C H A P T E R 21 Configuring Flex Links And Mac Address-Table Move Update - Cisco Catalyst 4500 Series Configuration Manual

About Flex Links
Flex Links
Flex Links are a pair of Layer 2 interfaces (switch ports or port channels) where one interface is
configured to act as a backup to the other. Users can disable STP and still retain basic link redundancy.
Flex Links are typically configured in service provider or enterprise networks where customers do not
want to run STP on some interfaces.
Note The Catalyst 4500 series switch supports a maximum of 16 Flex Links.
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. 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 fails. At any given time, only
one of the interfaces is in the forwarding state and forwarding traffic. If the primary link fails, the
standby link starts forwarding traffic. When the active link reactivates, it enters standby mode and does
not forward traffic. STP is disabled on Flex Links 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 shuts down, port 2 activates and starts
forwarding traffic to switch C. When port 1 reactivates, it enters standby mode and does not forward
traffic; port 2 continues forwarding traffic.
You can also choose to configure a preemption mechanism, specifying the preferred port for forwarding
traffic. In
after port 1 reactivates in the scenario, and it has greater bandwidth than port 2, port 1 begins forwarding
after a duration equal to the preemption delay; and port 2 becomes the standby. You do this by entering
the interface configuration switchport backup interface preemption mode bandwidth and switchport
backup interface preemption delay commands.
Figure 21-1
Uplink
switch B
If a primary (forwarding) link shuts down, a trap notifies the network management stations. If the
standby link shuts down, a trap notifies the users.
Flex Links are supported only on Layer 2 ports and port channels. Flex Links are compatible with
promiscuous trunks. It is not supported on isolated PVLAN trunks.
VLAN Flex Links Load Balancing and Support
VLAN Flex Links load balancing allows you to configure a Flex Links pair so that both ports
simultaneously forward the traffic for mutually exclusive VLANs. For example, if Flex Links ports are
configured for 1 to100 VLANs, the traffic of the first 50 VLANs can be forwarded on one port and the
Software Configuration Guide—Release IOS XE 3.3.0SG and IOS 15.1(1)SG
21-2
21-1, ports 1 and 2 on switch A are connected to uplink switches B and C. Because they are
Figure 21-1, for example, you can configure the Flex Links pair with preemption mode so that
Flex Links Configuration Example
Port 1
Switch A
Chapter 21 Configuring Flex Links and MAC Address-Table Move Update
Uplink
switch C
Port 2
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