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What will happen if number of vlans exceed switch limit ?

My network topo is like this:

6509A--3500xl--6509A

these two uplink is not bundle to etherchannel.

3500xl support 64 STP instance,if I define 70 vlan. So what will happen ? I am in doubt that if STP was disabled in exceed vlan and Will network loop happen ?

thanks!

4 REPLIES
Silver

Re: What will happen if number of vlans exceed switch limit ?

If your VLANs defined on that switch exceed 64 the following VLANs will have STP disabled. You can prune unneeded VLANs from your trunk andkeep VLANs local to the switch, unless there is a reason to have all the VLANs on that switch in particular. Since the 3500 has a maximum of 48 access ports I doubt that is the case.

Cisco Employee

Re: What will happen if number of vlans exceed switch limit ?

The CCO doc for this is at:

http://cco/en/US/products/hw/switches/ps637/products_configuration_guide_chapter09186a00800d9d3c.html#91917

"Each VLAN is a separate STP instance. If you have already used up all available STP instances on a switch, adding another VLAN anywhere in the VLAN Trunking Protocol (VTP) domain creates a VLAN that is not running STP on that switch. For example, if 250 VLANs are defined in the VTP domain, you can enable STP on 64 of those VLANs. The remaining VLANs must operate with STP disabled."

Regards,

Steve

New Member

Re: What will happen if number of vlans exceed switch limit ?

thanks for your reply .

If remaining VLANs must operate with STP disabled, is it locally or global ? I mean when it disabled in 3500, did it disable on 6509 , too ?

If it also disable on 6509,then there will be network loop and brought down whole network.

thanks!

New Member

Re: What will happen if number of vlans exceed switch limit ?

ooh,I got answer:

http://www.cisco.com/univercd/cc/td/doc/product/lan/c2900xl/29_35wc6/scg/swsyst.htm#xtocid47

Switches that are not running spanning tree still forward bridge protocol data units (BPDUs) that they receive so that the other switches on the VLAN that have a running STP instance can break loops. Therefore, spanning tree must be running on enough switches so that it can break all the loops in the network. For example, at least one switch on each loop in the VLAN must be running spanning tree. It is not absolutely necessary to run spanning tree on all switches in the VLAN; however, if you are running STP only on a minimal set of switches, an incautious change to the network that introduces another loop into the VLAN can result in a broadcast storm.

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