Help with a question on Edge port

[Steph1963]

Hi,

I have found that question for the Tswitch exam and not really sure if it makes sense.

What two things will occur when an edge port receives BPDU

   The port becomes a normal STP switch port

   The port immediately transisitions to a forwarding state

   The port immediately transitions to the err-disable state

   The switch generates a Topology Change Notification.

Answers: The port becomes a normal STP switch port & The switch generates a Topology Change Notfiicaton

Can somebody confirm me that an edge port is a concept that is more related to RSTP than STP and that portfast must be enable before a port can becomes and edge port.

If I am not mistaken, STP will send a TCN BPDU only on a port changed (up/down), I am not sure how would react Spanning-Tree if a port is up for a while and suddenly receives a BPDU. Does it really send a Topology Change Notification since the port was up before it receives the BPDU.

Thanks for your help

Stephane

[Peter Paluch]

Hi Manju,

Thank you, you are very kind! And please, do not ever call yourself or your questions stupid. Your questions are always welcome here, no matter how simplistic they may seem.

Regarding your question: On Sw1, I originally configured the spanning-tree bpdufilter enable on the port Fa0/1 connected towards Sw2 Fa0/1. I simply wanted to have the Fa0/1 on Sw2 up and not receive BPDUs for a while. When I captured all the output to show that this Fa0/1 on Sw2 is currently operating as PortFast port, I removed the BPDUFilter from Sw1 - and the rest is visible in the output.

So - no, a switchport does not send BPDUs when the BPDU Filter is configured on it, and I precisely made use of that feature: to make sure that at the beginning of the experiment, no BPDUs were sent from Sw1 to Sw2 towards the PortFast port. Then, after I made all verifications that the Fa0/1 on Sw2 is running as a PortFast port, I removed the BPDUFilter from Sw1 Fa0/1, and the Sw2 Fa0/1 began receiving BPDUs and losing its PortFast status in the process.

Please note that I have been running PVST (i.e. legacy STP) but the test with RSTP yielded similar results.

Best regards,

Peter

[manju.cisco]

Thanks Peter

This did helped me to understand something on portfast and PVST

[Jon Marshall]

Peter

Great post as usual, certanly deserves a rating from me.

*** Edit - wish they would fix this can't rate after posting a reply, getting a bit irritating

Jon

[cargiulorato@gmail.com]

Thank you very much Peter,

it was very clear and useful.

Carlo

[Jonathancert_2]

This was a informative post/discussion.  I am now have excellent understanding of edge ports.

Jonathan,

[Steph1963]

Hi Peter,

Thanks for that great work. This is so helpful in my efforts to have a decent understanding of STP & RSTP.

Can I summarize this by saying that the utilisation of bpduguard is an extra step of precaution when configuring a port as portfast since only using portfast does not guarantee that this port will not participate in the spanning-tree topology.

Thanks again for all your help

Stephane

[Peter Paluch]

Hi Stephane,

the utilisation of bpduguard is an extra step of precaution

Exactly.

only using portfast does not guarantee that this port will not participate in the spanning-tree topology

PortFast was never intended to guarantee such thing, nor cause a port to not participate in the spanning tree. A PortFast port still sends and receives BPDUs. It is just allowed to go immediately to Forwarding state after being connected, but after receiving a BPDU, the PortFast setting is disabled and the port becomes a normal (i.e. non-PortFast) STP port.

Best regards,

Peter