spanning tree design

[carl_townshend]

hi all, can anyone tell me about this setup, we are putting in 1 core switch and 4 other switches in room A and the same in Room B, Room A and room B are uplinked to each other via the gig fibre on the core. All switches are uplinked to each other and then the core, they are all uplinked at 1 gig, do I need to make one of the core switches a root primary as I dont want any of the switches that are uplinked to the core to to be blocked, but I want the uplinks to each switch to be blocked, what do I need to configure ?

thanks

Carlos

[gpulos]

it is a very good idea to determine which core switch you would like to make the root and then decrease it's STP Priority. this will make it the root bridge if no other bridges have a priority as low.

lets say you make the first bridge STP priority set at 4096. then make the second core bridge STP priority set at 8192. (all other switches should be default 32768; or whatever your STP design requires). this would create a core as the Root and allow the other core to take over as the Root if the first core died. this would also prevent dist/access switches from becoming the root in such a scenario.

it sounds like you stated the cores are connected and then each of the dist/access switches are connected to each other and the cores. if this is the case, there is no way to keep some ports from being blocked. what will not be blocked are the Root Port(s) and Designated Port(s) of your switches. all other ports should be in blocking mode if part of a loop.

[carl_townshend]

hi there, thanks for the reply, the idea was so that no of the uplink ports on the access switches are in use whilst connected to the core, is there no I gather by making the core switch one end a root it will work, but not on the other side ? am i right ?

[eric_chan]

Carl,

Sounds like this is your topology:

core0 - Switch1 - Switch2 - Switch3 - Switch4 - core0

core1 - Switch5 - Switch6 - Switch7 - Switch8 - core1

.. and core0 and core1 are connected.

if you assign STP priority value as follow:

STP Pri

HEX DEC

0x0000 0 core0

0x1000 4096 core1

0x2000 8192 Switch1,Switch4,Switch5,Switch8

0x3000 12288 Switch2,Switch3,Switch6,Switch7

then all ports will be forwarding except

Switch4's port to Switch3

and

Switch 8's port to Switch7

HTH

Eric

[carl_townshend]

Hi There, thanks for the reply, my design is as follows

core 0, sw1, sw2, sw3, sw4, each switch is connected to each other and the core, the same for the other comms room, and then both cores are connected to each other twice. ? how would i sort this ?

[eric_chan]

Ar.. i see... so you have something like a "pyramid" .. the 4 switches are meshed and each of them has a link to the core.

hmm.. well.. if you have the following STP priority assignment:

core0 -> 0x0000 ( 0 )

core1 -> 0x1000 (4096)

and as long as your access switches' STP priority are set up anything higher value than those two values... each of your access switches are going to receive STP BPDU from 4 ports ( 1 from the core, and 1 from each of the 3 links to the other peer access switches in the room. Assuming that all port cost are the same everywhere, on any given access switch, all 3 of the links the other access switches in the room will be in blocking state.

HTH

Eric

[eric_chan]

i need to correct myself:

It's not true that "all 3 of the links the other access switches in the room will be in blocking state.

"

Some port might be in forwarding state because it determines itself to the designated bridge on a port.

.. but at the end.. you will find that Access-to-access traffic will always go through the core first.

Eric :)