Cisco IE 4000 Software Configuration Guide

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Configuring STP
Information About Configuring STP
 Forwards frames switched from another interface
 Learns addresses
 Receives BPDUs
Disabled State
A Layer 2 interface in the disabled state does not participate in frame forwarding or in the spanning tree. An interface in
the disabled state is nonoperational.
A disabled interface performs these functions:
 Discards frames received on the interface
 Discards frames switched from another interface for forwarding
 Does not learn addresses
 Does not receive BPDUs
How a Switch or Port Becomes the Root Switch or Root Port
If all switches in a network are enabled with default spanning-tree settings, the switch with the lowest MAC address
becomes the root switch. In Figure 37 on page 320, Switch A is elected as the root switch because the switch priority
of all the switches is set to the default (32768) and Switch A has the lowest MAC address. However, because of traffic
patterns, number of forwarding interfaces, or link types, Switch A might not be the ideal root switch. By increasing the
priority (lowering the numerical value) of the ideal switch so that it becomes the root switch, you force a spanning-tree
recalculation to form a new topology with the ideal switch as the root.
Figure 37 Spanning-Tree Topology
When the spanning-tree topology is calculated based on default parameters, the path between source and destination
end stations in a switched network might not be ideal. For instance, connecting higher-speed links to an interface that
has a higher number than the root port can cause a root-port change. The goal is to make the fastest link the root port.
For example, assume that one port on Switch B is a Gigabit Ethernet link and that another port on Switch B (a 10/100
link) is the root port. Network traffic might be more efficient over the Gigabit Ethernet link. By changing the spanning-tree
port priority on the Gigabit Ethernet port to a higher priority (lower numerical value) than the root port, the Gigabit Ethernet
port becomes the new root port.
86475
DP
DP
RP
DP
RP
DP
RP = Root Port
DP = Designated Port
DP
RP
DA
CB

Cisco IE 4000 Specifications

General

Product Type	Switch
Form Factor	DIN Rail Mountable
MAC Address Table Size	8000
Jumbo Frame Support	9216 bytes
Operating Temperature	-40°C to 70°C
Mean Time Between Failures (MTBF)	Over 500, 000 hours
Memory	256 MB DRAM
Mounting	DIN Rail, Wall
Certifications	EN 50121-4
Ports	8 x 10/100Base-TX Ethernet Ports