STP / RSTP confusion...

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Apr 13th, 2009
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I am currently studying 802.1D and 802.1w, I have a few questions:


1. STP election is in the sequence of electing root bridge, root ports and than designated ports.

how does BPDUS are sent when the port at initial state is in blocking state ?


2.does this sequence counts for RSTP too?

If yes than when does proposal/agreement process comes into play for root/designated port roles?

from where does the proposal/agreement process starts? the books i have been referring , shows that the process starts from root bridge towards the edge switches. Kindly clearify...


3. following is an statement on cisco.com

"If the superior information received on the port causes the port to become a backup or alternate port,

RSTP sets the port to the blocking state but does not send the agreement message. The designated port

continues sending BPDUs with the proposal flag set until the forward-delay timer expires, at which time the port transitions to the forwarding state",but rstp doesnot use timers ?????????



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Francois Tallet Mon, 04/13/2009 - 08:15
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1. The blocking state only applies to data traffic. BPDUs are sent to a special destination mac address (in the "reserved range" of the IEEE spec) that are still received on blocked ports.

2. The proposal/agreement is purely local on a segment. Basically, when you are a designated port, you are proposing as long as you are blocking. If you receive an agreement, you go forwarding immediately else you keep proposing until the timers expire and go forwarding.

3. Modern networks are p2p. If you're a designated port connected to a backup or alternate port, it means that you're designated to a link that is not used. There is not much interest in going forwarding fast on this. However, the standard has been modified and the latest implementations should send back an agreement on alternate and backup ports too.

Regards,

Francois

Mukarram Jah Raheel Mon, 04/13/2009 - 09:04
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1. In blocking state BPDUs are only received and cannot be sent... ?

2. you mean after root bridge / root and designated port elections , the designated port starts the proposal/agreement process?

not during root and designated port election?

and which timers are you talking about,

"else you keep proposing until the timers expire and go forwarding"?

3. here the cisco.com says forward-delay timer is used , what is that for?

Francois Tallet Mon, 04/13/2009 - 09:09
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1. sent and receive of course

2. all this is local, it's not like all the bridges have to agree on a root and then we start deciding the roles of the ports. Ports have a role. When a port is selected as designated, if it is currently blocking, then it proposes in order to speed up its way out of the blocking state (a designated port is bound to be forwarding eventually). If it cannot go forwarding quickly, it uses the forward delay to do this slowly.

3. forward delay is used to put a port to forwarding in STP. RSTP just shortcuts the timers by adding some intelligence.

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