Written Lab 8 error

[iworms]

Hi, for this question in Chapter 8's written lab:
10. Which Cisco 802.1d extension stops BPDU from being transmitted out a port?The answer in the book is PortFast, but portfast alone is not enough. We need portfast plus bpdufilter to stop a port from sending BPDUs. (I tested and confirmed this on a 2950.)

[lammle]

hummm....


portfast stops BPDU from being sent out a port.
That is the Cisco definition.

Tell me more about your test.
Todd

[iworms]

I found this sentence here (http://www.cisco.com/en/US/docs/switches/lan/catalyst4000/8.1/configuration/guide/stp_enha.html#wp1047558):
By default, spanning tree sends BPDUs from all ports regardless of whether PortFast is enabled.

To test it is simple. Pick a port that has portfast on and bpdufilter off, and type:
show spanning-tree interface [INTERFACE] detail | include BPDU

You will see how many BPDU has been sent out that port. Repeat the command a few seconds later, you will see a larger number. After you turn bpdufilter on, the number stays the same.

(p.s. I'm just glad I've finished chapters 7 and 8 :D)

[iworms]

I found this sentence here (http://www.cisco.com/en/US/docs/switches/lan/catalyst4000/8.1/configuration/guide/stp_enha.html#wp1047558):
By default, spanning tree sends BPDUs from all ports regardless of whether PortFast is enabled.

To test it is simple. Pick a port that has portfast on and bpdufilter off, and type:
show spanning-tree interface [INTERFACE] detail | include BPDU

You will see how many BPDU has been sent out that port. Repeat the command a few seconds later, you will see a larger number. After you turn bpdufilter on, the number stays the same.

(p.s. I'm glad I've finished chapters 7 and 8 :D)

[iworms]

I found this sentence in a Cisco document (http://www.cisco.com/en/US/docs/switches/lan/catalyst4000/8.1/configuration/guide/stp_enha.html#wp1047558):
By default, spanning tree sends BPDUs from all ports regardless of whether PortFast is enabled.And to test it is simple:
show spanning-tree interface [INT-NAME] detail | include BPDU

This shows how many BPDUs have been sent. If bpdufilter is not on, a few seconds later the same command will show a larger number.

[jefss1]

After All, What is the correct answer?

I have the same doubt.

thanks in advance!!

[Fuzz]

BPDUFilter is required to stop sending BPDUs out of a port, regardless of PortFast. The clue is in the name - it filters out BPDUs in both directions, so you have to be careful about using it to make sure it only goes on edge ports. It effectively disables STP on that port (not the same as turning STP off, however.)

PortFast ports still send BPDUs, they just don't expect to receive them. Consider a downstream switch that gets connected to a PortFast port. How will it know that it is connected to another switch if BPDUs are not sent? It wouldn't, but the idea of PortFast is to transition through the STP states if it receives a BPDU, so by definition they must still send BPDUs.