12-7
Catalyst 2950 Desktop Switch Software Configuration Guide
78-14982-01
Chapter 12 Configuring RSTP and MSTP Understa nding MSTP
•Propagation—When an RSTP switch receives a TC message from another switch through a
designat ed or r oot por t, it pro pa gate s the to pology c hange to a ll of it s n onedge , edge , de signa te d
ports, and root port (excluding the port on which it is recei ved). The switch starts the TC-while timer
for al l su ch port s and f lushe s the infor mat ion lea rned on the m.
•Protocol migration—For backward compatibility with 802.1D switches, RSTP selectively sends
802.1D co nfigurati on BPDU s and T CN BPD Us on a per-port basi s.
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
processe s al l BPD Us re ceived on tha t p ort an d i g nores th e pr otoc ol type .
If the switch receives an 802.1D BPDU after the port’s migration-delay timer has expired, it assumes
that it is connected to an 802.1D switch and starts using only 802.1D BPDUs. Ho wev er , if the RSTP
switch i s us ing 8 02. 1D B PDUs on a po rt and r eceives an RSTP BPD U a fter t he ti mer has expi red,
it restarts the timer and starts using RSTP BPDUs on that port.

Understandin g MSTP

MSTP, which uses RSTP for rapi d convergence, enab les VLAN s to be grouped into a spann ing-tr ee
instan ce, wi th e ach inst anc e havin g a s pan ni ng-t ree t opol ogy inde pende nt o f othe r spa nning -t ree
instances. This architecture provides multiple forwarding paths for data traffic, enables load balancing,
and reduc es the numb er of sp anning -tree inst ances re quired to support a large num ber of VLA Ns.
These sec tions descr ibe how the MSTP works:
•Multip le Spanning -Tree Regions, page 12-7
•IST, CIST, and CST, page 12-8
•Hop Count, pa ge 12-10
For configurati o n information, see the “Configuring RSTP and MSTP Features” sect ion on pa ge 12-11.

Multiple Spanning-Tree Regions

For switches to participate in multiple spanning-tree (MST) instances, you must consistently configure
the switches with the same MST conf iguration in formation. A collection of in terconnected switch es that
have the same M ST configurati on compri ses an MST region as shown in Figure 12-3 on page 12- 9.
The MST configurati on determi nes to whi ch MST region each switch be longs. Th e configura tion
includes the name of the region, the revision number, and the MST instance-to-VLAN assignment map.
You configure the swit ch for a region by using the spanning-tree mst configuration globa l
configurati on comm and, afte r which the switch ente rs the MS T configurati on mode. From this mod e,
you ca n m ap VL ANs to a n M ST inst an ce by us in g th e 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 h ave one me mber or m ult iple me mb ers w it h the sam e M ST co nfigurat ion; ea c h m embe r
must be ca pabl e of pr oces sin g RSTP BPD Us . The re is no li mit t o the n umb er of MS T regi on s in a
network, but each region can support up to 16 spanning-tre e instances. You can assign a VLAN to only
one spanning-tree instance at a time.