OMG I just found I missed a small paragraph that has me confused about Max Age Timer and the Mac Address Timer.
When the switches receive this message they will reduce the aging time of the MAC address table from 300 seconds to 15 seconds (this is the forward delay timer). This message is called the TCN (Topology Change Notification).
So I am confused now, and need help understanding.
if the max age timer is the 20 seconds is this the 20 seconds blocking timer at the first.
**edited For my notes: the Max age timer has nothing to do with mac address aging. it has to do with the time that the switches wait before sending a topology change. basically the hello packets(BPDU) are sent every two seconds after 20 seconds it initiates a topology change. which starts the forward delay.**
If that is the 20 second block timer at the first then where is the timer for 15 seconds the mac age before its dropped.
if the entire process is 50 seconds max age timer(20s)>(listening(15s)>Learning(15s) then finally it starts forwarding and has is considered converged where does that mac address aging of 15 seconds fit in???
it obviously has to fit into either the listening or learning or the math does not add up someone please explain this as I just confuzzled myself!
ok may be on a lead just found the following:
The Forward Delay The Forward Delay is the time that is spent in the Listening and Learning state. When the port transitions to the Listening state, it indicates a change in the current Spanning Tree topology and that the port will go from a Blocking state to a Forwarding state. The Forward Delay is used to cover the period between the Blocking and Forwarding states, which includes the Listening and Learning states.
This time is set to 15 seconds (sec) by default but can be manually set to be between 4 and 30 seconds.
Tafa, Farai. Cisco CCNP SWITCH Simplified (Kindle Locations 2039-2043). Reality Press Ltd. Kindle Edition.
in addition, if we want to look at the forward delay default timer it can be seen and computed below:
Assuming all the default values, we can calculate the Forward Delay as follows:
Forward Delay = ((4 * hello) + (3 * Diameter)) / 2
Forward Delay = ((4 * 2) + (3 * 7)) / 2 Forward Delay = ((8) + (21)) / 2
Forward Delay = (29 / 2 Forward Delay = 29 / 2)
Forward Delay = 14.5 seconds (rounded up to 15 seconds)
Tafa, Farai. Cisco CCNP SWITCH Simplified (Kindle Locations 2045-2050). Reality Press Ltd. Kindle Edition.
that says the forward delay by default in a single switch is 15 seconds which would seem to say that the total time for listening and learning was 15 seconds on a single switch anyway. There is more information about message age timer that is increased per switch I am still banging my head against wall trying to figure out that detail. will report back findings.
The Message Age timer is used in conjunction with the Max Age timer.
Tafa, Farai. Cisco CCNP SWITCH Simplified (Kindle Location 2025). Reality Press Ltd. Kindle Edition.
so I cannot figure this out. it sounds like the default diameter which is what sets the message age timer has to do with covering the listening and learning stage and is part of the forward delay but I don’t know its confusing.
Whatever the case mac address aging is handled during this time and it seems that message age timer is in here as well and it has a default diameter of 7 but it depends on the amount of switches you go through on actual time. all that gobbly goop adds up to the listening and learning stage.
So uplinkfast basically cuts out most of that time allowing for very quick convergence. partially by getting rid of mac addresses which are part of the forward delay but also cuts out the rest by saying use me immediately basically in the code.
backbone fast on the other hand just cuts out the max age timer and does not wait on the loss of hello(BPDU) packets by sending RLQ to the root and once it gets its reply it moves the forward delay state immediately. It does nothing about the forward delay which comprises the mac address table and other gobbly goop like the message age timer.
someone should right a simple detailed engineering spec on that process so we have granular detail or maybe after we looked at it we would not want to know lol…