Cisco offers three solutions for eliminating STP. These comprise the stacking feature on 3750/2950 models, VSS on 6500s, and vPC on Nexus 5k/7k platforms. An important point to remember in comparison of these three methods is that in vPC, the control planes of the peer switches are separate, unlike Stacking/VSS where all switches are maintained by a single control plane.
Force10 Switches offer two solutions of this nature, which are discussed below.
- Force10 switches, specifically S4810, offer a feature known as VLT (Virtual Link Trunking). VLT Is a Dual Active Control Plane implementation, of a Multi-System LAG. Thus, in operation, it is very similar to Cisco vPC.
- The other option is the use of Stacking. Stacking maintains a single control plane, while the data plane obviously remains distinct on both switches. It provides the two switches as a single logical fabric, and the loss of one switch does not affect the forwarding of the data plane on the other switch. Thus, resilience is automatically accounted for. A Master failure, should result in the backup taking over the Master function. The current master continuously synchronizes its state with the backup during normal operation. The distributed forwarding tables are retained during the failover, as is the stack MAC address.
A few points to be mindful of when configuring VLT:
- STP, MSTP, and PVST are not supported. RSTP is a prerequisite. (Edit: 2017/02/05 – while this was correct when this blog post was committed, this is no longer the case. Owing to the many enhancements to VLT over the last few years, Per VLAN instances of STP can exist in conjunction with VLT. Always refer to the current user guides for the respective platforms, to confirm current capabilities and limitations)
- Configure the bridge priorities for deterministic behavior.
- Prefer static LAG between VLT peers and LACP towards hosts/switches.
- To become VLTi the port-channel must be in default mode (no switchport, no VLAN assigned).
- The backup link should be a different link than the VLTi and if possible following a diverse path. This could be the management interface IP address.
- You do not need to manually tag VLANs on the VLTi. The system does it by itself for any VLAN tagged on VLTs.
- Configure the primary VLT peer as the RSTP primary root device and configure the secondary VLT peer as the RSTP secondary root device.