Behavior Cisco switch when MAX STP instance allowed is exceeded

[CJF74]

Hi,

I understand that when a Cisco switch exceeds the maximum STP instances that the switch model supports, that STP is no longer running or protecting any additional vlans that are added to that switch. However I can not find answer anywhere that says if the Cisco switch still passes on BPDU frames that it receives from adjacent switches that are still running STP; or does it drop BPDU frames all together like a dumb/cheap switch?

A scenario to help illustrate my question:

6 switches are connected and trunked together as follows, where SW1 & 2 are at 1 site, SW 3 & 4 at another site, and the C3850s are like WAN transit switches. A physical loop exists for WAN redundancy between sites, but protected by RSTP per vlan for the first 128 vlans. But now the number of vlans has surpassed 128 the max supported limit of the C3850's.

Nexus_Sw1 <-> C3850 <-> Nexus_Sw3
^                                                             ^
|                                                              |
Nexus_Sw2 <-> C3850 <-> Nexus_Sw4

Will these new vlans create a bridging loop, or do the C3850's become like a transit hub for these vlans, but still protected by the Nexus switches on either side?

Any help appreciated, thanks,

Carl.

[balaji.bandi]

Make sure you have a clear definition here VLAN vs STP instance.

New Model have several VLAN support which is good enough for this generation network, if not we need more information.

STP Instance is different here - you can balance the STP priority 

Take example only

nexus Sw1 root bridge for odd VLAN XX

nexus Sw2 root bridge for even VLAN XX

Again if nexus deployed in vPC another discussion.

[Andrii Oliinyk]

hi Carl

topic is far aged & u might found n answer already. but just in case: when alike situation happens switch floods traffic w/o any processing on CPU. Still the link-local MC (specifically xSTP relevant) is just dropped.