rapid spanning tree convergence time

Unanswered Question
Jan 17th, 2010

Hi guys,

I have 2 6500 connected with a trunk (portchannel with 2 gigabit interfaces).

Now i want use beetwen them a 10 gigabit link.

I know i cant put the 10 gigabit intefaces in the portchannel so so i'm configuring 10 giga as a separeted trunk.

When i connect the two 10 giga interfaces what should be the convergence time?

I'm using rapid PVST.

I think beeing a new interfaces with a lower port cost convergence time is beetwen 20 and 30 sec?


I have this problem too.
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Ganesh Hariharan Sun, 01/17/2010 - 05:43


To prevent the delay when connecting hosts to a switch and during some topology changes, Rapid STP was developed and standardized by IEEE 802.1w, which allows a switch port to rapidly transition into the forwarding state during these situations.

Rapid Spanning Tree Protocol (RSTP), which provides for faster spanning tree convergence after a topology change. Standard IEEE 802.1D-2004 now incorporates RSTP and obsoletes STP. While STP can take 30 to 50 seconds to respond to a topology change, RSTP is typically able to respond to changes within 3*Hello (default is 6 seconds).

RSTP bridge port roles:

    * Root - A forwarding port that is the best port from Nonroot-bridge to Rootbridge
    * Designated - A forwarding port for every LAN segment
    * Alternate - An alternate path to the root bridge. This path is different than using the root port.
    * Backup - A backup/redundant path to a segment where another bridge port already connects.
    * Disabled - Not strictly part of STP, a network administrator can manually disable a port

RSTP is a refinement of STP and therefore shares most of its basic operation characteristics.

Hope that clear out your query !!

If helpful do rate the valuable post



Giuseppe Larosa Sun, 01/17/2010 - 05:44

Hello Alessandro,

if Rapid STP is used on both C6500 convergence time should be just the time to exchange BDPUs to perform the handshake: one side proposes its port as the designated port for segment the other side can agree on this or not.

no timers are involved and here is the great advantage of Rapid STP.

When adding a new link to an existing topology, switches use the so called synchronization wave: all links are freezed and on one single link the change is proposed to the other side.

Cisco implementation should use this freeze of other ports only when considering the root port.

It is very important to declare edge ports with spanning-tree portfast to reduce the total numbers of ports involved in a synchronization to achieve real fast convergence.



Hope to help



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