Topology Change and Switching Loop

I believe I posted a question in the recent past about a problem that happened in the Network.
A Vendor connected a server blades Chassis (Which has an internal Switch) that caused I believe a change in topology or less likely a Switching loop, since our company has a Network in SiteA and another failover Network in Site B (in case of Natural Catastrophe).

**Le 's start with topology change, I believe this can happen only if the Root Bridge is not already configured in the Network, the Root is determined by the oldest Mac address, or the lowest priority or manually configured as Root primary.

Correct?

I believe the only way to fix the topology Change is to go to the switch you want to be the Root and make it Primary


**Switching Loop
The only way in the scenario described above, that might have caused the Loop, could be if the Vendor has connected 2 ports from the blade chassis to 2 different ports on the same switch or on 2 ports on 2 different switches

I am not sure how you can fix a switching loop when it happens, even when STP is enabled

Any insights on this question are welcomed.

Thanks
jskfanAsked:
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JustInCaseCommented:
**Le 's start with topology change, I believe this can happen only if the Root Bridge is not already configured in the Network, the Root is determined by the oldest Mac address, or the lowest priority or manually configured as Root primary.

Correct?
That is incorrect. For start if STP is enabled, first thing is to elect root bridge in network. Network will not function properly without it. In fact, when you power on switch - every switch starts root bridge election with "I am root bridge", and then negotiate. If you insert new device in network root bridge election will be negotiated again.
Topology change notification is send when you attach any device to your network, to check is there network configuration change (including when you turn on PC if you don't configure portfast on port).
To prevent that some other switch is elected as root bridge you can manually configure switch that you want to be root bridge to have low value as priority.

Also, it is not only change in topology where are positioned primary and secondary root bridge, you can configure your topology so it cannot be changed by accident.
STP enchantments
Lets say that you can configure all ports that should not become root ports with root guard on those ports, so any of those ports cannot become root port. If superior BPDU is received on port - that port  goes to err-disabled state (root-inconsistent state).
You can use this mechanism to prevent change in network topology, but ...
I believe the only way to fix the topology Change is to go to the switch you want to be the Root and make it Primary
Not quite, if you did not previously set manually some switch as root bridge (and even if you did and new device had better priority value), if you remove device that is current root bridge (after topology change) from network or change priority of new device to be worse than original root bridge, network  will choose root bridge by the same criteria as it was before new device came into picture. That would lead to previous network topology as it was before.
But you don't want EVER to leave network devices to elect root bridge without you.  You always want to set primary and secondary root bridge. Network engineer's job is not about watching network, you need to understand it to be able controlling network and make it efficient.

Someone came and plugged in 2 cables in network and cause network loop?

All unused ports should be either disabled, or parts of black hole VLAN (VLAN that exists on that switch and is not part of any trunk ports on that switch) so if anyone plug cable and connects to switch without consulting with network engineer it simply will not work, and other cases that someone want to "be smart" and do more than just plug cable and establish that it does not work......

How to fix switching loop when it already happened?

"You see an network loop, you do what we do Neo. Run. You run your ass off."
Turn off switches and power on one by one, or pull out cables on trunks to see when it will stop.

So, you need to prevent those situations. When it happens in most cases you are cut of your network. You cannot telnet or ssh to switch, CPU is most likely 100% utilized, so no hope till you manually find problem.

So, with all being said, you need to understand STP otherwise - I don't need to tell you, you experienced it already, bad day ...
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jskfanAuthor Commented:
I said if the root bridge is not hard-coded , then when you plug in a new switch , it can cause topology change


To prevent  switching loop, you said you disable unused ports ... is it rule of thumb.?.....most of environments do not.

If I understand switching loops are prevented by STP, but even when STP is on , switching loops can happen. in this case are they caused by bad wiring ? if so, what is the bad wiring that can cause the switching loop?, for instance plugging 2 ends of a Cable to 2 ports on the same switch will cause a loop? what else can cause a loop ?
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jskfanAuthor Commented:
Most of Reading , I have done about STP and Switching loops, they talk about enabling STP to prevents loops.


STP is enabled by default, but still loops happen in many networks. the one thing they do not mention is the case by case that can cause a loop even with STP on,

other pro-active measures to take to prevent switching loops.. I know you mentioned Disable all unused ports, but before they use the unused ports either for trunking or access ports or etherchannel, what are the measures they should take ?
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gheistCommented:
STP is meant to fix switching loops, with it enabled it disables ports that are connected to other ports on same switch (essentially when it receives any STP packet in most cases). You can use patch cable or make software bridge between 2 blade ports - choice is yours how you implement it.

For root bridge - just make sure users cannot pull it to their ports and snoop on all traffic.
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mikebernhardtCommented:
It's possible that the bladecenter switch is running a different version of spanning tree than the switch it connects to (i.e. PVSTP vs MSTP), or the switch ports may be set up as host ports (spanning tree is running on the switch, but the ports go straight to forwarding- it's called "portfast" on a Cisco switch). If they are running the same type of spanning tree and all ports go through the listening/learning process, there's no way you should have loops.

Note: When you set a switch as "primary" all you are doing really is setting the bridge priority. The lower the number, the higher the likelihood that that it will be the lowest. The default on Cisco (I don't know what brands you're using) is 32768. Setting to primary automatically sets it to 8192. But if something else was manually set to a priority lower than that, it would still win. So I would check the settings on the blade center switch. I always set our intended root bridge to 0 to make sure nothing can be lower.
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jskfanAuthor Commented:
Thank you Guys!
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