3-15
Multiple Instance Spanning-Tree Operation
Configuring MSTP
Note on Path Cost RSTP and MSTP implement a greater range of path costs than 802.1D STP, and
use different default path cost values to account for higher network speeds.
These values are shown below.
Because the maximum value for the path cost allowed by 802.1D STP is 65535,
devices running that version of spanning tree cannot be configured to match
the values defined by MSTP, at least for 10 Mbps and 100 Mbps ports. In LANs
where there is a mix of devices running 802.1D STP, RSTP, and/or MSTP, you
should reconfigure the devices so the path costs match for ports with the same
network speeds.
Configuring MSTP
This section outlines the main pre-requisites for configuring MSTP in your
network, and describes MSTP settings at the global level, per individual port,
and per MST instance.
Planning an MSTP Application
Before configuring MSTP, keep in mind the following tips and considerations:
■ Ensure that the VLAN configuration in your network supports all of the
forwarding paths necessary for the desired connectivity. All ports con-
necting one switch to another within a region and one switch to another
between regions should be configured as members of all VLANs config-
ured in the region.
■ Configure all ports or trunks connecting one switch to another within a
region as members of all VLANs in the region. Otherwise, some VLANs
could be blocked from access to the spanning-tree root for an instance or
for the region.
Port Type 802.1D STP Path Cost RSTP and MSTP Path Cost
10 Mbps 100 2 000 000
100 Mbps 10 200 000
1 Gbps 5 20 000
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