Network topology , whether to use STP or not?

[srikanth ath]

Hello,

I have 4 Switches and now my organization want to deploy a new L3 switch betwreen L2 and router ( to create 15 Vlans for my organization) ...  all the four L2 switches connected to  L3 switch directly but these L2 switches are not connected to one another . so there is no point of using STP here..   BUt iam worried about the network we are going to use .. lets say we have 14 servers and all of them are used by internal employees ... if a server in switch 1 is accessed by switch 2 host through L3 ( and so Maximum employees where using it through L3 ) all the time will der be any performance degradation......i just need a justification whether its better to use STP for the network or Go on without using STP in network by not interconnecting all L2 switches...........please provide me all the dis advantages and i have without using STP in my network . as once i done deployment and configuration it should be for long run ....thanks in advance

[Joseph W. Doherty]

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Posting

The simple version is: you need STP to break L2 loops but since STP L2 loops can be by design or by accident, having it always enabled protects against the latter even when not required by the former.

The biggest disadvantage of having it always enabled, especially when not needed by design, is the time it takes for ports to go active.  This is mitigated by using a later version of STP (e.g. rapid-STP) and/or extra configuration settings (e.g. portfast for edge ports).

[srikanth ath]

I need,  will there be any performance degradation in long run ?  as the load in L3 switch will be more.. without using the STP and L2 switches are not interconnected...

[Joseph W. Doherty]

Disclaimer

The  Author of this posting offers the information contained within this  posting without consideration and with the reader's understanding that  there's no implied or expressed suitability or fitness for any purpose.  Information provided is for informational purposes only and should not  be construed as rendering professional advice of any kind. Usage of this  posting's information is solely at reader's own risk.

Liability Disclaimer

In  no event shall Author be liable for any damages whatsoever (including,  without limitation, damages for loss of use, data or profit) arising out  of the use or inability to use the posting's information even if Author  has been advised of the possibility of such damage.

Posting

As the other posters have noted, additional load on the switch is trivial.  The only time this might not be true is when you have a flapping L2 loop.  If you do have a loop, you would really, really want STP active then, because otherwise your network will generally meltdown.

[Jon Marshall]

In my opinion you should never turn off STP. There is no significant performance hit in running STP and if you use RSTP as Joseph suggests then it will converge very quickly. It doesn't matter whether you interconnect your L2 switches or not. Even if there are no loops in the network you should still run STP.

It is important to understand that in a working network STP does not add a lot of load to your switches. It is merely the exhange of BPDUs between switches and this is minimal traffic. As long as there are no changes to the network then there is no need for an STP recalculation. It is important to use spanning-tree portfast for end hosts.

If you turn off STP you gain very little but stand to lose an awful lot. You may not have redundant connections but it only takes a misconfiguration or a user connecting something up they shouldn't and you may well get a L2 loop. A L2 loop is not like a L3 loop in that a L3 loop generally just stops packets getting to where you want them to go. A L2 loop on the other hand can bring down your entire network.

I honestly can't see an advantage to not running STP.

Jon

[Peter Paluch]

Hello,

I completely agree with Joseph and Jon. The STP is in its principle a very lightweight protocol and when configured and deployed properly, there is no measurable impact on the CPU or memory footprint on your switches. With more active VLANs, I would suggest running MSTP to reduce the load but otherwise, the STP is one of the least load-creating processes on your switch.

Not running STP in a switched network is like being without a health insurance. You will eventually get ill some day, and you will need a medical care, and without a health insurance, it is going to cost you a lot. With STP, it is the same. Until the network is without physical loops, you may live well without STP, but any simple loop caused by an inadvertent connecting of the same cable back to the network will put it down on its knees - and then the real, very costly trouble will start.

There is no reasonable point in not running STP.

Best regards,

Peter