Overview Of Uplinkfast - Cisco 4500M Software Manual

Chapter 15 Configuring STP Features
Table 15-1 PortFast BPDU Filtering Port Configurations
Per-Port Configuration
Default
Default
Default
Disable
Enable
1. The port transmits at least 10 BPDUs. If this port receives any BPDUs, then PortFast and PortFast BPDU filtering are disabled.

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
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
Switch A
(Root)
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
approximately one to five seconds.
OL-6696-01
Global Configuration
Enable
Enable
Disable
Not applicable
Not applicable
shows an example of a topology with no link failures. Switch A, the root switch, is
L1
L2
Blocked port
Switch C
PortFast State
Enable
Disable
Not applicable
Not applicable
Not applicable
Switch B
L3
Figure
Software Configuration Guide—Release 12.2(25)EW
Overview of UplinkFast
PortFast BPDU Filtering State
1
Enable
Disable
Disable
Disable
Enable
15-3. This switchover takes
15-5