User's Manual

Table Of Contents
223
Users Manual of CS-6306R
1. CIST
Common and Internal Spanning Tree (CIST) means the spanning tree comprised by all single switches and
interconnected LAN. These switches may belong to different MST regions. They may be switches running
traditional STP or RSTP. Switches running STP or RSTP in the MST regions are considered to be in their own
regions.
After the network topology is stable, the whole CIST chooses a CIST root bridge. An internal CIST root bridge
will be selected in each region, which is the shortest path from the heart of the region to CIST root.
2. CST
If each MST region is viewed as a single switch, Common Spanning Tree (CST) is the spanning tree
connecting all “single switches”. As shown in Figure 2.1, region 1, 2 and 3 and STP switches make up of the
network CST.
3. IST
Internal Spanning Tree (IST) refers to part of CIST that is in an MST region, that is, IST and CST make up of
the CIST.
4. MSTI
The MSTP protocol allows different VLANs to be distributed to different spanning trees. Multiple spanning tree
instances are then created. Normally, No.0 spanning tree instance refers to CIST, which can be expanded to
the whole network. Every spanning tree instance starting from No.1 is in a certain region. Each spanning tree
instance can be distributed with multiple VLANs. In original state, all VLANs are distributed in CIST.
MSTI in the MST region is independent. They can choose different switches as their own roots.
19.2.1.4 Port Role
Ports in MSTP can function as different roles, similar to ports in RSTP.
1. Root port
Figure 2.2 Root port
Root port stands for the path between the current switch and the root bridge, which has minimum root path
cost.