Spanning Tree Reconfiguration - If Root Port is down.

[rajasekhar.bonthala]

Hello All,

I have some doubt on STP reconfiguration if Root port is down ...Can any one help...?

As per my understanding, if any port is down due to any reason, the corresponding bridge shall sends a TCN to the root bridge through the root port.

Lets say if a root port is down on a bridge, and so, the topology has to be re-established. As the root port itself is down, how does the Bridge convey the TCN to the root bridge ?

Thanks,

RajaSekhar                  

[Tom Watts]

Hi Raja, if the root port is down, the TCN notification can't make it to the root bridge since the link is down. The affected device will rely on the last known good bpdu. When the affected device has it's designated port down, it will remove the alternate port from discard once it hits the max age table then progress through the listening, learning, forwarding, at this time the new tcn is sent through the spanning tree topology. Once the TCN reached the root bridge, the root bridge will send a configuration bpdu then the whole spanning tree topology will update.

It can take up to 52 seconds to have the topology update in the entirety depending on spanning tree mode and size of topology.

-Tom
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[rajasekhar.bonthala]

I am trying to interpret your reply.

If root port is down, there is no immediate way to convey topology change information to the root bridge. So, the bridge waits for max age expiry and then restart the whole processor listening, learning, forwarding.

My doubt is how about the other bridges?

Thanks,

Raja

[Tom Watts]

The other bridges, once the  spanning-tree link state is down, the immediate affected bridge sends a TCN notification. If the TCN is unable to reach the root bridge, an election process happens for a new root bridge, via bridge priority or mac address. Once the new root bridge is elected, this device will send a configuration bpdu, the cascading switches will time out, update topology with the new configuration bpdu and move on to normal business.

-Tom
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[Juan Urti]

Great answer from Thomas.

The TCNs are send only for the Root Ports.

If the Root Port is down, there isn´t TCN. But, when the bridge place one port from blocked to forwarding, is here, where the TCN will be send to the Root Bridge, because one port pass from blocking to forwarding.

I have one question: in a ring topology, the switch have one root port and one designated port (one side of the ring, and the another side). If the root port is down, the designated port will be the new root port. Do this event generate a TCN?

Thanks from Argentina

Juan

[Tom Watts]

It must, the original root path will generate a TCN to the root bridge to let the root bridge know that the link is no longer present. When the block-state port becomes forwarding, another TCN must be sent to update the spanning-tree topology.

-Tom
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[BenBen]

There are two scenarios, if SW2 root port failed due to link failure:

1. SW2 will invalidate BPDU on the root port immediately.

2. SW2 will declare itself to be the root since it only receiving BPDU from the root. (Alternate/blocked port will not send BPDU).

3. SW3 will receive the BPDU and consider it is a inferior BPDU, SW3 ignores the BPDU for a (Max_Age - Msg_Age) period. During this period, the alternate port keeps as blocked status.

4. SW3 processes the inferior BPDU, change the port satus to listening and send BPDU to SW2 with SW1 as the root.

5. SW3 alternate port will be changed to designated port and forwarding, SW3 will send root a TCN.

6. SW2 agrees SW1 is the root.

7. SW2 change the port status to listening, then to learning and to forwarding, the port will be new root port.

8. When SW2 port becomes forwarding, it may send a TCN to root as well.

If SW3 root port failed, it will put alternate port to listening status, then to learning, then to forwarding. After the port is in forwarding, it will send a TCN to the root.

Didn't verify, but it should kind of like this.