STP between switches

[SJ K]

Hi

Switch 1 <-> Switch 2 <-> Switch 3

1) Does Switch 2 forward STP bpdu packets from Switch 3 to Switch 1 and vice versa in STP communication ?

2) Does Switch 2 forward bpdu of another STP type which is different from the current STP type the switch is running

e.g. Switch 2 is in MSTP - does it forward STP bpdu packets from Switch 3 to Switch 1 running in PVST ?

Thanks

[marce1000]

 -  In general it doesn't work that way : STP is 'organized' per Vlan and or configured-vlans following protocols long being in place before 'the dawn of mankind' :-)

 M.

[Joseph W. Doherty]

". . . STP is 'organized' per Vlan and or configured-vlans . . ."

Just a note, that would be if using one of Cisco's per-vlan STPs, other vendor ("standard") STP implementations often are not per VLAN, further if using the later MST variant, you select what VLANs are assigned per MST region.

[Cristian Matei]

Hi,

    1. A switch running let's say RSTP, will NEVER forward a BPDU between its trunks or access-ports; STP is hop-by-hop; each switch receives BPDU's, takes actions on them, and based on the end resulting port states and roles, it will generate its own BPDUs out on specific links; so generate, not flood/forward received BPDUs

   2. If a switch does NOT run STP, it's disabled, when it receives a BPDU, which is use a specific multicast destination address, it should forward it, treat it like a "data packet", flood it out as being multicast; based on some testing i performed, it's not always the case, and it kinda makes sense; why would you want to have a legacy switch in legacy access design, where without STP there is no switching as there will be loops.

   3. A switch running MSTP, it will detect if directly connected switches run MSTP (if so, it will send out MSTP BPDUs), or STP/RSTP (in which case it will send out STP/RSTP BPDUs).

Regards,

Cristian Matei.