5-18
Cisco MGX 8220 Installation and Configuration
Release 5.0, Part Number 78-6430-03 Rev. D0, November 2003
Chapter
Configuring the Clock Source
Configuring the Clock Source
IMATM has a T3/E3 interface and multiple T1/E1 inter face s. I t is supposed to replace a physical long
distance T3/E3 ATM trunk by a g r ou p of long distance T 1/ E 1 li nes. Had there been a physical T3/E3
line, the clock sync info automatically reaches from one end to other along with T3/E3 data traffic.
Since we are breaking this continuation and replacing it with a group of T1/E1 lines, we need to have
some mechanism in IMATM to pass the clo ck ac ross.
There are two commands in IMATM that can b e u s ed t o ch an ge th e c lock configuration.
1. The cnfclksrc command lets you configure a primary clock source and a secondary clock source.
An on-board PLL generates a clock, phase locke d to the pri ma ry cloc k sour ce ( If prim ary ha s gone
bad then phase is locked to the secondar y. It switches back to primary when primary clock be comes
OK. Should both secondary and primary becomes unusable, PLL switches to backplane). This
phase-locked o/p then drives the entir e car d ( fo r exa m pl e, bo th T 3 a nd N T1s on that card).
2. With the cnfln command you can configure the individual T1 li ne s as e ith er Lo o p C l ock o r Lo cal
Clock. When Local Clock is co nfigur ed , i t u ses th e cl o ck s el ect ed usi n g t h e cn fclksrc command,
whereas, Loop Clock is simply the clock recovered from T1/E1 receive. (Even when the T1/E1s
are configured in Loop Clock, the T 3 tr ans mit is sti ll dri v e n by t he pri mary /se cond ar y cloc k sou rc e
configured by cnfclksrc.)
3. The user needs to select which 10 sources should be used to drive all T1 and T3 lines on that card,
and then configure the clock source entering the cnfclksrc command.
Possible clock sources are 8-T1/E1 lines, T3/E3 line, or backplane 8 kKHz.
Note Primary from one of the DS1 lines an d se co nd ar y f rom T3 do es no t m ake sen s e, sin ce these two
mean different directi ons of passing cloc k s yn c .

Example 1

---------------
---------------
T3 --- N* T1 --------------- N*T1 ---- T3
---------------
---------------
END A E ND B
In order to pass clock info from END A to END B:
END A T1 lines need to derive the clock derived from the END A T3.
END B derives clock from on e of t he in coming N T1s and drives the T 3 transmit using this der ived
clock.
Hence, we should have:
cnfclksrc -pri T3 -sec B P8K (or T3) -cur P R I ; at END A
and
cnfclksrc -pri DS1_1 -sec DS1_2 -cur PRI ; at END B