12-8
Catalyst 2950 Desktop Switch Software Configuration Guide
78-14982-01
Chapter 12 Configuring RSTP and MSTP
Unders tan ding MSTP

IST, CIST, and CST

Unlike PVST+ in which all the spanning-tree instances are independent, the MSTP establishes and
maintain s two t ypes o f spanni ng-t ree s:
•An internal spanning tree (IST), which is the spanning tree that runs in an MST region.
Within each MST region, the MSTP maintains multiple spanning-tree instances. Instance 0 is a
special insta nce for a re gion, kno wn as the internal spanning tree (IST ). All other MST instances are
numbere d from 1 to 15.
The IST i s th e only sp an n ing- tr ee in st an ce t h at sends a nd r eceives B PDU s; all o f th e othe r
spann ing-tree ins tance inform ation is contain ed in M-reco rds, which are enc apsulated w ithin MSTP
BPDUs. B eca us e th e MST P B PDU c ar rie s in form ation for al l in s tan ce s , the n u mber o f BP DU s that
need to be processed by a switch to support multiple spanning-tree instances is significantly
reduced.
All MST instances within the same region share the same protocol timers, but each MST instance
has its own topology parameters, such as root switch ID, root path cost, and so forth. By default, all
VLANs are assigned to the IST.
An MST instanc e is local to the regi on; for example, MS T instance 1 in region A is independent of
MST inst ance 1 in region B, even if regions A and B are inter connec ted.
•A common and internal spanning tree (CIST), which is a collection of the ISTs in each MST region,
and the common spanning tree (CST) that interconnects the MST regions and single spanning trees.
The spann ing tree compute d in a r egi on appea rs as a su btre e in the CS T that encompas ses the en tire
switche d domain . The CIST is for med as a resul t of the spann ing-tr ee algo rithm ru nning betw een
switche s that supp ort the 802 .1W, 802. 1S, and 802 .1D prot ocols. Th e CIST insid e an MST region
is the same as the CST outside a region.
For more information, see the “Operations Within an MST Region” section on page 12-8 and th e
“Oper ati on s Be tw een MST R egions” se ction on pa ge 12-9.

Operations Within an MST Region

The IST connects all the MSTP switches in a region. When the IST converges, the root of the IST
becomes the IST master (shown in Figure 12-3 on page 12-9), which is the switch wi thin the re gion with
the lowest bridge ID and path cost to the CST root. The IST master also is the CST root if there is only
one region within the network. If the CST root is outside the region, one of the MSTP switches at the
boundary o f th e region i s sel ect ed as t h e IST m aste r.
When an MSTP switch initializes, it sends BPDUs claiming itself as the root of the CST and the IST
master, with both of the path costs to the CST root and to the IST master set to zero. The switch also
initializes all of its MST instances and claims to be the root for all of them. If the switch recei ves superior
MST root information (lower bridge ID, lower path cost, and so forth) than currently stored for the port,
it relinquishes its claim as the IST master.
During init iali zati on, a region m ight h ave many subregions, eac h wit h it s own IST mast er. As switc hes
receive superior IST information, they leave their old subregions and join the new subregion that m ight
conta in t he t r ue IST mast er. Thus al l s ubr egions sh rink, excep t for t he one t hat co nta in s the tr ue I ST
master.
For correct op eration, all switches in the MST region must agree on the same IST master. Theref ore, any two
switches in the region synchronize their port roles for an MST instance only if they converge to a common
IST master.