Hello timer – Periodic BPDU after every 2 sec by default – tune-able between 1 – 10 seconds.
Forward Delay – Listening + Learning state, equals to 15 seconds by default – tune-able between 4 – 30 seconds
Max Age – Maximum period of time that passes before a bridge port saves a configuration BPDU info, 20 sec by default – tune-able between 6 – 40 seconds.
Message Age – not a fixed value, the length of time that has passed since the root bridge initially originated the BPDU. When Root bridge sends it, its value is 0 and increments after every bridge-hop. It signified “how far is that bridge from Root”.
When a new configuration BPDU is received which is equal to or better than the previous configuration on that port, all the BPDU info is stored.
The age time starts at the message age that is received in that configuration BPDU. IF this age timer reaches max age before another BPDU is received that refreshes the timer, the info is aged out for that port.
Diameter of the STP domain (dia) – Maximum number of bridges between any 2 points on topology (any two points of end hosts attachment). IEEE recommends 7 with default timers.
Bridge Transit Delay (transit delay) – time taken by the bridge between reception and transmission of the same BPDU. Local latency. IEEE says it should be maximum 1 seconds.
BPDU transmission delay (BPDU delay) – delay between the time that a BPDU is received on a port and the time the configuration BPDU is effectively transmitted to another port. IEEE recommended 1 second maximum.
Message age increment over-estimate (msg overestimate) – This is the increment that each bridge adds to the message age before forwarding the BPDU. Cisco switches add 1 second.
Lost message (lost_msg) – number of BPDU that can be lost as the frame traverses from one end to the other end of the switched network.
Transmit halt delay (tx_halt_delay) – maximum amount of time that is necessary for a bridge to effectively move a port into the blocking state after the determination that the port needs to be blocked. IEEE recommendation is 1 second.
Medium access delay – med_access_delay – time that is necessary for a device to to gain access to the media for initial transmission. It is also the time between CPU decision to send a frame and the moment when the frame effectively begins to leave the bridge. IEEE says 0.5 seconds maximum.
End to end BPDU propagation delay: ((lost_msg + 1)XHello) + (BPDU_delayX(dia – 1))
Message age overestimate – age of BPDU since origination –
(Dia – 1)Xoverestimate per bridge = 6seconds
Maximum frame lifetime – maximum time a frame that was previously sent to the bridge network remains in the network before the frame reaches that destination. = (diaXtransmit_delay) + Med access delay = 7.5 ~ 8seconds.
Maximum transmission halt delay – time that is necessary in order to effectively block a port after the decision to block is made – 1 seconds (max transmission halt delay).
Max-Age – end to end BPDU propagation delay + message age overestimate = 14+6 = 20 s
Forward Delay – time from first bridge’s port enters listening state and stays there through the subsequent reconfiguration to when the last bridge in the topology hears of the change in the active topology. In addition, we need to count the same delay that we use to count max age
Time for last bridge to stop forwarding frame that are received on the previous topology (max trans halt delay), until the last frame that is forwarded on the previous topology disappears (max frame lifetime).
2Xforward delay = end to end bpdu prop delay + mssg age overestimate + max frame lifetime + max trans halt delay = 28.5/2 = 15 seconds.
The tune-able parameters are –
- Max age
- Forward delay
The other parameters should be considered fixed and non-tune-able. The formula comes out to be be:
Max_age = end to end propagation delay + message age overestimate
= 4XHello + 2XDia – 2
Forward_delay = (4XHello + 3XDia – 0.5) / 2 = (4XHello + 3XDia)/2
With this calculation –
If hello is 2 seconds – max_age = 14sec and forward_delay = 10 seconds for a dia of 4
If hello is 1 second – max_age= 10 sec and forward delay=8 seconds, for a dia of 4
Decrease the Hello timer to 1 second –
Easiest and surest way to reduce the timers value.
But, doubles the repletion and increases load on switches. Several switches and trunks will definitely add more load to the CPU.
Calculate the Dia –
Maximum switched hops from one end to the other end of the LAN.
Change the STP timers –
The timers changed on the root bridge is only required.
For redundancy, we should also configure them on the secondary/backup root bridge.