Advanced Traffic Management Guide K/KA/KB.15.15

Preconfiguring an MSTP regional topology
Starting in software release 13.X.X , the MSTP VLAN configuration enhancement allows you to
preconfigure an MSTP regional topology and ensure that the same VLAN ID-to-MSTI assignments
exist on each MSTP switch in the region.
CAUTION: When this software version is installed, the prior VLAN ID-to-MSTI mappings do not
change. However, this enhancement is not backward-compatible. If you install a software version
earlier than this version, and you have configured MSTI entries instances mapped to VLANs, they
will be removed from the configuration file when booting to the prior version of software. Do one
of the following to install or reload a prior version of the software:
1. Remove all MSTP mappings from the configuration file, then reconfigure the instance mapping
after running the desired software version.
2. Save the current configuration file before updating the software to a new version. If you later
reload this older version of the software, use this configuration file when you reload the older
version.
The default behavior of the spanning-tree instance vlan command changes so that, before
a static VLAN is configured or a dynamic VLAN is learned on the switch, you can preconfigure its
VLAN ID-to-MSTI mapping. Later, when the VLAN is created, it is automatically assigned to the
MSTI to which it was previously mapped.
By supporting preconfigured VLAN ID-to-MSTI topologies, the VLAN configuration enhancement
provides the following benefits:
• Scalability: In a network design in which you plan to use a large number of VLANs, you can
preconfigure identical VLAN ID-to-MSTI mappings on all switches in a single, campus-wide
MST region, regardless of the specific VLANs that you later configure on each switch. After
the initial VLAN ID-to-MSTI mapping, you can decide on the exact VLANs that you need on
each switch.
All switches in a region must be configured with the same VLAN ID-to-MSTI mappings and
the same MSTP configuration identifiers (region name and revision number).
• Flexibility: By preconfiguring identical VLAN ID-to-MSTI mappings on all switches in an MST
region, you can combine switches that support different maximum numbers of VLANs.
• Network stability: You can reduce the interruptions in network connectivity caused by the
regeneration of spanning trees in the entire network each time a configuration change in
VLAN-to-MSTI mapping is detected on a switch. The negative impact on network performance
is reduced if all newly created VLANs are pre-mapped to the correct MST instances. Later,
VLAN creation and deletion are ignored by MSTP and no interruption in spanning tree traffic
occurs.
• Usability: Dynamically learned GVRP VLANs can be mapped to MSTIs and support MSTP
load balancing.
Preconfiguring VLANs in an MST instance
When configuring an MSTP regional topology, multiple spanning tree instances are created. Each
MST instance provides a fully connected active topology for a particular set of VLANs.
Each switch in an MSTP region is configured with the following set of common parameters:
• Region name (spanning-tree config-name)
• Region revision number (spanning-tree config-revision)
• Identical VLAN ID-to-MSTI mapping (spanning-tree instance vlan)
Syntax:
[no] spanning-tree instance 1..16 vlan vid [vid..vid]
Preconfiguring an MSTP regional topology 95