tc bit, proposal bit in rstp

Answered Question
Jan 15th, 2009
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Hi every body!


In rstp,when link moves from discarding to fowarding state is considered as topology change.

Let say we have a switch sw1 which has root port f0/1 connected to sw2, and alternate port f0/2.For some reason, sw1 did not receive any bpdu for 6 seconds and immidiately puts its alternate port in fowarding which would be considered topology change.

1)Will sw1 sends the bpdu to sw2 with proposal and tc bit set to 1 or only proposal bit=1?


thanks a lot!



Correct Answer by Giuseppe Larosa about 8 years 2 months ago

Hello Sarah,

thanks for your kind remarks


on failure sw2 would try to claim to be the new root bridge.

sw3 will move the alternate port to forwarding and will handshake to make it its new root port.


then the two sw2 and sw3 will make the sync on their shared link.

I think sw3 leads the convergence because it is the one that can realize the root bridge is still alive.

when sw3 presents on the shared link a BPDU proposing its port as the designated port on the segment sw2 agrees and makes its port the new root port.


a blocked like an alternate port still receives BPDUs from the designated port so recovery is fast and takes the time of two negotiations that are made by exchanging two frames each (these are flash updates they don't need to wait 2 seconds to answer).

I think the one out the previous alternate port happens first.


Hope to help

Giuseppe


Correct Answer by Giuseppe Larosa about 8 years 2 months ago

Hello Sarah,

good question


see


Another form of immediate transition to the forwarding state included in RSTP is similar to the Cisco UplinkFast proprietary spanning tree extension. Basically, when a bridge loses its root port, it is able to put its best alternate port directly into the forwarding mode (the appearance of a new root port is also handled by RSTP). The selection of an alternate port as the new root port generates a topology change. The 802.1w topology change mechanism clears the appropriate entries in the Content Addressable Memory (CAM) tables of the upstream bridge. This removes the need for the dummy multicast generation process of UplinkFast.


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


It looks like that before the new port role is negotiated with the handshake and then the TC bit is set in another BPDU.

so it should send out a BPDU with only the proposal bit set.


Hope to help

Giuseppe



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Correct Answer
Giuseppe Larosa Fri, 01/16/2009 - 01:25
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Hello Sarah,

good question


see


Another form of immediate transition to the forwarding state included in RSTP is similar to the Cisco UplinkFast proprietary spanning tree extension. Basically, when a bridge loses its root port, it is able to put its best alternate port directly into the forwarding mode (the appearance of a new root port is also handled by RSTP). The selection of an alternate port as the new root port generates a topology change. The 802.1w topology change mechanism clears the appropriate entries in the Content Addressable Memory (CAM) tables of the upstream bridge. This removes the need for the dummy multicast generation process of UplinkFast.


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


It looks like that before the new port role is negotiated with the handshake and then the TC bit is set in another BPDU.

so it should send out a BPDU with only the proposal bit set.


Hope to help

Giuseppe



sarahr202 Fri, 01/16/2009 - 08:07
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one more question if you don't mind.

sw1d----rsw2d-----rsw3alter---------

Let say the link between sw1 and sw2 goes down.

The following take place:

1)sw2 sends inferior bpdu to sw3,

2) sw3 receives inferior bpdu and concludes connection to root bridge via sw1 is down, it immediately puts its alternate port in forwarding state.


Now the question is when sw3 receives inferior bpdu , will it retires its root port and makes it designated port and put it in discarding state, then perfoorm the synch with sw2?

As according to one link:

Or sw3 upon receipt of inferior bpdu, retires its root port and makes it designated port and then send bpdu to sw2.

Sw2 receives the bpdu and concludes its superior and makes the port root port.

Sw3 now perform synch with sw2.


thanks a lot


Correct Answer
Giuseppe Larosa Fri, 01/16/2009 - 09:12
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Hello Sarah,

thanks for your kind remarks


on failure sw2 would try to claim to be the new root bridge.

sw3 will move the alternate port to forwarding and will handshake to make it its new root port.


then the two sw2 and sw3 will make the sync on their shared link.

I think sw3 leads the convergence because it is the one that can realize the root bridge is still alive.

when sw3 presents on the shared link a BPDU proposing its port as the designated port on the segment sw2 agrees and makes its port the new root port.


a blocked like an alternate port still receives BPDUs from the designated port so recovery is fast and takes the time of two negotiations that are made by exchanging two frames each (these are flash updates they don't need to wait 2 seconds to answer).

I think the one out the previous alternate port happens first.


Hope to help

Giuseppe


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