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Catalyst 3550 Multilayer Switch Command Reference
OL-8566-01
Chapter 2 Catalyst 3550 Switch Cisco IOS Commands
switchport backup interface
Usage Guidelines With Flex Links configured, one link acts as the primary interface and forwards traffic, while the other
interface is in standby mode, ready to begin forwarding traffic if the primary link shuts down. The
interface being configured is referred to as the active link; the specified interface is identified as the
backup link. The feature provides an alternative to the Spanning Tree Protocol (STP), allowing users to
turn off STP and still retain basic link redundancy.
• This command is available only for Layer 2 interfaces.
• You can configure only one Flex Link backup link for any active link, and it must be a different
interface from the active interface.
• An interface can belong to only one Flex Link pair. An interface can be a backup link for only one
active link. An active link cannot belong to another Flex Link pair.
• A backup link does not have to be the same type (Fast Ethernet or Gigabit Ethernet, for instance) as
the active link. However, you should configure both Flex Links with similar characteristics so that
there are no loops or changes in behavior if the standby link takes over traffic forwarding.
• Neither of the links can be a port that belongs to an EtherChannel. However, you can configure two
port channels (EtherChannel logical interfaces) as Flex Links, and you can configure a port channel
and a physical interface as Flex Links, with either the port channel or the physical interface as the
primary link.
• If STP is configured on the switch, Flex Links do not participate in STP in all valid VLANs. If STP
is not running, be sure that there are no loops in the configured topology.
Examples This example shows how to configure two interfaces as Flex Links:
Switch# configure terminal
Switch(conf)# interface fastethernet1/0/1
Switch(conf-if)# switchport backup interface fastethernet1/0/2
Switch(conf-if)# end
This example shows how to configure the Fast Ethernet interface to always preempt the backup:
Switch# configure terminal
Switch(conf)# interface fastethernet1/0/1
Switch(conf-if)# switchport backup interface fastethernet1/0/2 preemption forced
Switch(conf-if)# end
This example shows how to configure the Fast Ethernet interface preemption delay time:
Switch# configure terminal
Switch(conf)# interface fastethernet1/0/1
Switch(conf-if)# switchport backup interface fastethernet1/0/2 preemption delay 150
Switch(conf-if)# end
This example shows how to configure the Fast Ethernet interface as the MMU primary VLAN:
Switch# configure terminal
Switch(conf)# interface fastethernet1/0/1
Switch(conf-if)# switchport backup interface fastethernet1/0/2 mmu primary vlan 1021
Switch(conf-if)# end
You can verify your setting by entering the show interfaces switchport backup privileged EXEC
command.
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