ANNOUNCEMENT - The community will be down for maintenace this Thursday August 13 from 12:00 AM PT to 02:00 AM PT. As a precaution save your work.
Showing results for 
Search instead for 
Did you mean: 

RSTP - Using ring nodes more than 'Max Age'

Hello people,

  1. Consider the following scenario -
  2. When configuring RSTP, 'Max Age' is set to 40.
  3. The root bridge's priority is set to 0.
  4. All other bridges have the same priority.
  5. The topology used is a ring.
  6. There are a total of 79 devices in the ring, all supporting RSTP.
  7. All port costs are the same, all segment speeds are the same.

My assumptions -

  1. When the network starts up, the the bridge with priority 0 will be elected as root. 
  2. At all the switches which are upto 39 hop counts away from the root in each direction in the ring, the received BPDUs are still valid (Message age < Max Age) . Hence the port on the 39th switch with the higher MAC ID than it's neighbor will have the 'Alternate' role. With this an ethernet loop is prevented.

Will the network converge as described in point 2 above or is my assumption flawed?


Only if it is valid, consider the second scenario - suppose there is a link failure between the 2nd and 3rd switch from the root bridge in one direction. Now instead of a ring, we have a long daisy chain of 79 switches. There will be 2 switches at one side of the root switch, and 76 switches on the other side. At the switch at a hop count of 40 from the root bridge, the BPDUs will be discarded, since the Message Age will be equal to the Max Age.


Is there a chance of a stable network convergence in this network?


What happens to the switches #40 to #76 and their ports? Switch #41 will not see the BPDUs indicating the root bridge with priority 0. So it will send BPDUs in both directions with Max Age set to 40. How will switch #40 and #39 react to these BPDUs?

Everyone's tags (3)

Re: RSTP - Using ring nodes more than 'Max Age'

This is quite an interesting question that I'm also always confused about. Thank you for bringing this up and looking forward to learning from others.
Content for Community-Ad