IBM 12.1(22)EA6 How a Switch or Port Becomes the Root Switch or Root Port

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Cisco Systems IntelligentGigabit Ethernet Switch Modules for the IBM BladeCenter, Software Configuration Guide

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Chapter9 Configuring STP

Understanding Spanning-Tree Features

A Layer 2 interface in the disabled state does not particip ate in frame forwarding or in the spanning tree.

An interface in the disabled state is nonoperational.

A disabled interface performs as follows:

•Discards frames received on the port

•Discards frames switched from another interface for forwar ding

•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 9-2, 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 topol ogy with the

ideal switch as the root.

Figure9-2 Spanning-Tree Topology

When the spanning-tree topology is calculated based on default p arameters, 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 100 Mbps 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 interface to a higher

priority (lower numerical value) than the root port, the Gigabit Ethernet interface becomes the new root

port.

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RP = Root Port

DP = Designated Port