I have a design issue which I'm not too sure how to solve.

Essentially, I am in the process of setting up a single network to spread over two physical sites which are linked together by a long distance link.

Each site has a VSS Cisco and my idea was to implement an LACP trunk across both intersite links - see attachment.

My concern is that the intersite switching devices do not support Link Loss Forwarding, so should one of the intersite links go down, I'm not sure how the LACP would manage because the interface on the Cisco is still up (just no traffic possible).

In the attached schematic, the link between Intersite A-1 and Intersite B-1 has gone down (red line), yet the link on the Cisco switches (Te-A1 and Te-B1) are still up.

What is the best way of managing this? L3 routing between the sites would be tricky because each side has devices which need to be on the same VLAN and receive broadcast traffic so I guess routing is not easy. Devices connected on Cisco VSS Site A need to see devices connected to Cisco VSS Site B on the same VLAN.

I hope that makes sense.

Thanks in advance for any recommendations.

Xavier