STP basic question

Unanswered Question
Apr 22nd, 2010

Hi,


I've seen comments where people say you don't necesserially need to run STP on switches. I'm confused as to why people say that, a few questions on this


1. If they don't run STP then what do they run

2. How do people take care of redundant paths and automatically unblock paths once a failure is detected.

3. What situtations don't require STP?


Thanks

Dan

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ycae Thu, 04/22/2010 - 04:22

I think that STP is not 100% used when you have VSS configuration. But I think the underlying mechanism is still STP as a double protection.


In my opinion I think that STP or better rstp should be used to ensure that you will not have any broadcast storm in the network. If I remember right, You can disable STP on cisco switches but as soon as a cisco switch detects that the attached device is a switch, it will ensure that there will be no storm....it is somewhat a last resort protection.


Yves

KARUPPUCHAMY MA... Thu, 04/22/2010 - 04:24

Hi,


1. If they don't run STP then what do they run


No...we can run only STP.


2. How do people take care of redundant paths and automatically unblock paths once a failure is detected.


If you configure STP properly in your network, that will take care of  redundant path.no need of any people's intervention


3. What situtations don't require STP?


If you are not having redundant path, then no need to run STP


for more info,just have a look into the below URL


http://www.cisco.com/en/US/tech/tk389/tk621/technologies_configuration_example09186a008009467c.shtml


Thanks

Karuppu

dan_track Thu, 04/22/2010 - 04:33

Thanks for that.


You mentioned: "If you configure STP properly in your network, that will take  care of  redundant path."


Don't the switches automatically take care of the STP, why do I need to configure it? And what do you mean by configure it? Which scenarios would configuring it be useful.


Thanks

Dan

ycae Thu, 04/22/2010 - 04:38

Configuring it gets useful if you would like to specify which of your switches is the root bridge.

You can trust the protocol to do this by its own but it is always better if you fully understand your network and know what is the normal path of a packet and what would be the path if one of the links would fail. By specifying the root bridge, you can exactly say how your packets flow through the network.


Another option would be to specify portfast. It helps end points to connect to the network in a quick way without the listening and learning phase.


Yves

Jon Marshall Thu, 04/22/2010 - 04:42

Dan


dan_track wrote:


Thanks for that.


You mentioned: "If you configure STP properly in your network, that will take  care of  redundant path."


Don't the switches automatically take care of the STP, why do I need to configure it? And what do you mean by configure it? Which scenarios would configuring it be useful.


Thanks

Dan


Switches run PVST+ by default. You can change it to MST or Rapid-Pvst+ but if you don't do anything then the switches will still be running STP and will still block redundant paths.


If you run PVST+ you may want to configure portfast, uplinkfast, backbonefast. You may also want to manually specificy which switch will be the STP root.


If you run MST or Rapid-PVST+ you don't need to worry about uplinkfast/backbonefast but you still need to tell the switch which ports are portfast etc.


So it can be a plug and play protocol in it's most basic sense but it is recommended to optimise it for your environment.


6500 VSS & Nexus VPCs relegate STP to a failsafe rather than an active participant but it should still be run. And even if you no redudant paths you should still run it just in case one is added.


Note the above applies to Cisco switches. Other vendors can do things slightly differently eg. Nortel do not utilise STP for redundant links.


Jon

KARUPPUCHAMY MA... Thu, 04/22/2010 - 04:48

Hi,


By default in all the cisco switches PVST+(Per VLAN spanning tree) is enabled.


But you have to optimize or fine-tuning the PVST+ configuration to make STP  works fine.


Take a example that,


In your switching network you need to elect root bridge based on the priority value.If priority value is same (if you have not fine tuned, the default priority value 32768) then it MAC address will comes to picture for calculation..


In this scenario, if someone is inserted the new switch with the priority value of less than 32768, then the new switch will act as root bridge and your network will be unstable.


So that you need to fine tune the STP config.



Hope you got it.


Thanks

Karuppu

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