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Community Member

Confusion about the need for port roles in RSTP

Hi,

I am training for my CCNA exam and currently studying STP and RSTP.

On the online document titled

Understanding Rapid Spanning Tree Protocol (802.1w) (http://www.cisco.com/en/US/tech/tk389/tk621/technologies_white_paper09186a0080094cfa.shtml
)

it is stated that it was impossible to determine the port role once the port goes into forwarding state in STP. Therefore, in RSTP, role and state of a port was decoupled.

As far as I understand, only one port is selected as root port per switch/bridge, and only one designated port per segment, and those ports are at forwarding state. In this case, why do we need to know the role of the port? As I understand, the port roles change only when a topology change occurs, and only ports at forwarding state transmit user data, regardless of their role. So, why the decoupling?

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Cisco Employee

Confusion about the need for port roles in RSTP

Hi,

it is stated that it was impossible to determine the port role once the  port goes into forwarding state in STP. Therefore, in RSTP, role and  state of a port was decoupled.

This is not what the document meant. What the document says is that STP officially recognizes only port states, and from simply telling that the port is Forwarding, you do not exactly know why - is it a Designated or a Root port? In addition, even though a port has its role immediately clear after receiving and evaluating the received BPDU, it may still need to progress through port states to reach the forwarding state. For example, if a switch is newly connected via a single link to the remainder of the network, a single received BPDU on its port is enough to determine that the port shall become a Root port - it may receive the best BPDU on that port. Still, however, even though the port is in the Root role, it needs to progress through the various states until it reaches the Forwarding state.

Splitting the port role and state is about making the operation of RSTP more well-defined. The most important use for this is when a port in the Root or Designated role can alternate between Forwarding and Discarding/Learning states as a result of RSTP Sync operation.

I am not sure if this answers your question - please ask further!

Best regards,

Peter

2 REPLIES
Cisco Employee

Confusion about the need for port roles in RSTP

Hi,

it is stated that it was impossible to determine the port role once the  port goes into forwarding state in STP. Therefore, in RSTP, role and  state of a port was decoupled.

This is not what the document meant. What the document says is that STP officially recognizes only port states, and from simply telling that the port is Forwarding, you do not exactly know why - is it a Designated or a Root port? In addition, even though a port has its role immediately clear after receiving and evaluating the received BPDU, it may still need to progress through port states to reach the forwarding state. For example, if a switch is newly connected via a single link to the remainder of the network, a single received BPDU on its port is enough to determine that the port shall become a Root port - it may receive the best BPDU on that port. Still, however, even though the port is in the Root role, it needs to progress through the various states until it reaches the Forwarding state.

Splitting the port role and state is about making the operation of RSTP more well-defined. The most important use for this is when a port in the Root or Designated role can alternate between Forwarding and Discarding/Learning states as a result of RSTP Sync operation.

I am not sure if this answers your question - please ask further!

Best regards,

Peter

Community Member

Confusion about the need for port roles in RSTP

Thank you. This correctly answers my question.

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