Layer 2 redundant path question

[switched switch]

hi all,

i have SWITCH A connected to SWITCH B with two parallel links between them. Both links are trunks.

SWITCH A is the root for all vlans.

on one of the links between switches I have a device inline -a tcp accelerator device and as such I need to have traffic symmetrically down this parallel link as a primary link and The other parallel link becomes the backup in case the tcp accelerator path fails (appliance dies etc).

what do I need to do to accurately set the accelerator path as primary like this?

i thought it was setting cost higher on both interfaces of the backup path but I understand this causes the STP path to reconverge and it may not be an optimal solution. I am also aware of port priority but it appeared this wasn't having any impact with influencing traffic path flow.

I am doing some testing, and I find that port priority isnt influencing the traffic down through the tcp accelerator, when i set the interface port priority low on SWITCH A through the accelerator, it doesnt swing the traffic. I guess I would have to shut the ports on the bypass path to force it. This isnt ideal, as by default if the path through the primary path SWITCH A to SWITCH B is up, I want traffic to be forced down there.

[Mark Malone · Accepted solution]

what do I need to do to accurately set the accelerator path as primary like this?

Hi set the stp cost to 5000 at each end of the link that does not have the accelerator this should force it to always be the backup , if the link is already backup and traffic is going through the accelerator it should be a minimal blip while its set

[switched switch]

Hi mark,

thank you for your valuable input. One question, what impact does setting the cost on SWITCH A on port away from the root have towards SWITH B? I was under the impression the cost is used to influence the cost to the root bridge, and in this case it is SWITCH A?

[Mark Malone]

so if you want switch a to be the root manually set it a lot lower than switch b say 4096 for all vlans and you can even leave switch b as default and altering the cost should not effect it being the root , also you can set the cost on one end of the link and it will still work it doesn't have to be  both ends but I always do it that way on redundant links to be sure there path cost at l2 is much higher so its always in blocking state ,I use cost quite frequently at l2 to build redundant paths between switches when necessary as backup links , between just 2 switches like this convergence will be minimal should not even notice it but always change stp in a planned window or out of hours just to be sure not to effect local switch users , when stp is manually controlled and well planned it cuts down convergence times even on larger networks , when you have just 2 switches directly connected like this on the switch a is set as root correctly making a backup path should not be an issue