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New Member

Totally confused with STP timer and topology changes.

Totally confused with STP timer and topology changes.

Can someone please explain them ?

Regards, Chandu
Cisco Employee

Totally confused with STP timer and topology changes.


This is an extremely vast question. Can you be more specific? Perhaps start with an example or a scenario that confuses you.

Best regards,


New Member

Totally confused with STP timer and topology changes.

Hi Peter,

Sure i will be back here with a scenario or point-to-point question



Regards, Chandu
VIP Purple

Totally confused with STP timer and topology changes.

Hi Chandrakanth,

It will be good if you can provide the scenario:

But is the little explanation about STP Timer and Topology Change:

Hello timer

Hello timer is the time between each Bridge Protocol Data Unit(BPDU) that is sent on a port. This is equal to two seconds by default, but can be adjusted between 1 ~ 10 seconds.

Max Age

Max age timer controls the maximum length of time a bridge port saves its configuration BPDU information. This is 20 seconds by default and can be turned to adjusted between 6 ~ 40 seconds

Forward Delay

Forward delay timer is the time spent in the listening and learning stage. 15 seconds is default, but it can be adjusted between 4 ~ 30 seconds.

The network topology can happen in a network due to different reasons like a link failure, a Switch (Bridge) failure, or a port transitioning to forwarding state.

The topology change must be notified to all Switches (Bridges) in the network and the process involves two steps:

  • The Switch (Bridge) notifies the topology change to Root Bridge
  • The Root Switch (Bridge) bridge broadcasts the topology change information into the whole network.

When a Switch (Bridge) discovers topology change, it generates a TCN (Topology Change Notification) BPDU (Bridge Protocol Data Unit) and sends the TCN BPDU on its root port. The upstream Switch (Bridge) responds back the sender with TCA (Topology Change Acknowledgment) BPDU (Bridge Protocol Data Unit) and TCA (Topology Change Acknowledgment) BPDU (Bridge Protocol Data Unit)

The upstream Switch (Bridge) (bridge which received the TCN BPDU) generates another TCN BPDU and sends out via its Root Port. The process continues until the Root Switch (Root Bridge) receives the TCN BPDU.

When the Root Switch (Root Bridge) is aware that there is a topology change in the network, it starts to send out its Configuration BPDUs with the topology change (TC) bit set. Configuration BPDUs are received by every Switch (Bridge) in the network and all bridges become aware of the network topology change.

Switches (Bridges) keep its MAC address table entries for 300 seconds (5 minutes, known as aging time), by default. When a network topology change happens, the Switch (Bridge) temporarily lowers the aging time to the same as the forward delay time (15 seconds) to relearn the MAC address changes happened because of topology change.

This is important because normally only after five minutes an entry is aged out from the MAC address table of the switch and the network devices could be unreachable for up to 5 minutes.  This is known as a black hole because frames can be forwarded to a device, which is no longer available.

Hope you got the point.