A couple easy spanning tree questions

[wrkemmer]

Just looking for some clarification on a couple of spanning tree items.

1) Do all switches transmit and receive BPDUs regardless of redundant links? For example, in a simple setup ---- switch A connected to switch B--- I'm assuming they still transmit and receive BPDUs, even though there is no redundant link.

2) I have 3 switches interconnected ( A to B to C ) in a triangle-like formation. They will converge and one link will block to stop the loop. If I add a fourth switch off of one of the existing switches will spanning tree need to reconverge for all the switches (and stop user data transmission for 50 secs. for everyone) or will the existing switches keep their spanning tree information and the new switch just learn the formation and join in after 50 secs of blocking, listening and learning?

Any information is appreciated.

[milan.kulik]

Hi,

to your questions:

1) The root switch sends BPDUs on all ports every Hello interval (2 seconds default).

Any other switch sends BPDUs on it's designated ports and listens for BPDUs on all ports. There is no relationship to redundant/non-redundant links.

2) Connecting a new switch doesn't disable user communication in the whole network. The blocking, listening, learning process is always local for the new switch ports (and the connected neigbour ports, of course). So generally there might be user data transmission breaks in the case you connect a new switch in the middle of current spannin-tree (daisy chained switches A--B--C--D--E and you connect a new switch between C and D, e.g.) or you connect a new root switch (lowest bridge priority) to the network.

There are some features (Uplinkfast, Backbonefast) which can decerase the convergence time depending on your topology.

See http://www.cisco.com/cgi-bin/Support/browse/psp_view.pl?p=Internetworking:Spanning_Tree&viewall=true for lots of STP details.

Regards,

Milan

[rsissons]

All switches will transmit and receive BPDUs and any change in the network topology will cause spanning tree to reconverge on all switches.