Spanning Tree blocked port

[grapevine]

Assume that there are two switches and there are two ports connecting each other and one of the ports is in blocking state. Does the other switch know which port is blocked on its neighbor and how does the traffic flow?

[m.schwarz2016]

in order to provide the desired path redundancy, as well as to avoid a loop condition, STP defines a tree that spans all the switches in an extended network. STP forces certain redundant data paths into a standby (blocked) state and leaves other paths in a forwarding state. If a link in the forwarding state becomes unavailable, STP reconfigures the network and reroutes data paths through the activation of the appropriate standby path.

Here is the full article:

http://www.cisco.com/c/en/us/support/docs/lan-switching/spanning-tree-protocol/5234-5.html

[grapevine]

My question is would a switch know if the port on neighboring switch is in 'blocking state' for a specific vlan?

[Scott Strickland]

Technically, yes.  So assume you've got your two switches connected, SW1 is the RB and SW2 is downstream.  SW1 puts both interfaces as designated forwarding.  SW2 declares one interface as the root port in forwarding state.  The other interface is altn blocking.  Both interfaces are receiving BPDUs from the RB, but in the event of an STP change, only the RP on SW2 would send a BPDU to the root bridge.

For example, assume you attached SW3 as a down stream switch from SW2.  This puts a new interface on SW2 into the forwarding state as a designated port, SW2 has to send a TC BPDU out its forwarding interfaces.  So it sends this BPDU out its root port to SW1, but not the blocking port.  This can be validated by looking at 'show spanning-tree interface x/x detail'.  If your in a lab doing this, clear your spanning-tree counters on SW1 and SW2 so you strart with clean numbers.

So yes, the RB does know, although I'm not sure that it cares because it puts all ports in the forwarding state anyway.

Scott

[pwwiddicombe]

On Nexus, be aware there are 3 types - network, edge, and normal.  While subtle, there are some instances where connecting nexus to other families will behave differently because of "assurance".  Network type on a nexus will send bpdu's on a blocked port, which can confuse, say, a 6509 in some instances.