ROOT Bridge timer value

[prashantdwivedi]

. If we change timer(hello)value in root bridge what cud be the effect in topology?

i found non root bridge still have 2 sec as their hello timer may i know why?

[andtoth]

Hi,

The timer values are sent in the STP BPDU packets from root towards the downstream switches. The downstream switches will use those values configured on the root.

When you are checking the 'show spanning-tree' output, there are 2 sections, one starts with Root ID, other starts with Bridge ID. If you have changed the hello timer on the root, downstream switches should indicate that value at Hello Time in the Root ID section and their own default in the Bridge ID section.

  Root ID    Priority    32921              Address     000c.30ae.6280              Cost        4              Port        2 (GigabitEthernet1/0/2)              Hello Time   1 sec  Max Age 20 sec  Forward Delay 15 sec   Bridge ID  Priority    32921  (priority 32768 sys-id-ext 153)              Address     0012.01c7.6800              Hello Time   2 sec  Max Age 20 sec  Forward Delay 15 sec              Aging Time  300 sec

Note that Hello Time is different in the 2 sections. Also, they will not propagate this value to their running configuration.

Regarding the effects of changing the timers, I would recommend you to check the following guides:

Understanding and Tuning Spanning Tree Protocol Timers

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

Best Practices for Catalyst 6500/6000 Series and Catalyst 4500/4000 Series Switches Running Cisco IOS Software

http://www.cisco.com/en/US/products/hw/switches/ps700/products_white_paper09186a00801b49a4.shtml

Andras