Understanding Flex Links And The Mac Address-Table Move Update; Flex Links; Vlan Flex Link Load Balancing And Support - Cisco 3845 - Security Bundle Router Software Manual

Understanding Flex Links and the MAC Address-Table Move Update

Note STP is enabled by default on the switch ports.
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. You cannot configure Flex Links on ports configured with an
Ethernet virtual connection (EVC) service instance. 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.
You can also choose to configure a preemption mechanism, specifying the preferred port for forwarding
traffic. In
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 17-1
Uplink
switch B
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, Layer 3 ports, or ports
configured with service instances.

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
port comes back up, it resumes forwarding traffic in the preferred vlans. This way, apart from providing
the redundancy, this Flex Link pair can be used for load balancing. Also, Flex Link VLAN
load-balancing does not impose any restrictions on uplink switches.
Cisco ME 3800X and 3600X Switch Software Configuration Guide
17-2
Chapter 17
17-1, ports 1 and 2 on switch A are connected to uplink switches B and C. Because they are
Figure 17-1, for example, you can configure the Flex Link pair with preemption mode so that
Flex Links Configuration Example
Port 1
Switch A
Configuring Flex Links and the MAC Address-Table Move Update Feature
Uplink
switch C
Port 2
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