stp timers

[sarahr202]

Hi everybody.

Please consider the following

SW1---------------R SW2-D---------------BSw3

sw1 is root port and transmitting its timers as:

hello 4 sec

forward delay 35 sec

max-age 50 sec.

Sw2 receives the bpdu containing these timers and relays it sw3.

Sw3 has default settting i.e hello timer 2 sec, forward delay 15 sec, max age 20 sec

My question will sw3 follow the timers received in bpdu from root switch or follow its own timer? for e.g  will it consider the neighbor dead if no hello bpdu is received in 20 sec( the default setting of max age on sw3) or will it consider the neighbor dead if it does not receives any hello bpdu in 50 sec ( the advertised max age)

thanks and have a great day

[Reza Sharifi · Accepted solution]

Hi Sarah,

Sw3 will use the timers that are configured on the root bridge (sw2) not local timer.

As the

Spanning Tree Protocol Timers

section mentions, each BPDU includes the hello, forward delay, and max age STP       timers. An IEEE bridge is not concerned about the local configuration of the       timers value. The IEEE bridge considers the value of the timers in the BPDU       that the bridge receives. Effectively, only a timer that is configured on the       root bridge of the STP is important. If you lose the root, the new root starts       to impose its local timer value on the entire network. So, even if you do not       need to configure the same timer value in the entire network, you must at least       configure any timer changes on the root bridge and on the backup root       bridge.

more info:

http://www.cisco.com/en/US/tech/tk389/tk621/technologies_tech_note09186a0080094954.shtml

HTH

[sarahr202]

thanks Reza

[Mert OZKUL · Accepted solution]

Hi Sarah,

Additionally to Reza,

In STP, Switches use the Root Bridge's timer which propagating with Configuration BPDUs.

If the root bridge fail, STP will use the timer which the new elected root bridge.

For example,

SwitchA (Root ||| Hello 4, Max Age 50)

SwitchB (Not-Root ||| Hello 2, Max-Age 20)

In this case, Timers will be "Hello 4, Max-Age 50"

Assume the Switch A failed and Switch B is root

So the new STP timers will be"Hello 2, Max-Age 20"

STP has only one point-of-view so all other switches depend on the root switch's configuration.

BR,

-Mert