Information About Flex Links
Flex Links are a pair of Layer 2 interfaces (ports or port channels), where one interface is configured to act as a backup to the other. Flex Links are typically configured in service-provider or enterprise networks where customers do not want to run STP. Flex Links provide link-level redundancy that is an alternative to Spanning Tree Protocol (STP). STP is automatically disabled on Flex Links interfaces.
Release 15.1SY supports a maximum of 16 Flex Links. Flex Links are supported only on Layer 2 ports and port channels, not on VLANs or on Layer 3 ports.
To configure the Flex Links feature, you configure one Layer 2 interface as the standby link for the link that you want to be primary. With Flex Links configured for a pair of interfaces, only one of the interfaces is in the linkup state and is forwarding traffic. If the primary link shuts down, the standby link starts forwarding traffic. When the inactive link comes back up, it goes into standby mode.
In Figure 12-1, 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 and the other one 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 to forward traffic.
Figure 12-1 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.When a primary link fails, the feature takes these actions:
Detects the failure.
Moves any dynamic unicast MAC addresses that are learned on the primary link to the standby link.
Moves the standby link to a forwarding state.
Transmits dummy multicast packets over the new active interface. The dummy multicast packet format is:
– Destination: 01:00:0c:cd:cd:cd
– Source: MAC address of the hosts or ports on the newly active Flex Link port.
In Figure 12-2, ports 1 and 2 on switch A are connected to switches B and D through a Flex Link pair. Port 1 is forwarding traffic, and port 2 is in the blocking state. Traffic from the PC to the server is forwarded from port 1 to port 3. The MAC address of the PC has been learned on port 3 of switch C. Traffic from the server to the PC is forwarded from port 3 to port 1.
If port 1 shuts down, port 2 starts forwarding traffic. If there is no traffic from the PC to the server after failover to port 2, switch C does not learn the MAC address of the PC on port 4, and because of that, switch C keeps forwarding traffic from the server to the PC out of port 3. There is traffic loss from the server to the PC because port 1 is down. To alleviate this problem, the feature sends out a dummy multicast packet with the source MAC address of the PC over port 2. Switch C learns the PC MAC address on port 4 and start forwarding traffic from the server to the PC out of port 4. One dummy multicast packet is sent out for every MAC address.
Flex links interface preemption specifies one of the ports in a flex links pair as preferred for traffic forwarding. The preference can be unconditional or it can be based on bandwidth availability. See the “How to Configure Flex Links” section.
Figure 12-2 Flexlink Dummy Multicast Packets Example