question on RSTP

Unanswered Question

Hi !

I currently studing for my CCNP certification .

Questions on STP :

1) I not sure I'm correctly understand the difference between alternate and backup port. When I had read on it I had suppose wasd correctly understood, but in lab (I use 4 switchs 2 2950 and 2 2960 connected each others) and none port as mount as backup role all blocking was alternated.

2) Was I understand correctly ? The root RSTP network flood BPDU on his designated port at regular basic (HELLO-TIME) is non-root switch received BPDU from root it relayed this BPDU but if it not received BPDU from root it generate BPDU and this one ?? what this BPDU contain (information about the root it self for example ?) If it is what this switch is made I suppose the switch it not send thoose BPDU on his root port ?

3) Is exist some differences between TCN BPDU, TC BPDU and TCA BPDU those appear the sames in some documents for me and differents in others... I one document I was understant 802.1d is able to understabt 802.1w TC/TCA BPDU but 802.1w send also TCN BPDU for compatibility issue with 802.1d....

4) I understand a ROOT bridge send a BPDU evrery hello time on his all designated port as a keep alive messages, when connected designated bridge receive those BPDU they are relayed those on there designated port an so on... When I switch miss 3 BPDU in row that connection as consider lost and the relection process can restart... What append when designated switch did not received BPDU on his root port is it generate BPDU it self and send it on his designated port as a keep alive message ? What is avertising in thoses kind of BPDU as root bridge for exemple ??

5) In case of indirect link failiure should I suppose to able to view RLQ packets pass between siwtches with some sniffer software? I was connected between them on a hub I was see STP packet to orphelin switch advertsign it self as a new root but nothing about RLQ (I was connected on hub between each switch with my sniffer software...)

6) What system are use to notifying Topology Change and what is different when a RSTP switch is connected with STP legacy switch ? When the TC timer is use and is not. I my documentation some time they are talk about TCN BPDU for legacy switch and TC BPDU is it the same thing or not what is the difference ? And if they are different why 802.1d understand

for my studdy I read the following Cisco documentation :

Understanding Rapid Spanning Tree

Protocol (802.1w)

Understanding and configuring STP

Understanding and configuring Backbone Fast on Catalyst Switches doc ID 12014

And finally BCMSN official document from Cisco Press.

Excuse me it maybe my English, I'm a French peoples, sometimes English documents are confuse for me I had to aks someone to explain to me in others words...

I have this problem too.
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shrikar.dange Fri, 02/01/2008 - 00:09


1) a. alternate port: when your root port fails this port immediately takes over as a root port.In other words this is the "NEXT BEST" root port.

b. Backup port: If you have 2 ports on the same bridge connected to the same segment then the one of the port becomes the backup port.Back up port is the "NEXT BEST" designated port for that particular segment.

It depends on your topology why you do not see alternate and back up port.Its not mandatory they will show up each time you enable RSTP.

2)Yes you are partly correct here.The RSTP bridge relays the BPDUs on all DESIGNATED ports not on root ports.The bridge will send the BPDUs with current information (current root BID) for 3 hello times if they do not recive BPDU from root or neighbouring bridge after that they will clear all protocol information try to converge as fast as possible.

3)TCN bpdu is used only in 802.1D.The TC and TCA bpdus are the configuration bpdus with different bits used for differnt purposes in the flag field of the BPDU frame format.

In 802.1D there are two different type pf message formats of the bpdus as 00(configuration) and 80(TCN) which is not the case with 802.1w wchich uses only 1 msg format 802.1W sends TCN bpdu for compability issue to its 802.1d neighbour.

4)same as ans 2

5)I did not understand your question properly.But with sniffer you should be able to see the RLQ frames.It depends on your topology and configuration.

6)For TC while timer the "ANY" RSTP bridge which notices the topology chagne sends configuration bpdus with TC flag bit set.In case of STP this is done "ONLY" by root bridge.

TCN bpdu is only used in 802.1D it is different message type all together (80) while TC and TCA are the configuration message types (00) with different bit set in the flag field of the configuration BPDU.

In case you are connecting RSTP bridge to legacy or 802.1D bridge then 802.1w will send TCN bpdu to this legacy switch as on that port both the switches will be running 802.1D



shri :)

Hi !

I had posted this on before you had response to me on other discussion. Because I did not received any answer back on this post I had ask to you same question on the other discussion. I the first time I had posted my question I had limitation of 4000 caracters it was the reason I had split my question in 2 post... because you and we have more historic on other post I will continue on the other one....

thanks a lot !!


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