Common Spanning Tree - Cisco Catalyst 4500 Series Configuration Manual

Chapter 20 Configuring STP and MST
Figure 20-2
SST
Region
F/f = Forwarding
B/b = Blocking
R = Root Bridge
r
To STP running in the SST region, an MST region appears as a single SST or pseudobridge, which
operates as follows:
•
•
•

Common Spanning Tree

CST (802.1Q) is a single spanning tree for all the VLANs. In a Catalyst 4500 series switch running
PVST+, the VLAN 1 spanning tree corresponds to CST. In a Catalyst 4500 series switch running MST,
IST (instance 0) corresponds to CST.
OL-25340-01
Network with Interconnected SST and MST Regions
MST
Region
B
F
F
r
b r
F
r
F
F
= Root port
Although the values for root identifiers and root path costs match for all BPDUs in all
pseudobridges, a pseudobridge differs from a single SST bridge as follows:
– The pseudobridge BPDUs have different bridge identifiers. This difference does not affect STP
operation in the neighboring SST regions because the root identifier and root cost are the same.
– BPDUs sent from the pseudobridge ports may have significantly different message ages.
Because the message age increases by one second for each hop, the difference in the message
age is measured in seconds.
Data traffic from one port of a pseudobridge (a port at the edge of a region) to another port follows
a path entirely contained within the pseudobridge or MST region. Data traffic belonging to different
VLANs might follow different paths within the MST regions established by MST.
The system prevents looping by doing either of the following:
Blocking the appropriate pseudobridge ports by allowing one forwarding port on the boundary
–
and blocking all other ports.
Setting the CST partitions to block the ports of the SST regions.
–
B
r
B
F B
F
F
F
R
MST
Region
Software Configuration Guide—Release IOS XE 3.3.0SG and IOS 15.1(1)SG
F
F
F
r
r
r
SST
b
F
Region
B
About MST
20-25