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Author Topic: Confused on STP with trunks  (Read 1457 times)

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Offline JohnnyLaw

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Confused on STP with trunks
« on: April 20, 2016, 01:48:15 PM »
The more I keep reading on STP with trunks the more I get confused honestly.
Right now all of my equipment is single trunked to other equipment.  From Avaya to Avaya single trunks should I be running STP on those ports?  When I enable MLT on these switches should they continue to use STP? From Avaya to junk low end switches should I run STP off on that port?

Ive read the summary of recommendations posted by avaya that contained a ton of this information but Im not quite sure when I should and should NOT be using STP I guess.  My current layout is basically branched all over from switch to switch, none of it really forms a triangle at any point, obviously no loops, I just need to make sure my MLTs are setup correctly.
Just another network guy


Offline Telair

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Re: Confused on STP with trunks
« Reply #1 on: April 20, 2016, 05:38:08 PM »
Unless you have a great reason, typically you do not want to run STP over trunk links.  MLT's shouldn't run STP either.  Spanning Tree is typically run on edge ports to make sure users don't take a cable and plug it in to the other port in their office and cause a loop.

Offline JohnnyLaw

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Re: Confused on STP with trunks
« Reply #2 on: April 21, 2016, 02:11:54 PM »
I was under the impression that you only disable STP on ports that run SMLT?  If Im just trunking switch to switch the reading Ive done looks like you still need to use it to be safe.  :-\
Just another network guy

Offline Telair

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Re: Confused on STP with trunks
« Reply #3 on: April 22, 2016, 12:09:44 AM »
I guess the question is would there be a chance for someone to loop the network between two switches?  If no, then only run STP on the local ports of each switch.  But not between switches.  The problem with spanning tree is that it tends to cut the trunk port sometimes when a loop develops.  That tends to drop whole switch stacks when it should have just shutdown a single edge port.  Hard to tell what STP will do.

If switch A is on the first floor and switch B is on the second, there is no way for someone to create a loop between them.  Then there is zero point in running STP between the two switches.  And you don't want to run STP back through your core.  Bad things can happen when a spanning tree re-convergence happens in the middle of your core network because someone reboot their PC on the edge.  I know there are ways to stop it, but it's best to never give it the chance by not running it back in to your core.  I have seen that scenario play out first-hand.  Lots of screaming and hand waving occurs very quickly.

Also, Avaya has the SLPP protocol which detects loops and can disable individual VLAN's instead of cutting whole ports.  Maybe read up on SLPP to see if it might work better for you.  And it can work on MLT links.
« Last Edit: April 22, 2016, 12:13:38 AM by Telair »