
the stp mode rstp command.
■ MSTP mode: In this mode, the protocol packets sent out of the ports of the switch
are MSTP packets, or STP packets if the ports have STP-enabled switches
connected. In this case, the multiple spanning tree function is enabled as well.
Configuration procedure
Configuration example
1 Configure the current switch to operate in the STP mode.
<S4200G> system-view
System View: return to User View with Ctrl+Z.
[4200G] stp mode stp
MST Region Maximum
Hops Configuration
The maximum hops values configured on the region roots in an MST region limit the
size of the MST region.
A configuration BPDU contains a field that maintains the remaining hops of the
configuration BPDU. And a switch discards the configuration BPDUs whose remaining
hops are 0. After a configuration BPDU reaches a root bridge of a spanning tree in a
MST region, the value of the remaining hops field in the configuration BPDU is
decreased by 1 every time the configuration BPDU passes a switch. Such a mechanism
disables the switches that are beyond the maximum hops from participating in
spanning tree generation, and thus limits the size of an MST region.
With such a mechanism, the maximum hops configured on the switch operating as
the root bridge of the IST or an MSTI in a MST region becomes the network diameter
of the spanning tree, which limits the size of the spanning tree in the current MST
region. The switches that are not root bridges in the MST region adopt the maximum
hops settings of their root bridges.
Configuration procedure
Note that only the maximum hops settings on the switches operating as region roots
can limit the size of the MST region.
Table 89 Configure MSTP operation mode
Operation Command Description
Enter system view system-view —
Configure the MSTP
operation mode for the
switch
stp mode { stp | rstp | mstp } Required
A MSTP-enabled switch operates in
the MSTP mode by default.
Table 90 Configure the maximum hops for an MST region
Operation Command Description
Enter system view system-view —
Configure the
maximum hops for the
MST region
stp max-hops hops Required
By default, the maximum hops of
an MST region is 20.