STP Convergence Optimizations

Posted: September 1, 2015 in STP
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STP Topology Change Notifications

  • In normal STP operation, a bridge keeps receiving configuration BPDUs from the root bridge on its root port. But, it never sends out a BPDU toward the root bridge. In order to achieve that, a special BPDU called the topology change notification (TCN) BPDU has been introduced. Therefore, when a bridge needs to signal a topology change, it starts to send TCNs on its root port. The designated bridge receives the TCN, acknowledges it, and generates another one for its own root port. The process continues until the TCN hits the root bridge.
  • The TCN is a very simple BPDU that contains absolutely no information that a bridge sends out every hello_time seconds (this is locally configured hello_time, not the hello_time specified in configuration BPDUs). The designated bridge acknowledges the TCN by immediately sending back a normal configuration BPDU with the topology change acknowledgement (TCA) bit set. The bridge that notifies the topology change does not stop sending its TCN until the designated bridge has acknowledged it. Therefore, the designated bridge answers the TCN even though it does not receive configuration BPDU from its root
  • With legacy STP 802.1D , ports that are facing away from the root bride and are connected to devices that are not participating in STP ( PCs ) 
  • If a port connecting a PC by default goes down in STP , the port sends a TCN up towards the root bridge and the root bridge is then going to send a TCN Ack down to the other devices , in the case of legacy STP what does the STP TCN do ? 
  • It tells the other bridges to change the MAC address aging time ( 300 sec ) to the maximum age time ( 20 sec ) , this means in a legacy Spanning Tree Design  for any of your edge ports that are not configured as Edge ports (portfast ) when PC powers on & off  , its gonna cause the entire Layer network to to age out the entire MAC address table in 20 secs.
  • Basically you end up having the large spikes in broadcast traffic in unknown traffic because the MAC address get flushed out  & the network needs to re-learn them.
  • In Rapid STP the issue is even worse  because when TCN is generated  it  causes the switches to immediately Flush out the MAC table as opposed to wait for the MAX Age timer ( RSTP is event driven ).

# spanning tree portfast default – enables portfast on all switch ports that are not receiving BPDUs.

# spanning tree portfast bpdufilter default – enable both portfast and BPDU filter on all ports that are not Receiving BPDUs.

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