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18-2
Cisco ME 3400 Ethernet Access Switch Software Configuration Guide
OL-9639-06
Chapter 18 Configuring Flex Links and the MAC Address-Table Move Update Feature
Understanding Flex Links and the MAC Address-Table Move Update
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.
Note STP is enabled by default on network node interfaces (NNIs). It is disabled on enhanced network
interfaces (ENIs), but you can enable it. STP is not supported on user network interfaces (UNIs).
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 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 18-1, ports 1 and 2 on switch A are connected to uplink switches B and C. Because they are
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.
You can also choose to configure a preemption mechanism, specifying the preferred port for forwarding
traffic. In
Figure 18-1, for example, you can configure the Flex Link pair with preemption mode so that
after port 1 comes back up in the scenario, if it has greater bandwidth than port 2, port 1 begins
forwarding after 60 seconds; 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 18-1 Flex Links Configuration Example
If a primary (forwarding) link goes down, a trap notifies the network management stations. If the standby
link goes down, a trap notifies the users.
Flex Links are supported only on Layer 2 ports and port channels, not on VLANs or Layer 3 ports.
VLAN Flex Link Load Balancing and Support
VLAN Flex Link load-balancing allows users to configure a Flex Link pair so that both ports
simultaneously forward the traffic for some mutually exclusive VLANs. For example, if Flex Link ports
are configured for 1-100 VLANs, the traffic of the first 50 VLANs can be forwarded on one port and the
rest on the other port. If one of the ports fail, the other active port forwards all the traffic. When the failed
Switch A
Uplink
switch B
Port 1 Port 2
Uplink
switch C
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