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802.1d topology change behavior

This question is simply a mental exercise I was running through. When I wasn't 100% sure of the answer I thought..."NetPro!"

(see attached pic)

Assume I have a stable STP tree, then I add gig link between SwitchC and SwitchD. My question revolves around exactly what SwitchC does once port 1/3 hits the learning state, bare with me here….

From switch C's perspective:

The link between SwitchC and SwitchD is brought up.

Switch C generates a TCN towards the root bridge on the designated port. The designated switch (SwitchB) generates TCA to SwitchC and sends the TCN towards the root bridge on port 1/1. The root bridge receives the TCN and generates a configuration BPDU with the TCN bit set to all downstream switches. Switches that receive the config BPDU from the root bridge set their CAM table timeout to the value of forward_delay as advertised in the config BPDU.

At the same time all of this happening SwitchC has port 1/3 in the blocking state. Eventually port 1/3 will hit the learning state. Once it does hit the learning state it is going to receive a configuration BPDU from switch D with a root path cost of 8, which is less then SwitchC's current root port path cost.

How does SwitchC respond at this point? My gut is telling me that SwitchC immediately moves the current RP into the blocking state, and proceeds with moving port 1/3 through learning and eventually forwarding.

1 ACCEPTED SOLUTION

Accepted Solutions
Hall of Fame Super Blue

Re: 802.1d topology change behavior

Hi Jeremy

Your gut feeling is pretty much right :) as i have just tested this in our lab.

Please see attached txt file for details (sorry too big to post as a message).

HTH

Jon

5 REPLIES
Hall of Fame Super Blue

Re: 802.1d topology change behavior

Hi Jeremy

Your gut feeling is pretty much right :) as i have just tested this in our lab.

Please see attached txt file for details (sorry too big to post as a message).

HTH

Jon

Re: 802.1d topology change behavior

Uplinkfast on switch C would hold the new root port 1/3 in blocking state while keeping the old root port 1/1 forwarding for some 35 seconds in order to wait for port 1/2 on bridge D to go forwarding. Then it would switchover in one shot between the old root port and the new one.

RSTP/MST would switchover the root port immediately (putting 1/3 forwarding immediately) and then trigger a topology change to flush the cam tables. An agreement would be sent to port 1/2 on D so that it can go to forwading immediately.

Regards,

Francois

Community Member

Re: 802.1d topology change behavior

The question is specific to 802.11D, and in this 'exercise' uplinkfast isn't being used.

Community Member

Re: 802.1d topology change behavior

Wow! This is a most excellent response! And as all good responses brings more questions :)

So then on to the next question. How much of the events is SwitchB aware of? (short of TCN/TCA behavior).

It's also interesting to note that Switch3 sends a TCN on the new root port. Is this TCN effectively signaling that F0/21 (old root port) is now blocking?

Community Member

Re: 802.1d topology change behavior

Nevermind...I worked it all out.

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