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Cisco uBR7200 Series Universal Broadband Router Software Configuration Guide
OL-2239-03
Chapter3 Configuring Cable Modem Interface Features
Configuring the Upstream Cable Modem Interface
When considering whether to adjust backoff values, keep the following considerations in mind:
• The cableinterface reconnection time after a power outage is related to the following factors:
–
DHCP, ToD, and TFTP servers often operate well below 1 percent load under normal situations, but
can jump to over 100 percent after an outage.
–
Adjusting the backoffs to larger numbers slows cableinterface reconnection and reduces server load.
–
Backoffs that are too small result in cableinterfaces failing to range the upstream RF levels correctly
and cycling to maximum power, thus increasing connection time and reducing network performance.
–
Backoffs that are too large result in increased recovery time after a large service outage.
–
There is significant variation in cableinterface performance (brand to brand) in cableinterface restart
time.
• All cableinterfaces should recover in 0 to 10 minutes after all services are restored
(CiscouBR7200series, RF transport, DHCP, TFTP, and ToD servers). A CM that takes longer than
10minutes could be experiencing a problem with the modem itself, a problem with CMTS settings,
or a problem in the DOCSIS provisioning servers.
Note Upstream segments serving a relatively large number of cableinterfaces (for example, more
than 1600) might suffer recovery times greater than 10minutes.
Verifying Upstream Data Backoff
To verify backoff window settings, enter the show controllers cable command for the upstream port you
have just configured:
Router# show controllers cable5/0 u0
Cable5/0 Upstream 0 is up
Frequency 24.016 MHz, Channel Width 1.600 MHz, QPSK Symbol Rate 1.280 Msps
Spectrum Group is overridden
SNR 33.2560 dB
Nominal Input Power Level 0 dBmV, Tx Timing Offset 2288
Ranging Backoff automatic (Start 0, End 3)
Ranging Insertion Interval automatic (60 ms)
Tx Backoff Start 0, Tx Backoff End 4
Modulation Profile Group 1
part_id=0x3137, rev_id=0x03, rev2_id=0xFF
nb_agc_thr=0x0000, nb_agc_nom=0x0000
Range Load Reg Size=0x58
Request Load Reg Size=0x0E
Command Purpose
Step1
Router(config-if)# cable upstream
usport data-backoff start end
or
Router(config-if)# cable upstream
usport data-backoff automatic
Optimizes the automatic setting for as many as 250 cableinterfaces per
upstream port. Sets manual values for data backoff windows only when
operating with more than 250 cableinterfaces per upstream port.
Configures the default backoff window values of 0 and 4.
Step2
Router(config-if)# cable upstream
usport range start end
or
Router(config-if)# cable upstream
usport range automatic
Optimizes the automatic setting for as many as 250 cableinterfaces per
upstream port. Sets manual values for data backoff windows only when
operating with more than 250 cableinterfaces per upstream port.
Configures the default backoff window values of 0 and 4.