Difference between CST and 802.1D

PStefanov

Hello guys,

I am currently preparing for the switching part of my IE exam and reading the BCMSN self-study guide. I just finished reading the whole STP part of the book and I am kind of confused - I don't see any difference between the 802.1Q CST and 802.1D. Is there any difference at all and if so what is it?

Thanks in advance

Paul Boz

http://www.cisco.com/en/US/tech/tk389/tk621/technologies_white_paper09186a0080094cfc.shtml

CST is just a single spanning tree for an entire switched network, like 802.1D. 802.1S addresses this.

Cucumber

AFAIK, 802.1D Was created when there were no VLANs as there were no switches, only bridges. So only one Spanning Tree was a logical thing, for a single big LAN.

Now, when using switches, you can have many VLANs, CST accomodates all the VLANs in a switched network inside a single Spanning Tree.

So, the way I see it:

802.1D= one Spanning Tree for one physical LAN.
CST = one Spanning Tree for many VLANs

Im sure I will get corrected if Im wrong.

tech-airman

littlegrave wrote:

Hello guys,

I am currently preparing for the switching part of my IE exam and reading the BCMSN self-study guide. I just finished reading the whole STP part of the book and I am kind of confused - I don't see any difference between the 802.1Q CST and 802.1D. Is there any difference at all and if so what is it?

Thanks in advance

littlegrave,

First, keep in mind that this is a Cisco exam so you have to start with the "Cisco is better" mindset. Incidentally, in this case they are.

802.1d is the basic "Spanning Tree Protocol" or STP for short. This deals with how the root bridge/switch is elected, then the designated bridge/switches are elected, then finally the process of blocking and forwarding specific ports on the designated bridge/switches. However, this is for the case of the single VLAN.

So enter 802.1q. Not only is this a trunking protocol but 802.1q CST provides a "STP-like" election of a single root switch, then the desingated bridge/switches are elected, then finally the process of blocking and forwarding specific ports on the designated bridge/switches. However, the downside of 802.1q CST is that there is only a single root switch for all VLANs. That means if that single switch goes down, then a whole new root switch election occurs, assuming there's a backup switch to take over as the new root switch.

Enter Cisco's relatively superior solution called "PVST+." PVST stands for "Per VLAN Spanning Tree." It differs from 802.1q CST because instead of having a single root switch for all VLANs, the "Per VLAN" part of PVST means STP is run in each VLAN. That means VLAN1 might elect Switch1 as root switch, VLAN2 might elect Switch3 as root switch, and VLAN3 might elect Switch 2 as root switch. Now, the benefit of this compared to 802.1q CST is obvious because if say Switch 1 fails, then VLAN1 can run STP again and maybe elect Switch2 as it's root switch WITHOUT running STP for VLAN2 and VLAN3. With 802.1q CST, if the root switch fails, then ALL VLANS would need to run STP which would possibly almost bring the switched network to it's knees with all the BPDUs flying around with the re-election process.

I hope this helps.

Source:

1. Building Cisco Multilayer Switched Networks (BCMSN) (Authorized Self-Study Guide), 4th Edition - http://www.ciscopress.com/title/1587052733
2. Per VLAN Spanning Tree (PVST) - Cisco Systems - http://www.cisco.com/en/US/tech/tk389/tk621/tk846/tsd_technology_support_sub-protocol_home.html
3. Per VLAN Spanning Tree Plus (PVST+) - Cisco Systems - http://www.cisco.com/en/US/tech/tk389/tk621/tk847/tsd_technology_support_sub-protocol_home.html

PStefanov

tech-airman wrote:

littlegrave wrote:

Hello guys,

I am currently preparing for the switching part of my IE exam and reading the BCMSN self-study guide. I just finished reading the whole STP part of the book and I am kind of confused - I don't see any difference between the 802.1Q CST and 802.1D. Is there any difference at all and if so what is it?

