Spanning tree blocking port

[Patrick McHenry]

Hi,

I just need to clear this up in my head. If a blocking port only receives bpdus and no data, why is the interface on the other end forwarding? I know that a designated port can receive and forward traffic but, in the case of it being connected to a blocking port, can it really be forwarding? Is it only forwarding bpdus and not any data?

Thanks, Pat.

[JohnTylerPearce]

Basically BPDUs will always be forwarded from what I understand. But there is a difference between BPDUs

and a users data. Basically every segment will need a designated port in which to send traffic. If a port is blocked

on a segment, that basically means a switching loop can be formed and spanning tree took care of that by blocking

the port so only a loop-free path exists in the entire switched network (per vlan if you will). Just remember the whole

point of spanning tree is to create a loop free path within the network for frames to traverse in which they can not

be looped. Just remember if a switch has a blocked port then there is a RP and or DP somewhere else in which it

can traverse, so it will not traverse out the blocked port.

[Jon Marshall · Accepted solution]

Pat

No it will be forwarding data for any devices on that segment although if it is a point to point link between switches there won't be any devices.

sw1 -> sw2

|           |

|           |

sw3 -> sw4

there is a loop here. So lets say sw4 port to sw2 is blocking. Now lets say you connect a hub between sw2 and sw4 with devices attached. If sw2 does not forward data onto that segment then how would those devices get any traffic.

If the link is a P2P interconnect then you are right no data should be sent because sw2 will learn of no end device mac-addresses on it's DP.

Edit - the reason it is not intuitive is probably because most switch interconnect links are P2P so there are never end devices on that segment. So there seems little point in forwarding data. But it is important to realise that the function of the DP is to forward packets onto a segment. And if there are end devices on that segment the switch obviously will have a record of the mac-address(es) reachable via the DP.

Jon

[Patrick McHenry]

Jon,

Thanks for clearing that up. You knew where I was confused. The hub explaination was the ticket.

Pat.