Cisco Catalyst 3550 Series User Manual

18-5
Catalyst 3550 Multilayer Switch Software Configuration Guide
78-11194-09
Chapter 18 Configuring Optional Spanning-Tree Features
Understanding Optional Spanning-Tree Features
Figure 18-3 shows an example 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 a blocking state.
Figure 18-3 UplinkFast Example 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 18-4. This change takes
approximately 1 to 5 seconds.
Figure 18-4 UplinkFast Example After Direct Link Failure
Understanding Cross-Stack UplinkFast
Cross-stack UplinkFast (CSUF) provides a fast spanning-tree transition (fast convergence in less than
1 second under normal network conditions) across a stack of switches that use the GigaStack GBICs
connected in a shared cascaded configuration (multidrop backbone). During the fast transition, an
alternate redundant link on the stack of switches is placed in the forwarding state without causing
temporary spanning-tree loops or loss of connectivity to the backbone. With this feature, you can have
a redundant and resilient network in some configurations. You enable CSUF by using the spanning-tree
stack-port interface configuration command. The CSUF feature is supported only when the switch is
running PVST+. It is not supported when the switch is running rapid PVST+ or MSTP.
CSUF might not provide a fast transition all the time; in these cases, the normal spanning-tree transition
occurs, completing in 30 to 40 seconds. For more information, see the “Events that Cause Fast
Convergence” section on page 18-7.
L1
L2 L3
Switch C
Switch A
(Root)
Switch B
Blocked port
43575
L1
L2 L3
Switch C
Switch A
(Root)
Switch B
UplinkFast transitions port
directly to forwarding state.
Link failure
43576

Cisco Catalyst 3550 Series Specifications

General

Form Factor	Rack-mountable
Manageable	Yes
RAM	64 MB
Device Type	Switch
MAC Address Table Size	12000 entries
Routing Protocol	RIP
Features	VLAN support
Power Supply	AC 120/230 V (50/60 Hz)
Ports	24 or 48 x 10/100Base-TX, 1 x GBIC, 2 x 10/100/1000Base-T
Operating Temperature	32 to 113°F (0 to 45°C)