Spanning-Tree Operation

802.1D Spanning-Tree Protocol (STP)

In figure 5-13, STP is enabled and in its default configuration on all switches, unless otherwise indicated in table 5-5, below:

Table 5-5. STP Parameter Settings for Figure 5-13

STP Parameter

Switch “1”

Switch “2”

Switch “3”

Switch “4”

Switch Priority

01

12

32,768 (default)

32,768 (default)

(Fast) Uplink

No

No

No

Ports 3 & 5

1This setting ensures that Switch “1” will be the primary root switch for STP in figure 5-13. 2This setting ensures that Switch “2” will be the backup root switch for STP in figure 5-13.

With the above-indicated topology and configuration:

Scenario 1: If the link between switches “4” and “2” goes down, then the link between switches “4” and “3” will begin forwarding in as little as ten seconds.

Scenario 2: If Switch “1” fails, then:

Switch “2” becomes the root switch.

The link between Switch “3” and Switch “2” begins forwarding.

The link between Switch “2” and the LAN begins forwarding.

Operating Rules for Fast Uplink

A switch with ports configured for fast uplink must be an edge switch and not either an interior switch or the STP root switch.

Configure fast-uplink on only the edge switch ports used for providing redundant STP uplink connections in a network. (Configuring Fast-Uplink STP on ports in interior switches can create network performance prob­ lems.) That is, a port configured for STP uplink should not be connected to a switch that is sequentially further away from the STP root device. For example, switch “4” in figure 5-13 (page 5-33) is an edge switch.

Configure fast uplink on a group (two or more) of redundant edge-switch uplink ports where only one port in the group is expected to be in the forwarding state at any given time.

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