This article explains some important Cisco facts and nuances when connecting with EX-series Switch for PVST+ and LACP compatibility.
Problem or Goal:
Connecting Cisco and EX-series switches for PVST+ and LACP
For PVST+ running on Cisco catalyst switch and STP, RSTP or MSTP running on EX:
Some Cisco Catalyst Switch platforms with newer IOS allow vlan 1 minimization, or the ability to remove or disallow vlan 1 from trunk ports. However when this is done, the cisco switch does not send out untagged CST (common spanning tree) BPDUs for VLAN 1 to the EX Switch, nor interpret / receive such BPDUs from EX Switch. In this case the EX Switch does not interpret any STP BPDUs from Cisco, and the 2 Cisco switches interconnected via an EX Switch think that they have a direct point to point connection. Hence, it is important to allow native vlan VLAN 1 on the cisco trunk ports.
For LACP running between cisco and EX:
Some Cisco Catalyst Switch platforms with newer IOS allow the native vlan on trunk ports to always be tagged through configuration. Control protocols like CDP, VTP, LACP, PAgP etc are always transmitted on VLAN 1 (even if VLAN 1 is removed or disallowed from the trunk). If VLAN 1 is tagged, either because native vlan is tagged or native vlan is not vlan 1, then these control protocol PDUs are also tagged. A tagged LACP frame violates the standard, and the EX Switch cannot interpret such LACP frames.