Cisco vPC provides distributed port channels across two separate devices, providing redundant and loop-free topologies. Although Cisco vPC was designed and developed for intra-data center use cases, its ability to bundle links belonging to separate devices into a single logical port channel provides a possible solution to stretch VLANs between separate physical sites interconnected with DWDM and dark fibre.

 

The primary advantage of bundling physical point-to-point links interconnecting the two data centers is being able to provide stretched Layer 2 domains without creating Layer 2 looped topologies. To achieve this, VMware recommends filtering the spanning tree BPDUs across the port channel established between the two data centers, providing the ability to isolate the STP domains, one of the main technical challenges of any LAN extension technology. In essence, the idea is to replace STP with LACP as the control plane protocol.

One of the challenges of this architecture is the lack of ability to provide Layer 2 and Layer 3 communication across the same Cisco vPC bundled links. The reason for this is the lack of support for dynamic IGP peering establishment across a Cisco vPC connection. One solution or workaround is to leverage an extra pair of Layer 3 links to specifically be used for routed communication, as shown in Figure 17.