Thanks in advance

littlegrave,

First, keep in mind that this is a Cisco exam so you have to start with the "Cisco is better" mindset. Incidentally, in this case they are.

802.1d is the basic "Spanning Tree Protocol" or STP for short. This deals with how the root bridge/switch is elected, then the designated bridge/switches are elected, then finally the process of blocking and forwarding specific ports on the designated bridge/switches. However, this is for the case of the single VLAN.

So enter 802.1q. Not only is this a trunking protocol but 802.1q CST provides a "STP-like" election of a single root switch, then the desingated bridge/switches are elected, then finally the process of blocking and forwarding specific ports on the designated bridge/switches. However, the downside of 802.1q CST is that there is only a single root switch for all VLANs. That means if that single switch goes down, then a whole new root switch election occurs, assuming there's a backup switch to take over as the new root switch.

Enter Cisco's relatively superior solution called "PVST+." PVST stands for "Per VLAN Spanning Tree." It differs from 802.1q CST because instead of having a single root switch for all VLANs, the "Per VLAN" part of PVST means STP is run in each VLAN. That means VLAN1 might elect Switch1 as root switch, VLAN2 might elect Switch3 as root switch, and VLAN3 might elect Switch 2 as root switch. Now, the benefit of this compared to 802.1q CST is obvious because if say Switch 1 fails, then VLAN1 can run STP again and maybe elect Switch2 as it's root switch WITHOUT running STP for VLAN2 and VLAN3. With 802.1q CST, if the root switch fails, then ALL VLANS would need to run STP which would possibly almost bring the switched network to it's knees with all the BPDUs flying around with the re-election process.

I hope this helps.

Source:

1. Building Cisco Multilayer Switched Networks (BCMSN) (Authorized Self-Study Guide), 4th Edition - http://www.ciscopress.com/title/1587052733
2. Per VLAN Spanning Tree (PVST) - Cisco Systems - http://www.cisco.com/en/US/tech/tk389/tk621/tk846/tsd_technology_support_sub-protocol_home.html
3. Per VLAN Spanning Tree Plus (PVST+) - Cisco Systems - http://www.cisco.com/en/US/tech/tk389/tk621/tk847/tsd_technology_support_sub-protocol_home.html

My question has nothing to do with Cisco or any exam at all. I think I know most if not all of the STP types. I have almost all of the Cisco Press books and I don't think I can find the answer to this question in them.

I'd rather agree with Cucumber and accept this assumption. I know the operation of the two protocols is absolutely but there must be a slight difference between those two.

Anyway, it's not so important I guess so I am moving on.

Webmaster

That's one way to respond to a great reply from someone trying to help... who obviously can't know how much you know about STP, but based on your question you don't know the different STP types as well as you claim:

littlegrave wrote:

I don't see any difference between the 802.1Q CST and 802.1D. Is there any difference at all and if so what is it?

tech-airman's reply to your question doesn't contradict Cucumber's reply so your comment regarding rather agreeing with Cucumber's assumption is odd to say least... and in contrary to tech-airman's reply not very useful to anyone else reading this topic. Also, the difference between the two is covered in any Cisco Press book that covers STP and CST. The answer is actually in the name CST as well. For more details you could google for the "802.1D-1998.pdf" which still contains the original STP standard (removed from 2004 version, when was superseded by RSTP, at least in the 802.1D IEEE standard).

optimus

Unbelievable.


tech-airman goes way out of his way and posts what I thought was a really good reponse which I actually learned something from, and gets a ration of (fill in your favorite adjective) from littlegrave. You will be so lucky littelgrave to get a good response from somebody next time.

PStefanov

OK, guys. I am sorry if you misunderstood what I meant. I just wanted to say that these two protocols are IEEE and Cisco Press books are not the right source to find the answer to this questions because there are lots of mistakes in them and you know this.

Anyway, I reread the post 5-6 times all over again and finally managed to track the difference between 802.1Q CST and 802.1D.

I am sorry again!

Thanks for the response tech-airman!