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STP Basics

Well, basically, as I understand that, STP finds shortest paths from each switch to the root switch using port weights and makes ports along each path - forwarding and all others - blocking. Now I'm trying to understand what was the point of introducing concepts of root ports and designated ports. Effectively, root port is a port which looks towards the root switch and designated port is a port which looks from the root switch. Do they have any other differences?Or we can say that this terminology is intentionally overly redundant?

--- Nikita Andreev
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Cisco Employee

Re: STP Basics

Hi Nikita,

The main different between the root port and the destination port is that the root port recieves BPDU [superior BPDU] from root switch on that port and thus avoiding this switch to become the root. Through the designted port, the switch forwards BPDU to inform the peer switch that this has the superior BPDU and thus avoiding the connected blocked port becoming forwarding.

Let me know if the above answers your question.

Regards,

Hari

9 REPLIES

STP Basics

Harmont12345 wrote:

Well, basically, as I understand that, STP finds shortest paths from each switch to the root switch using port weights and makes ports along each path - forwarding and all others - blocking. Now I'm trying to understand what was the point of introducing concepts of root ports and designated ports. Effectively, root port is a port which looks towards the root switch and designated port is a port which looks from the root switch. Do they have any other differences?Or we can say that this terminology is intentionally overly redundant?


Hello ,

The root port is the port the switch chooses for its overall best path to the root switch. The designated port is chosen between switches that share a network segment; it is based on each switch advertised cost to the root.

Root port is the closes to the root on the switch itself while designated port is the closest to the root on the link and are forwarding ports that are generally upstream facing towards the root bride.  Designated ports are downstream facing, forwaring ports away from the root bridge.

Hope to Help !!

Ganeshh Iyer

Rate if it Helps ...

New Member

Re: STP Basics

How is it different from what I said? Yeah, I know that a root port is a port which is closest to the root and designated port is a port with a smallest cost on a segment. But generally both port types lies on the path from the non-root switch to the root switch. Just from the opposite sides.

--- Nikita Andreev

--- Nikita Andreev

Re: STP Basics

Harmont12345 wrote:

How is it different from what I said? Yeah, I know that a root port is a port which is closest to the root and designated port is a port with a smallest cost on a segment. But generally both port types lies on the path from the non-root switch to the root switch. Just from the opposite sides.

--- Nikita Andreev


Nikita,

I just tried to provide my way of explanation ..I know there is no difference what you have written... to answer to your query " the difference what i can see is the role for each port in stp to converge fast with respect stp design " apart from that rest of the things is know to you ..

Ganesh

New Member

Re: STP Basics

the difference what i can see is the role for each port in stp to converge fast with respect stp design

Can you explain that a little bit further. How their roles are different in STP convergence process?

--- Nikita Andreev

--- Nikita Andreev

Re: STP Basics

Harmont12345 wrote:

the difference what i can see is the role for each port in stp to converge fast with respect stp design

Can you explain that a little bit further. How their roles are different in STP convergence process?

--- Nikita Andreev

There are four distinct port roles that switch ports are automatically configured for during the spanning-tree process out of which i am just want to convey for root and designated port. It just a role defination ...

Root Port

The root port exists on non-root bridges and is the switch port with the best path to the root bridge. Root ports forward traffic toward the root bridge

Designated Port

The designated port exists on root and non-root bridges.

          a- For root bridges, all switch ports are designated ports.

          b-For non-root bridges, a designated port is the switch port that receives and forwards frames toward the root bridge as needed. Only one designated port is allowed per segment.


Ganesh

New Member

Re: STP Basics

I'm sorry, but how your answer is related to my qestion? To be more clear, I want to understand the following. What is RP role in convergence process? What is DP role in convergence process? And what is the core difference between them from that viewpoint?

--- Nikita Andreev

--- Nikita Andreev
Cisco Employee

Re: STP Basics

Hi Nikita,

The main different between the root port and the destination port is that the root port recieves BPDU [superior BPDU] from root switch on that port and thus avoiding this switch to become the root. Through the designted port, the switch forwards BPDU to inform the peer switch that this has the superior BPDU and thus avoiding the connected blocked port becoming forwarding.

Let me know if the above answers your question.

Regards,

Hari

New Member

Re: STP Basics

That makes much more sense. I'll review that. This made me realize that DPs lie not only across the shortest path, but exist also on segments blocked from one side and prevent it from starting to forward.

--- Nikita Andreev

--- Nikita Andreev
Cisco Employee

Re: STP Basics

Hi Nikita,

Just add one more bit of informaiton, spanning tree find the best cost path rather than shortest path.

For example, let's say I have two links to the root switch. Through first link, I have to cross 1 100MB link to reach the root switch and through the second link I have to cross two 1Gig link to reach the switch. Though the first link seem to be the shortest path, the switch will still choose the second link as it has better cost path.

Regards,

Hari

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