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4th T1 clock 4Hz faster in a 4-T1 multilink on 3640


We are an electric coop and run a bunch of private T1s over our own microwave links. The main site is a 3640 with slot0 and slot1 populated with two vwic-2mft-T1 cards each, and slot2 with a 8-port ATM IMA mux riding on another 8 T1s. The first 3 T1s in slot0 form a PPP multilink to  Power plant J (1760 w/ two vwic-2mft-T1), the 4th T1 port of slot0 and the first vwic-2mft-1 in slot1 form another 3-T1 multilink to another power plant. The eight IMA over T1 form various multilinks to other sites as well.

All has been well until we added a 4th T1 at the 2nd vvwi-2mfg-t1 in slot 1 to the bundle for Power Plant J and started to experience network performance issues. The new T1 at Controller T1 1/2  at the 3640 and its other end both show numerous slip secs and other errors. When we looked at the four T1s with a T-Bird at the main site, which are all configured to "clock source internal", we found that the three old T1s are running in lock-step at a freqency of 1.543967 MHz and the 4th T1 is 4 cycles faster at 1.543971 MHz. Testing on the other end, which is configured to use "clock source line" showed the same exact result.

Why is the 4th T1 clock running 4Hz faster? Is this vwic a counterfeit? Is this caused by a limitation of the vwic card or 3600?

I rebooted the routers but it didn't help. On the remote end, I was able to make all four T1 recover from the same clock by using "clock source line primary" on one line and "clock source internal" on the rest. But I cannot figure out a way to do the same at the main site. All four T1 at the 3600 are set to use "clock source internal", resulting in "Current port master clock: osc on this network module".

Help, please!


2 Replies 2

paolo bevilacqua
Hall of Fame Master Hall of Fame Master
Hall of Fame Master

Being practical, with such an old (15 years) and unsupported router, I think you would be fighting a lost battle.

Better would be to look at faster wireless/microwave options, with ethernet interface, and new routers.

Unfortunately, our entire microwave system is running on something that old or even older. You are talking about 3 DS3 and dozens and dozens of micrwave sties here. We are slowly introducing a few high-speed radios here an there but we are running on a shoe-string budget. Good thing the Cisco devices are still going and reliable, which is the reason why we are still a Cisco shop.

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