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How to use spanning tree

Posted on 2011-09-16
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we have a network that looks like a hub a spoke. The main spoke is the ISP's drop into the firewall. that connects to a stacked pair of L3 switches. attached to that pair are various other switches. I'm certain none of those switches are plugged into each other in any loop.

Off many of the switches are servers and iSCSI SAN appliances. with iSCSI, it is recommended that STP be turned off on those ports. I have a few questions:

1. Should STP be turned off on all ports but the trunk ports between the hub and spoke switches?
2. Our firewall drops into each switch in the stack and there's an HA firewall that does the same. Do I leave STP turned on for those 4 ports?
3. Most of our servers do not yet have dedicated iSCSI NIC ports. There are 2 and they are the iSCSI ports and the regular traffic ports. do I still turn STP off for those ports on the switch?
4. Some documents say if I have to leave STP on, turn on RSTP. Is that just as good as it being off or is there still some risk?

Thanks!
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Question by:MrVault
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Soulja earned 167 total points
ID: 36551760
You didn't state what type os switches you were running, so my answers will assume Cisco.

1. Should STP be turned off on all ports but the trunk ports between the hub and spoke switches?

Essentially STP is applied globally on the switch. You can disable it per vlan, but I still wouldn't recommend it. The ports connected to endpoints can have portfast enabled but never on the trunk ports connected to other switches. Also, on the endpoint ports you want to enable BPDU Guard, this way if a switch accidently gets plugged into the port it go into err disable.

2. Our firewall drops into each switch in the stack and there's an HA firewall that does the same. Do I leave STP turned on for those 4 ports?

STP is enabled globally on a switch. As stated above you can disable it per vlan, but I wouldn't recommend it.

3. Most of our servers do not yet have dedicated iSCSI NIC ports. There are 2 and they are the iSCSI ports and the regular traffic ports. do I still turn STP off for those ports on the switch?

Same answer as question one.

4. Some documents say if I have to leave STP on, turn on RSTP. Is that just as good as it being off or is there still some risk?

Rapids Spanning Tree is an improved version of STP, so use it. It is much quicker in regards to convergence.
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by:Epic7
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1. Should STP be turned off on all ports but the trunk ports between the hub and spoke switches?
 - STP is used to prevent loops. I would advise leaving STP on all access ports. We currently don't have STP on trunk ports

2. Our firewall drops into each switch in the stack and there's an HA firewall that does the same. Do I leave STP turned on for those 4 ports?
-Yes

3. Most of our servers do not yet have dedicated iSCSI NIC ports. There are 2 and they are the iSCSI ports and the regular traffic ports. do I still turn STP off for those ports on the switch?
- Leave stp on
4. Some documents say if I have to leave STP on, turn on RSTP. Is that just as good as it being off or is there still some risk?

- RSTP is Rapid spanning-tree protocal. I would advise not having a network with out some type of STP
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by:MrVault
ID: 36551832
Sorry, we're running Brocade/Foundry switches. Servers are mainly Windows Server 2008 R2 and Dell Equallogic SAN arrays.

The reason I asked about disabling per port is that you can do this on each port's settings in Brocade.

The documentation such as this link say to turn it off, but if it has to be on, enable portfast. But it sounds more ideal to have it off on the endpoints (servers and storage attached). Does it matter? http://www.cisco.com/en/US/prod/collateral/switches/ps5718/ps6021/white_paper_c11-563477.html

I don't know if Brocade has an equivalent BPDU Guard.

Eventually we'll be moving the iSCSI traffic to separate NICs on the servers and then onto different VLANs and whole switches.

@Epic7: can you explain "access ports"? If you don't have it on for trunk ports and you don't have it on for endpoints (hosts, devices), then what is left?

It seems as if the recommendation is to use STP, but yet vendors of iSCSI devices are asking that we NOT use it due to issues they see with it.
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by:Epic7
ID: 36551933
@Epic7: can you explain "access ports"? If you don't have it on for trunk ports and you don't have it on for endpoints (hosts, devices), then what is left?

access ports and endpoint ports are the same thing, just different way of saying it.

I have't worked with Brocade, i would assume they would have something similar to BPDU. It's nothing more than a security feature that. What it does is remembers the mac address of the device, if it's unplugged and a new device is plugged in it shuts down the port.
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by:Epic7
ID: 36551968
I believe my BPDU desription wasn't correct. i was thinking of port sticky.. please disregard that statement.
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Author Comment

by:MrVault
ID: 36551985
thanks.

is the concern with STP/RSTP that someone would connect switch A to B and B to C and then later accidentally connect C to A (directly or though another switch)? We have a relatively small setup, so I feel like if the iSCSI vendors say it is causing problems, and I'm the only only connecting switches right next to each other, then I feel pretty confident we're not going to introduce a loop.

I'm reading Brocade docs and they say it's global too. it's weird that in the GUI I can turn it on or off in each port's settings.
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by:Epic7
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ID: 36552029
i hope this example helps..


Car A is attempting to leave the city. He drives down the road and is able to turn right but not left. The reason he can turn right is because it leads twords a bridge that will bring him to another set of roads. The left turn would have brought him back to his house.

What STP/RSTP does is creates a loop free network. This makes doesn't allow for collisions on the network. Having a loop can cause nasty affects to your network which is why those two protocals were created to stop the issue. It doesn't stop someone from accessing a server or network device per say, that's where Access control lists come into play.

I don't think this is going to be an issue if you keep STP/RSTP on.
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by:MrVault
ID: 36552058
I see what you're saying. Should I just disregard the vendor who is saying our SAN issues could be caused by us having STP turned on for those ports?
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by:Epic7
ID: 36555036
I don't think it's the issue but turn it off. If you find that the problem is still there, than turn it back on.

Test that out and let me know what you find out.
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by:MrVault
ID: 36816813
Thanks everyone.
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