FlexLinks are an alternative to the looped access layer topology. FlexLinks provide an active-standby pair of uplinks defined on a common access layer switch. After an interface is configured to be a part of an active-standby FlexLink pair, spanning tree is turned off on both links and the secondary link is placed in a standby state, which prevents it from being available for packet forwarding. FlexLinks operate in single pairs only, participate only in a single pair at a time, and can consist of mixed interface types with mixed bandwidth. FlexLinks are configured with local significance only because the opposite end of a FlexLink is not aware of its configuration or operation. FlexLinks also has no support for preempt, or an ability to return to the primary state automatically after a failure condition is restored.
The main advantage of using FlexLinks is that there is no loop in the design and spanning tree is not enabled. Although this can have advantages in reducing complexity and reliance on STP, there is the drawback of possible loop conditions that can exist, which is covered in more detail later in this chapter. Other disadvantages are a slightly longer convergence time than R-PVST+, and the inability to balance traffic across both uplinks. Failover times measured using FlexLinks were usually under two seconds.
NNote: When FlexLinks are enabled on the access layer switch, it is locally significant only. The aggregation switch ports to which FlexLinks are connected do not have any knowledge of this state, and the link state appears as up and active on both the active and standby links. CDP and UDLD packets still traverse and operate as normal. Spanning tree is disabled (no BPDUs flow) on the access layer ports configured for FlexLink operation, but spanning tree logical and virtual ports are still allocated on the aggregation switch line card. VLANs are in the forwarding state as type P2P on the aggregation switch ports.
Figure 6 shows the FlexLinks access topology.
Figure 6 FlexLinks Access Topology
The configuration steps for FlexLinks are as follows. FlexLinks are configured only on the primary interface.