## bridge loop problem

Nov 28th, 2004

Hi,

The switch diameter of my customer network environment have been more than 7, and sometimes my customer network have bridge loop problem, ( core switch backplane utilization up to 90%, the packet loss, HSRP have duplicate IP address error message ) but this problem will be recover automatically after 10 mins. So I think it is the spanning-tree topology convergent problem. I think if I increase the STP timer, although the STP convergent time will increase, but it will prevent the bridge loop problem. If switch diameter are 10, what are the suitable value for hello, forward delay & mac age timer? And any consideration for increase STP timer?

Best Regards,

Jackson Ku

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## Replies

konigl Sun, 11/28/2004 - 11:37

To calculate the appropriate IEEE 802.1D STP timer settings for Max Age (MA) and Forward Delay (FD), you only need to know the Network Diameter (ND) and Hello Time (HT). Plug the values into these two formulas:

MA = 4*HT + 2*ND - 2

FD = 0.5*(4*HT + 3*ND -0.5), and round UP to next whole number or integer

For STP defaults, HT=2 and ND=7, so MA=20 and FD=15 (14.25 rounded up to next second is 15). STP reconvergence will take from 2*FD=30 to 2*FD+MA=30+20=50 seconds.

In your case, HT=2 and ND=10 results in MA=26 and FD=19, and reconvergence in 38 to 64 seconds.

To speed up reconvergence, you could lower the HT to 1 second. That would lead to HT=1 and ND=10, MA=22 and FD=17, with reconvergence in 34 to 56 seconds. It's not really much of an improvement, though. You would be better off minimizing the size of ND.

What switches is your customer using? Maybe IEEE 802.1w Rapid STP would be an option.

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Posted November 28, 2004 at 3:14 AM
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