When Should I Use Spanning Tree?

[NERIC]

We have a pretty bust LAN here (traffic wise). We are trying to track down potential issues and I need some Spanning Tree advice. I'll start off by saying there are definitely NO loops in our network.
Is there a standard for enabling or disabling spanning tree? Should it be used only on certain device types? We have only one VLAN and all switches link back into the same main switch. I have read conflicting suggestions. Should it be on always, off always, or depending on the device connected? And which setting for which devices? Any help would be greatly appreciated.

 

[svermill]

You should always leave it on. You never know when someone is unintentionally going to create a loop. Meltdown is a bad thing. You should mostly leave default settings. However, I like to set the root deterministically. That can only be done by careful traffic flow analysis, though.

 

[NERIC]

I'm not arguing with you. But, that does contradict almost every article I've read. It also seems to be the opinion of amny people here. Which may explain the LAN problems we are having lately. I dunno. Does anyone have another opinion?

 

[svermill]

What articles? And what people on this forum. I've been around here for a while (couple of years? I can't remember for sure) and have never once seen a recommendation to turn off spanning tree.

The last time I saw spanning tree turned off in a network that ABSOLUTELY didn't have any loops, every single switch failed within 5 minutes. There were loops in multiple places in the network. Fortunately, especially since I was the guilty party that turned STP off, the network was not yet turned over to the users and therefore wasn't in production.

What are you trying to solve? STP sends out tiny little BPDUs every couple of seconds. It doesn't eat up resources on the CPU either.

Now if your saying that there's an argument to go layer 3 and not have a large layer 2 spanning tree network, I would agree in the majority of cases. But you didn't ask about L2 vs. L3. I assumed you have a L2 network and for some unnkown reason want to turn off STP.

I know you were seeking other opinions, but it would be helpful if you were clear on your real issue. Besides, what good does it do to ask for opinions until you get the one you want?

Just trying to be helpful...

 

[svermill]

I should also mention that STP doesn't block ports unless there is a loop. So there is no risk of having an unused link that isn't looped. STP won't cause resources to go unused. Some folks have that misconception.

 

[Vectra16]

We have spanning-tree disabled on certain ports due to certain pc's are unable to log onto the network. The general consensous is not to disable Spanning-tree. It has worked for our network but we basically use our switch's as edge devices. We dont disable it on our distribution Switches.

I ho[pe this can be of some Help

 

[byteya]

Spanning-tree can be disabled on ports that you know PC's are going to be plugged into and for sure not another switch. Is that was NERIC is talking about?

We do this on many of our switches so that if a device gets disconnected due to errors or user intervention it skips all the negotation and connects immediately. Otherwise there is that 30 second delay. As PC's get faster and faster it also gets rid of problems with the PC trying to access resources before the port is up.

As for the ports (GBIC's in our network) that are connected to other switches, always leave spanning tree on. At some point you or another tech will create a loop that as svermill said (and I've done it too,on a live network ), it stops traffic real quick.

*J*

 

[wybnormal]

I also would like to know what articles say to disable spanning tree? IMHO, thats a very bad idea. You dont disconnect something just because you do not understand how and why it functions.

I agree with Servmill to set the root switch manaually. This also follows Cisco's best practices for deploying a switched network. Dont forget that with VLANs, you need to set the root switch for EACH VLAN.

For some well written but a bit dense prose about spanning tree, get a copy of Cisco LAN Switching by Cisco Press. You will read more about spanning tree then the average geek wants to know. Those with access to certificationzone.com, they have a nice tutorial on spanning tree, I should know since I was the author

MikeS


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Sun Tzu

 

[svermill]

Mike,

It was wise to offer some STP resources (give a man a fish, blah, blah). I would add to your list that Priscilla Oppenheimer's "Troubleshooting Campus Networks" is about as detailed a look I've ever found into the interworkings of STP.

Vectra16, jyschaefer,

Good catch! I hadn't thought of port fast for PCs. That's almost an absolute must for most DHCP-enabled machines these days. The machines quit trying to reach the DHCP server long before STP enables the ports.

Regards,

Scott

 

[hitender]

STP should always be enable at any cost.Since its BPDUS are not much BW consumptions.
I n very small network itt is goodto keepit off Hitender (BE.CCNA)
Best Of luck
Try & Try Till U succedd and update others on u r achievments

hit_singh@rediffmail.com

 

[criticalUpdate]

I'm amazed how little the portfast feature that is on many cisco switches is mentioned in the cisco switches forum.

Best practice leave spanning-tree on the switch leave trunk ports and non-pc connected ports alone. Set portfast on for those ports with PC's and especially dhcp ones. setting the root is fine but defaults will work for most people except the ones with dhcp problems. Jeff