RSTP when access port get down

[Sharanjeet_Kumar]

Hi all,

Can someone tell me "How RSTP handles when access ports get down, because RSTP don't send TCN BPDU to let inform the Root bridge"?

TIA

[Gopinath_Pigili]

• Topology changes are handled slightly different from STP.
• First, the goal of RSTP is fast re-convergence. Since ports are assumed to transition to forwarding
  relatively fast, simply increasing MAC address aging speed is not enough. Thus, when a topology change is detected, RSTP instructs the bridge to flush all MAC address table entries.
• With Ethernet, this process results in unconstrained flooding until the moment MAC addresses are re-learned. The bridge detecting a topology change sets the TC (Topology Change) bit in all outgoing BPDUs and starts sending BPDUs with the TC bit set upstream through the root port as well. This marking lasts for TCWhile=2xHelloTime seconds and allows the detecting bridge the start the flooding process.
• Every bridge that receives a BPDU with TC bit set, should receive it on either root port (coming from upstream) or designated port (coming from downstream).
  The receiving bridge performs the following:
• Flushes all MAC addresses associated with all ports with except to the port where the TC BPDU was received
• Repeats the flooding procedure by starting TCWhile timer and setting the TC bit for all BPDUs sent upstream or downstream. The receiving port is excluded from flooding, in order to ensure flooding procedure termination.

Best regards

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[M02@rt37]

Hello @Sharanjeet_Kumar

assuming it is an access port then portfast or "edge port" is configured. If the link of that access port goes Down, RSTP detects this as a link down event and transitions the port to the blocking state almost immediately. 

Alternate and Backup ports are designated as potential paths that can rapidly transition to forwarding if the primary path (through the root port) fails. RSTP ensures that alternate and backup ports are ready to take over if needed due to a link down event. When a link goes Down, RSTP recalculates the patch cost for affected ports and reevaluates the STP topology. This enables RSTP to deetermine the best path forward and make rapid adjustments.

[Joseph W. Doherty]

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@Sharanjeet_Kumar wrote:

Hi all,

Can someone tell me "How RSTP handles when access ports get down, because RSTP don't send TCN BPDU to let inform the Root bridge"?

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That depends whether RSTP believes the access port is an edge port.

An edge port doesn't need to send a TCN because you've configured it such that it's logically a dead end, i.e. it going up or down should not effect the L2 topology.