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New Member

Stp blocking ports

If a stp segment has 3 switch ports connected to it and one is the dp and two are blocking, what is the process for the two blocking ports to 1. recognize the dp has failed 2. Decide which one of them will be the new dp

Thanks,

Brad

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Cisco Employee

Stp blocking ports

Hi everyone,

Brad is asking about shared segments that are well recognized by all STP versions, and yes - they can be frequently spotted in networks using hubs, unmanaged switches, WiFi interconnections and similar. While not the best practice indeed, such networks do exist and STP has to deal with them. So - Brad, you're posing some very good questions.

what is the process for the two blocking ports to 1. recognize the dp has failed

The DP is the only port that sends BPDUs onto the segment. The remaining blocking ports depend on receiving these BPDUs, and in fact, it is precisely because of these BPDUs that the remaining ports know that they should remain blocking. Remember that the DP is the port that sends the best BPDUs on the segment. If a port receives a superior BPDU than its own (and is not a root port itself), it becomes a blocking port.

Now, if the DP has failed, it has also stopped sending BPDUs. The remaining blocking ports will keep the BPDU stored for max_age seconds after which it expires.

2. Decide which one of them will be the new dp

After the BPDUs from the former DP stop arriving at the blocking ports and expire after max_age seconds, both ports will move into Listening state and start sending BPDUs themselves. One of these ports will send superior BPDUs and will gradually proceed to become the new DP while the other port will receive these superior BPDUs and again see that it is not the author of the best BPDUs itself, so it will revert back to the Blocking state.

Feel welcome to ask further!

Best regards,

Peter

9 REPLIES
New Member

Stp blocking ports

I'm curious how you plan on getting 3 switches on the same STP segment... are you planning on using a hub? or unmanaged switch that isn't running STP? Generally best practices dictate only direct links between switches.

New Member

Stp blocking ports

Thanks for taking the time to reply. The reason I asked the question is because I'm trying to learn about the workings of stp not setup a network so best practices are not really relevant here. Just assume the network is as described.can you answer the question?

New Member

Stp blocking ports

@jcarvaja, I just assumed CCSPs knew everything. I guess you guys are actually human after all.

Stp blocking ports

That is correct, there are areas that we need to improve as every other human being That is just common sense

Looking for some Networking Assistance? Contact me directly at jcarvaja@laguiadelnetworking.com I will fix your problem ASAP. Cheers, Julio Carvajal Segura http://laguiadelnetworking.com
New Member

Stp blocking ports

@jcarvaja, why did you remove your original post?

Cisco Employee

Stp blocking ports

Hi everyone,

Brad is asking about shared segments that are well recognized by all STP versions, and yes - they can be frequently spotted in networks using hubs, unmanaged switches, WiFi interconnections and similar. While not the best practice indeed, such networks do exist and STP has to deal with them. So - Brad, you're posing some very good questions.

what is the process for the two blocking ports to 1. recognize the dp has failed

The DP is the only port that sends BPDUs onto the segment. The remaining blocking ports depend on receiving these BPDUs, and in fact, it is precisely because of these BPDUs that the remaining ports know that they should remain blocking. Remember that the DP is the port that sends the best BPDUs on the segment. If a port receives a superior BPDU than its own (and is not a root port itself), it becomes a blocking port.

Now, if the DP has failed, it has also stopped sending BPDUs. The remaining blocking ports will keep the BPDU stored for max_age seconds after which it expires.

2. Decide which one of them will be the new dp

After the BPDUs from the former DP stop arriving at the blocking ports and expire after max_age seconds, both ports will move into Listening state and start sending BPDUs themselves. One of these ports will send superior BPDUs and will gradually proceed to become the new DP while the other port will receive these superior BPDUs and again see that it is not the author of the best BPDUs itself, so it will revert back to the Blocking state.

Feel welcome to ask further!

Best regards,

Peter

New Member

Re: Stp blocking ports

@Peter, Thanks for the great info. I had always assumed the TCN BPDU is what initiated the process for the blocking ports to elect a new DP but in reading Cisco Press' "LAN Switching" it seemed to be saying that the TCN is really only used to get switches to lower their aging timers so the network converges faster.

I really appreciate 1. you promoting questions and discussion and 2. taking the time to give a detailed answer.

Thanks,

Brad

Cisco Employee

Stp blocking ports

Hi Brad,

You're very much welcome! Thank you for your kind words.

I had always assumed the TCN BPDU is what initiated the process for the  blocking ports to elect a new DP but in reading Cisco Press' "LAN  Switching" it seemed to be saying that the TCN is really only used to  get switches to lower their aging timers so the network converges  faster.

The book is correct. The TCN is used in the STP to inform the root bridge that somewhere in the network, a topology change occured, and as a result, the MAC addresses in the switching tables may point towards incorrect ports. The root bridge will then start sending BPDUs with the TC flag set to inform all other bridges to cut down the MAC table aging time to 15 seconds instead of the default of 300 seconds.

You may be interested in reading the following article - it helped me to understand the topology change process in STP very much.

http://www.cisco.com/en/US/tech/tk389/tk621/technologies_tech_note09186a0080094797.shtml

Best regards,

Peter

New Member

Stp blocking ports

@Peter, That link was very helpful. It directly addressed the issue that stumped me. I'm just starting out on my path to get my CCIE R&S so I'm sure I will have many more questions. Thanks.

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