question:when a blocked port receive inferior BPDU

szokp

when a blocked port receive inferior BPDU, will it turn into listening state immediately ? or it will do noting until max time age out ?

tndfr

szokp wrote: »

when a blocked port receive inferior BPDU, will it turn into listening state immediately ? or it will do noting until max time age out ?

switching 101, what book are you reading?

JaCkNiFe

szokp wrote: »

when a blocked port receive inferior BPDU, will it turn into listening state immediately ? or it will do noting until max time age out ?

Here is a great Cisco resource that explains the Basics of STP

szokp

JaCkNiFe wrote: »

Here is a great Cisco resource that explains the Basics of STP

thank for sharing that resource! it is very useful for learning STP.

but my question is a little more complicated than the SWF. think about a classic scene of STP:three SW is connect to each other and form a triangle topology, which A is the root birdge, port1/2 of B connect to C with a blocked state in the affect of STP converged.
if C lost connection to A, after the max-age timer out ,it will announce itself as the new root bridge . in that case , port1/2 of B will receive a inferior BPDU compare to BPDU from A . i want to know : whether port1/2 of B will wait for the max-age timer out or it will immediately begin transition to forwarding mode ?

Forsaken_GA

szokp wrote: »

thank for sharing that resource! it is very useful for learning STP.

but my question is a little more complicated than the SWF. as show in the attached figure, if C lost connection to A, after the max-age timer out ,it will announce itself as the new root bridge . in that case , port1/2 of B will receive a inferior BPDU compare to BPDU from A . i want to know : whether port1/2 of B will wait for the max-age timer out or it will immediately begin transition to forwarding mode ?

You have to wait the timer out. The switch will keep the previous BPDU associated with the port until the timer expires, then it will begin considering the inferior BPDU's, and then it'll begin changing states.

szokp

Forsaken_GA wrote: »

You have to wait the timer out. The switch will keep the previous BPDU associated with the port until the timer expires, then it will begin considering the inferior BPDU's, and then it'll begin changing states.

both CST and RSTP will act like that ?

as my opinion，transition to forwarding mode should begin right after receiving inferior BPDU form the same SW sending the previous BPDUs. since inferior BPDU means that SW has already spent its time for the max-age, local SW should not "waste" the same time for local max-age to respond for the topology change.

/usr

switching 101

I would hardly call knowing such details of STP "switching 101". There's a reason this is the CCNP forum...

chmorin

/usr wrote: »

I would hardly call knowing such details of STP "switching 101". There's a reason this is the CCNP forum...

CCNA Exam Topics wrote:

<li type="ul">Describe enhanced switching technologies (including: VTP, RSTP, VLAN, PVSTP, 802.1q)
<li type="ul">Configure, verify, and troubleshoot RSTP operation

Basic CCNA studies will pretty much tell you this:

http://www.cisco.com/en/US/tech/tk389/tk621/technologies_tech_note09186a00800c2548.shtml wrote:

The usual behavior in regards to the Institute of Electrical and Electronics Engineers (IEEE) specifications is to simply ignore any inferior BPDUs.

More advanced configuration, however, can allow you to configure Backbone Fast.

http://www.cisco.com/en/US/tech/tk389/tk621/technologies_tech_note09186a00800c2548.shtml wrote:

Backbone fast uses them because as soon as one is received, it is certain that a failure occurred on the path to the root and that you must age out at least one port.

When an inferior BPDU is detected on a non-designated port, the second phase of backbone fast is triggered. Instead of passively waiting max_age to age out ports that can be affected by the failure, a proactive way to test them immediately is introduced by means of the RLQ PDU. The RLQ is used to achieve a kind of ping for the root on a non-designated port and allowed to quickly confirm if the BPDU stored on a port is still valid or needs to be discarded.

Ref: Understanding and Configuring Backbone Fast on Catalyst Switches - Cisco Systems

Forsaken_GA

szokp wrote: »

both CST and RSTP will act like that ?

as my opinion，transition to forwarding mode should begin right after receiving inferior BPDU form the same SW sending the previous BPDUs. since inferior BPDU means that SW has already spent its time for the max-age, local SW should not "waste" the same time for local max-age to respond for the topology change.

I actually don't know if the behavior changes between the two. If you lab it up and find out, please share the results!

szokp

thank to chmorin very much! the URL you given is very helpful for me to understand the use of BackboneFast as well as RSTP. now i can get the answer from these URL , maybe it can answer Forsaken_GA's question as well.

1、 a blocked port in CST will just ignore the received inferior BPDU.
Understanding and Configuring Backbone Fast on Catalyst Switches - Cisco Systems


2、a discarding port in RSTP , on the contrast, will respond to inferior BPDU immediately.
Understanding Rapid Spanning Tree Protocol (802.1w) [Spanning Tree Protocol] - Cisco Systems

logicmyfoot

JaCkNiFe wrote: »

Here is a great Cisco resource that explains the Basics of STP

awesome tutorial repped! is there a database or link where i can find a list of such tutorials??