Spanning tree in Blocking but there are input and output packets

[c.ong]

Hi Sir,

Good day to you.

When I issued show spanning-tree of an interface, the state is in blocking mode. However, when I issued show interfaces of the same interface, I can notice there is input and output packets traversing the interface. WHy is it so? I thought spanning will not allow any packet to pass through if it is in blocking mode. I am afraid this will cause a potential loop.

Thank you.

Delon

[farkascsgy]

In blocking state BPDU packets are received, don't forget this issue.

Anyway clear the counter and check again the inout and output counters.

switch#clear counters gigabitethernet0/1

bye

FCS

Please rate me if I helped.

[globalnettech]

Hello,

in addition to Csaba's post, check out this explanation of what a port in the blocking state does:

A port in the blocking state performs as follows:

Discards frames received from the attached segment.

Discards frames switched from another port for forwarding.

Does not incorporate station location into its address database. (There is no learning at this point, so there is no address database update.)

Receives BPDUs and directs them to the system module.

Does not transmit BPDUs received from the system module.

Receives and responds to network management messages.

HTH,

GNT

[laurentElens]

Hi Delon,

no way to make a sampling?

(Ethereal is for free.)

Regard

[laurentElens]

and maybe a little "monitor sessions ..." command.

(I wonder if it can mirror the frames to a lower speed interface.)

[Kevin Dorrell]

When a port is blocking, it blocks at the point that the interface joins the bridge, not at transmitter/receiver of the interface.

What does this mean in practice?

It means that the interface can still receive packets, and will still count them. Don't forget that the other end of this link will still be forwarding - mainly broadcasts and flooded unicasts. The bridge port will discard everything except the link-local stuff - CDP, BPDUs, etc.

In terms of transmission, the interface will not be getting any traffic from the bridge. But it will still be sending link-local protocols such as BPDUs, CDP, etc. Expect some transmitted traffic, but not as much as is being received.

It is precisely because the BPDUs are still being transmitted and received that the bridge knows to keep the (bridge-)port blocked, and so avoids the forwarding loop.

Does that help?

Kevin Dorrell

Luxembourg

[c.ong]

Thanks Kevin,

Delon