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

[lap]

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

[Peter Paluch]

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]

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