2 ISP Circuitos same switch

[beconnect]

Hi All

we have Cisco 3850 in one site ( A) and Cisco Sg350 on remote site (B).

Between the 2 sites we have a link that has 2 ISP circuits ( principal and backup)

We need to automate , as best possible, if the principal goes down , backup comes up..

Can it be done with STP priorities on the ports ont both sides?

REgards

[Joseph W. Doherty]

"Can it be done with STP priorities on the ports ont both sides?"

Should be, especially if switches "see" failed path as "physically" down.

If switches don't see failed path as "physically" down, believe STP will eventually conclude path no longer present, and move to remaining path, but moving over will probably take longer.

[beconnect]

Thanks for replying

So  I need to configure STP on the respective ports , assuming the higher priority to the main circuit and a lower to the backup.

if its ok backup should be " put down " by stp. Correct?

Regards

[Joseph W. Doherty]

Yes, although to be honest, without looking it up, cannot remember whether STP prefers the higher or lower metric for a path.

Also, BTW, by "put down", we mean blocked.

[beconnect]

yes being blocked..

as soon as the main circuit fails for any reason the backup should come up..

like port 1/23 handles the main circuit cable and port 1/24 handles the backup circuit cable. 

Regards

[Giuseppe Larosa]

Hello @beconnect 

you can use spanning-tree cost to increase the cost of the secondary path.

First of all, enforce the root bridge role on the C3850

Then

you need to consider that STP uses cost on the receiving port so the spanning-tree vlan x cost 5000 is really meaningful on the non root bridge.

If using Rapid PVST it should take up to 3 hello timers to promote the backup alternate uplink in service ( if missing three STP hellos from root bridge in a row on primary link). 6 seconds

Edit:

if the primary link fails the alternate uplink is promoted immediately to forwarding state.

Hope to help

Giuseppe

[beconnect]

Hi Giuseppe

But this spanning tree config is made at the switch level?

or could it be done at port level only?

Like, enable STP on those ports that have the links and leave all the rest to the defaults..