I am having a Switch

Cisco IOS Software, C2960 Software (C2960-LANBASEK9-M), Version 12.2(50)SE4, RELEASE SOFTWARE (fc1)

I have interface FastEthernet0/1 up to interface FastEthernet0/48 are configured as switchport access vlan , switchport mode access, no cdp enable.


spanning-tree mode pvst spanning-tree extend system-id

My question is, whether it is possible to bridge two ports without one of the port goes into Blocking State? I mean why should it when it's a PER vlan spanning-tree.


No, not in the configuration you described. One port will be in a BLK (blocking) state.

spanning-tree mode pvst will globally enable PVST. spanning-tree extend system-id affects how the Bridge IDs are created. More details about the spanning-tree extend system-id command can be found here.

If only a single VLAN exists, any bridge between any two ports will cause a bridging loop. BPDUs will be detected and the port will change to a blocking state.

Both ports will be in a FWD (forwarding) state if:

  • Port are in two different VLANs. Bad things can happen as two different VLANs are bridged.

  • spanning-tree bpdufilter enable is configured at the interface or globally configured. This will create a bridging loop and subsequently cause a Broadcast Storm (<--very bad).

Note that enabling BPDU Filters by itself is typically very dangerous to stability of the network.

The Cisco 12.2SX Software Configuration Guide provides detailed guidance on STP (Spanning Tree Protocol) and its optional features such as BPDU Filtering.

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