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Catalyst2950 Desktop Switch Software Configuration Guide
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Chapter11 Configuring RSTP and MSTP Understanding MSTP
•Propagation—When an RSTP switch receives a TC message from another switch through a
designated or root port, it propagates the topology change to a ll of i ts n onedge , edge , designa te d
ports, and root port (excluding the port on which it is received). The switch starts the TC-while time r
for all such ports and flushes the information learned on the m.
•Protocol migration—For backward compatibility with 802.1D switches, RSTP selectively sends
802.1D configuration BPDUs and TCN BPDUs on a per-port basis.
When a port is initialized, the migrate-delay timer is started (specifies the minimum time during
which RSTP BPDUs are sent), and RSTP BPDUs are sent. While this timer is active, the switch
processes all BPDUs received on that port and ignores the protocol type .
If the switch receives an 802.1D BPDU after the por t’s migration-delay timer has expired, it assumes
that it is connected to an 802.1D switch and starts using only 802.1D BPDUs. H owever, if the RSTP
switch is using 802.1D BPDUs on a port and receives an RSTP BPDU after the ti mer has expi red,
it restarts the timer and starts using RSTP BPDUs on that port.
Understanding MSTP
MSTP, which uses RSTP for rapid convergence, enables VLANs to be grouped into a spanning-tree
instance, with each instance having a spanning-tree topology inde pende nt o f othe r spanning -t ree
instances. This architecture provides multiple forwarding paths for data traffic, enables load balancing,
and reduces the number of spanning-tree instances required to support a large number of VLANs .
These sections describe how the MSTP works:
•Multiple Spanning-Tree Regions, page 11-7
•IST, CIST, and CST, page 11-8
•Hop Count, page 11-10
For configuration information, see the “Configuring RSTP and MSTP Features” se cti on on page 11-11.

Multiple Spanning-Tree Regions

For switches to participate in multiple spanning-tree (MST) instances, you must consistently configure
the switches with the same MST configuration information. A collection of interconnected switches that
have the same MST configuration comprises an MST region as shown in Figure 11-3 on page 11-9.
The MST configuration determines to which MST region each switch belongs. The configuration
includes the name of the region, the revision number, and the MST instance-to-VLAN assignme nt map.
You configure the switch for a region by using the spanning-tree mst configuration global
configuration command, after which the switch enters the MST configuration mode. From this mode,
you can map VLANs to an MST instance by using the instance MS T co nfigur ati on comm a nd, sp ec ify
the region name by using the name MST configuration command, and set the revision number by using
the revision MST configuration command.
A region can have one member or multiple members with the sam e M ST co nfigurat ion; ea c h m embe r
must be capable of processing RSTP BPDUs. There is no limit t o the numb er of MS T regi on s in a
network, but each region can support up to 16 spanning-tree instances. You can assign a VLAN to only
one spanning-tree instance at a time.