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lap
New Member

Behavior Cisco switch when MAX STP instance allowed is exceeded | For Cisco TAC

Hi,

I would like to know the exact behavior of a C2950 or another switch when  MAX STP instances allowed is exceeded. for example on a C2950 which has Max STP instance 64. What happen if you have 71 VLANs for example? What will the switch do exactly?

Regards,

Laurent

2 REPLIES
Cisco Employee

Behavior Cisco switch when MAX STP instance allowed is exceeded

Laurent,

I am not a Cisco TAC member. However, from experimenting at our lab, I know that the STP will be running only in the first 64 VLANs in your case. The remaining 7 VLANs will not be STP-protected: essentially, STP will not run in them. If there is a physical loop in your network, you will have switching loops in the 7 unprotected VLANs.

Does this answer your question? Please feel welcome to ask further!

Best regards,

Peter

lap
New Member

Behavior Cisco switch when MAX STP instance allowed is exceeded

Hi Peter,

Thanks for your reply. I was asking because our customer is running with 71 VLANs on C3500XL and C2950. On the core switch running HSRP we were getting duplicate HSRP IP for random VLANs: VLANs 19,53,60,18, etc. On the

C3500XL and C2950  there were 64 STP instances instead of 70 (all VLANs allowed on trunks towards core) and the list of VLANs was from 2 to 66. So VLAN 19,53,60,18 was running STP on these switches but we were getting duplicate for VLAN 18,19,53,60 so I was thinking that the behavior of the switch when exceeding 64 STP instances will not only prevent STP to run for the 7 remaining VLANs but also randomly changing STP on VLANs. I want to say that these switches have randomly enable/disable STP on the 64 VLANs. What do you think?

Regards,

Laurent

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