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Cisco Catalyst 4500 Series Administration Guide

Cisco Catalyst 4500 Series
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22-25
Software Configuration Guide—Release IOS XE 3.5.0SG and IOS 15.2(2)SG
OL-27597-01
Chapter 22 Configuring STP and MST
About MST
Figure 22-2 Network with Interconnected SST and MST Regions
To STP running in the SST region, an MST region appears as a single SST or pseudobridge, which
operates as follows:
• 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.
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.
MST
Region
MST
Region
SST
Region
SST
Region
F
F
F
F
F
F
R
F
F
F
F
F
F
F
B
B
B
B
B
r
r
r
r
r
r
r
b
b
F/f
= Forwarding
B/b = Blocking
R
= Root Bridge
r
= Root port
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Cisco Catalyst 4500 Series Specifications

General IconGeneral
SeriesCatalyst 4500 Series
CategorySwitch
Layer SupportLayer 2, Layer 3
Form FactorModular chassis
StackableNo
Chassis Slots3, 6, 7, 10
Power Supply OptionsAC, DC
RedundancyPower supply, Supervisor engine
Network ManagementCisco IOS Software CLI, SNMP, Cisco Prime Infrastructure
FeaturesSecurity, QoS
Port DensityUp to 384 ports per chassis
Security Features802.1X, ACLs, DHCP Snooping, Dynamic ARP Inspection, IP Source Guard
Supervisor Engine8-E

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