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Configuring Flex Links
This chapter describes how to configure Flex Links, a pair of interfaces on the Catalyst 3750 switch that
are used to provide a mutual backup. Unless otherwise noted, the term switch refers to a standalone
switch and to a switch stack.
For complete syntax and usage information for the commands used in this chapter, refer to the command
Note
reference for this release.
The chapter includes these sections:
•
•
•
Understanding Flex Links
Flex Links are a pair of a Layer 2 interfaces (switchports or port channels), where one interface is
configured to act as a backup to the other. The feature provides an alternative solution to the Spanning
Tree Protocol (STP), allowing users to turn off STP and still provide basic link redundancy. Flex Links
are typically configured in service provider or enterprise networks where customers do not want to run
STP on the switch. If the switch is running STP, it is not necessary to configure Flex Links because STP
already provides link-level redundancy or backup.
You configure Flex Links on one Layer 2 interface (the active link) by assigning another Layer 2
interface as the Flex Link or backup link. The Flex Link can be on the same switch or on another switch
in the stack. When one of the links is up and forwarding traffic, the other link is in standby mode, ready
to begin forwarding traffic if the other link shuts down. At any given time, only one of the interfaces is
in the linkup state and forwarding traffic. If the primary link shuts down, the standby link starts
forwarding traffic. When the active link comes back up, it goes into standby mode and does not forward
traffic. STP is disabled on Flex Link interfaces.
In
Figure
configured as Flex Links, only one of the interfaces is forwarding traffic; the other is in standby mode.
If port 1 is the active link, it begins forwarding traffic between port 1 and switch B; the link between
port 2 (the backup link) and switch C is not forwarding traffic. If port 1 goes down, port 2 comes up and
starts forwarding traffic to switch C. When port 1 comes back up, it goes into standby mode and does
not forward traffic; port 2 continues forwarding traffic.
78-16180-02
Understanding Flex Links, page 20-1
Configuring Flex Links, page 20-2
Monitoring Flex Links, page 20-3
20-1, ports 1 and 2 on switch A are connected to uplink switches B and C. Because they are
C H A P T E R
Catalyst 3750 Switch Software Configuration Guide
20
20-1
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Troubleshooting
