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Maximum Numbers of Switches on STP

How many switches can be connected on a huge STP domain. Is there any recommended figures for the numbers of switches. I checked the web that containes the maximum numbers of diameter. How many switches can be seated on one huge switched network.

Please help to figure out the standard and the STP rules.

Thanks & Best Regards

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Gold

Re: Maximum Numbers of Switches on STP

Well, network diameter (ND) of 7 does restrict you to how strung out your L2 switches can be. Depending on how the VLANs are distributed, it is conceivable that you could interconnect 100 or 200 switches or more; but keeping to a ND=7 would force you to introduce L3 separation to keep things manageable. No VLAN could be allowed to traverse the entire system of switches, unless it had no STP redundant links associated with it.

As to the topic of adjusting STP timers, Max Age (MA) and Forward Delay (FD) are ultimately derived from the Hello Time (HT) and Network Diameter (ND). Minimum HT is 1 second, default is 2 seconds; minimum ND is 2, default is 7.

If you know your ND and your HT (which is configured on the root bridge or switch, per VLAN), you can calculate your optimum MA and ND:

MA = 4*HT + 2*ND - 2

FD = 0.5*(4*HT + 3*ND -0.5)

If you get a value with a fraction, just round it up to the next whole number. Also, since HT can't go lower than 1, the only way you can really minimize your STP timers is to make the ND as small as possible.

While you can add switches and create networks with ND greater than 7, that increases the time required for STP to (re)converge. And, even the MA and FD values have maximums on Cisco switches: I have seen MA max at 200, FD max at 200. So there are some practical limits. (If you want to see what they are, plug these maximums into the formulas above, along with your HT of choice, and solve for ND; the smaller of the two ND's is your limit.)

FWIW, you can read all about Spanning Tree for free if you go to http://www.ciscopress.com, look up Cisco LAN Switching by Kennedy Clark and Kevin Hamilton. Last time I checked, two sample chapters are the ones devoted to STP. Everything you ever wanted to know.

Except the formula for calculating STP port cost of a FEC or GEC connection, which can be found here:

http://forum.cisco.com/eforum/servlet/NetProf?page=netprof&CommCmd=MB%3Fcmd%3Dpass_through%26location%3Doutline%40%5E1%40%40.ee9c473/0#selected_message

Hope this helps.

6 REPLIES
New Member

Re: Maximum Numbers of Switches on STP

Concerning 802.1d spanning tree:

Size of STP domain isn't restricted but

1) Default STP timers' values are computed basing on assumption that the diameter of network is 7 switches. If you have larger network you should adjust timers' values. There're formulae in Kennedy Clark's "LAN switching".

2) Larger STP domain is difficult to manage and troubleshoot.

New Member

Re: Maximum Numbers of Switches on STP

So as far as the diameter of 7 switches rule, I may put as many switches as I can. I would like to put more than 100 switches but the diameter is not more than 7. Is this still acceptable configuration? Of course, it may be difficult to manage and trouble-shoot.

New Member

Re: Maximum Numbers of Switches on STP

Diameter is computed as follows:

it is the longest possible path through the network.

So if you have 100 switches, and you have a path from first switch to 100th switch throughout all others switches, than the diameter would be 100.

New Member

Re: Maximum Numbers of Switches on STP

Hi,

look to the "Exceeding diameter 7 of STP" conversation. (Aug.11).

best regards

VBala'zs

New Member

Re: Maximum Numbers of Switches on STP

So, as far as following the rule of Diameter 7 of STP, it does not matter if I have 100 or 200 switches connected.

Gold

Re: Maximum Numbers of Switches on STP

Well, network diameter (ND) of 7 does restrict you to how strung out your L2 switches can be. Depending on how the VLANs are distributed, it is conceivable that you could interconnect 100 or 200 switches or more; but keeping to a ND=7 would force you to introduce L3 separation to keep things manageable. No VLAN could be allowed to traverse the entire system of switches, unless it had no STP redundant links associated with it.

As to the topic of adjusting STP timers, Max Age (MA) and Forward Delay (FD) are ultimately derived from the Hello Time (HT) and Network Diameter (ND). Minimum HT is 1 second, default is 2 seconds; minimum ND is 2, default is 7.

If you know your ND and your HT (which is configured on the root bridge or switch, per VLAN), you can calculate your optimum MA and ND:

MA = 4*HT + 2*ND - 2

FD = 0.5*(4*HT + 3*ND -0.5)

If you get a value with a fraction, just round it up to the next whole number. Also, since HT can't go lower than 1, the only way you can really minimize your STP timers is to make the ND as small as possible.

While you can add switches and create networks with ND greater than 7, that increases the time required for STP to (re)converge. And, even the MA and FD values have maximums on Cisco switches: I have seen MA max at 200, FD max at 200. So there are some practical limits. (If you want to see what they are, plug these maximums into the formulas above, along with your HT of choice, and solve for ND; the smaller of the two ND's is your limit.)

FWIW, you can read all about Spanning Tree for free if you go to http://www.ciscopress.com, look up Cisco LAN Switching by Kennedy Clark and Kevin Hamilton. Last time I checked, two sample chapters are the ones devoted to STP. Everything you ever wanted to know.

Except the formula for calculating STP port cost of a FEC or GEC connection, which can be found here:

http://forum.cisco.com/eforum/servlet/NetProf?page=netprof&CommCmd=MB%3Fcmd%3Dpass_through%26location%3Doutline%40%5E1%40%40.ee9c473/0#selected_message

Hope this helps.

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