RSTP processes

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Nov 17th, 2008
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Hi all, with RSTP, when I plug another switch in with a lower cost to the route, does RSTP, block all ports on the switch, then it puts the old root port into the alternate port state ?


whereas STP puts all ports in the blocking state and all ports goes through the listen, learn transitions?


cheers

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Giuseppe Larosa Mon, 11/17/2008 - 12:39
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Hello Carl,

the biggest difference is the capability of two RSTP switches to isolate themselves from the network and to exchange BPDUs about the role that each side of the new link has to take.

This is called synchronization all other non-edge ports are blocked during the time of these BPDU exchange.


Old STP switches haven't this capability to really exchange messages on the link.

So they need to wait the expiration of the timers to be able to perform a safe transition to a new loop-free topology


We can say that RSTP is more similar to the way IP routing protocols work.



Hope to help

Giuseppe


Mohamed Sobair Mon, 11/17/2008 - 22:33
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Hi,


RSTP speeds Network convergence by having three port states:


1- Discard

2- learning

3- Forwarding


With normal STP, the ports goes through 5 states in order to be in forwarding STP state:


1- Disabled

2- blocking

3- listening

4- learning

5- forwarding


Enhanced RSTP features are:

1- Alternate port

2- Backup port.


Alternate and Backup Port Roles:


These two port roles correspond to the blocking state of 802.1D. A blocked port is defined as not being the designated or root port. A blocked port receives a more useful BPDU than the one it sends out on its segment. Remember that a port absolutely needs to receive BPDUs in order to stay blocked. RSTP introduces these two roles for this purpose


please refer to the bellow link for more info about RSTP 802.1 w:


http://www.cisco.com/en/US/tech/tk389/tk621/technologies_white_paper09186a0080094cfa.shtml


HTH

Mohamed

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