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15-5

Software Configuration Guide—Release 12.2(25)EW

OL-6696-01

Chapter 15 Configuring STP Features

Overview of UplinkFast

Note UplinkFast is most useful in wiring-closet switches. This feature might not be useful for other types of

applications.

Spanning Tree UplinkFast provides fast convergence after a direct link failure and uses uplink groups to

achieve load balancing between redundant Layer 2 links. Convergence is the speed and ability of a group

of internetworking devices running a specific routing protocol to agree on the topology of an

internetwork after a change in that topology. An uplink group is a set of Layer 2 interfaces (per VLAN),

only one of which is forwarding at any given time. Specifically, an uplink group consists of the root port

(which is forwarding) and a set of blocked ports, except for self-looping ports. The uplink group provides

an alternate path in case the currently forwarding link fails.

Figure 15-2 shows an example of a topology with no link failures. Switch A, the root switch, is

connected directly to Switch B over link L1 and to Switch C over link L2. The Layer 2 interface on

Switch C that is connected directly to Switch B is in the blocking state.

Figure 15-2 UplinkFast Before Direct Link Failure

If Switch C detects a link failure on the currently active link L2 on the root port (a direct link failure),

UplinkFast unblocks the blocked port on Switch C and transitions it to the forwarding state without

going through the listening and learning states, as shown in Figure 15-3. This switchover takes

approximately one to five seconds.

Table 15-1 PortFast BPDU Filtering Port Configurations

Per-Port Configuration Global Configuration PortFast State PortFast BPDU Filtering State

Default Enable Enable Enable

Default Enable Disable Disable

Default Disable Not applicable Disable

Disable Not applicable Not applicable Disable

Enable Not applicable Not applicable Enable

1. The port transmits at least 10 BPDUs. If this port receives any BPDUs, then PortFast and PortFast BPDU filtering are disabled.

L2 L3

Switch C

Switch A

(Root)

Switch B

Blocked port

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