Cisco WS-C2980G-A Software Guide - Page 159
Understanding How UplinkFast Works
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Chapter 8 Configuring Spanning Tree PortFast, BPDU Guard, BPDU Filter, UplinkFast, BackboneFast, and Loop Guard Understanding How UplinkFast Works Understanding How UplinkFast Works UplinkFast provides fast convergence using uplink groups in the network access layer after a spanning tree topology change. An uplink group is a set of ports (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 (not including self-looped ports). The uplink group provides an alternate path in case the currently forwarding link fails. Note UplinkFast is most useful in wiring-closet switches that have a limited number of active VLANs. This enhancement might not be useful for other types of applications and should not be enabled on backbone or distribution layer switches. Figure 8-1 shows an example UplinkFast network topology. Switch A, the root switch, is connected directly to Switch B over link L1 and to Switch C over link L2. The port on Switch C that is connected to Switch B over link L3 is in blocking state. Figure 8-1 UplinkFast Example Before Direct Link Failure Switch A (Root) Switch B L1 L2 L3 Blocked port 11241 Switch C If Switch C detects a link failure on the currently active link L2 (a direct link failure), UplinkFast unblocks the blocked port on Switch C and transitions it to the forwarding state immediately, without transitioning the port through the listening and learning states (as shown in Figure 8-2). This switchover takes approximately 1 to 5 seconds. Figure 8-2 Example of UplinkFast After Direct Link Failure Switch A (Root) Switch B L1 L2 Link failure Switch C L3 UplinkFast transitions port directly to forwarding state 11242 78-15486-01 Catalyst 4500 Series, Catalyst 2948G, Catalyst 2980G Switches Software Configuration Guide-Release 8.1 8-3
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