Root port in forwarding state

Answered Question
Jun 7th, 2007

Hi,


Can a root port in forwarding state send a BPDU with proposal flag ? I guess no. Only a designated port in non forwarding state would use the proposal flag. Root ports use only agreement flags ?



thanks,

Navid

Correct Answer by ariela about 9 years 8 months ago

Yes, on RSTP only a designated port would use the proposal flag, and root port only agreement flags.


In contrast to the 802.1D standard, the 802.1W standard does not have any bridge specific timers. All timers in the CLI are applied on a per-port basis, even though they are configured under bridge parameters.


802.1W state machines attempt to quickly place the ports into either a forwarding or discarding state. Root ports are quickly placed in forwarding state when both of the following events occur:


* It is assigned to be the Root port.


* It receives an RST BPDU with a proposal flag from a Designated port. The proposal flag is sent by ports with a Designated role when they are ready to move into a forwarding state.


When a the role of Root port is given to another port, the old Root port is instructed to reroot. The old Root port goes into a discarding state and negotiates with its peer port for a new role and a new state. A peer port is the port on the other bridge to which the port is connected.


A port with a Designated role is quickly placed into a forwarding state if one of the following occurs:


* The Designated port receives an RST BPDU that contains an agreement flag from a Root port


* The Designated port is an Edge port


However, a Designated port that is attached to an Alternate port or a Backup port must wait until the forward delay timer expires twice on that port while it is still in a Designated role, before it can proceed to the forwarding state. Backup ports and Alternate ports are quickly placed into discarding states.


for more infos:

http://www2.rad.com/networks/2005/bridges/morebri/rstp.htm

http://www.cisco.com/en/US/products/hw/switches/ps646/products_configuration_guide_chapter09186a008014f34d.html#wp1008347


HTH

Regards

Andrea

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Correct Answer
ariela Thu, 06/07/2007 - 05:20

Yes, on RSTP only a designated port would use the proposal flag, and root port only agreement flags.


In contrast to the 802.1D standard, the 802.1W standard does not have any bridge specific timers. All timers in the CLI are applied on a per-port basis, even though they are configured under bridge parameters.


802.1W state machines attempt to quickly place the ports into either a forwarding or discarding state. Root ports are quickly placed in forwarding state when both of the following events occur:


* It is assigned to be the Root port.


* It receives an RST BPDU with a proposal flag from a Designated port. The proposal flag is sent by ports with a Designated role when they are ready to move into a forwarding state.


When a the role of Root port is given to another port, the old Root port is instructed to reroot. The old Root port goes into a discarding state and negotiates with its peer port for a new role and a new state. A peer port is the port on the other bridge to which the port is connected.


A port with a Designated role is quickly placed into a forwarding state if one of the following occurs:


* The Designated port receives an RST BPDU that contains an agreement flag from a Root port


* The Designated port is an Edge port


However, a Designated port that is attached to an Alternate port or a Backup port must wait until the forward delay timer expires twice on that port while it is still in a Designated role, before it can proceed to the forwarding state. Backup ports and Alternate ports are quickly placed into discarding states.


for more infos:

http://www2.rad.com/networks/2005/bridges/morebri/rstp.htm

http://www.cisco.com/en/US/products/hw/switches/ps646/products_configuration_guide_chapter09186a008014f34d.html#wp1008347


HTH

Regards

Andrea

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