Configuring Headend Broadband Access Router Features
Configuring the Upstream Cable Interface
You can set fixed start and end values for data backoff on the upstream ports, or you can set the upstream ports for automatic data backoff. You have the same options for ranging backoff. For both backoff windows, the default start value is 0; the default end value is 4. Valid values are from 0 to 15.
To set data or ranging backoff values for an upstream port, use one or more of the following commands, beginning in cable interface configuration mode:
| Command | Purpose |
Step 1 |
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The automatic setting is optimized for up to 250 | ||
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| cable modems per upstream port. Set manual values |
| or | for data backoff windows only when operating with |
| more than 250 cable modems per upstream port. | |
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| |
| Configures the default backoff window values of 0 | |
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| |
| and 4. | |
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Step 2 |
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|
The automatic setting is optimized for up to 250 | ||
| end | cable modems per upstream port. Set manual values |
| or | for ranging backoff windows only when operating |
| with more than 250 cable modems per upstream port. | |
|
| |
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| Configures the default backoff window values of 0 |
| automatic | |
| and 4. | |
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| |
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When considering whether or not to adjust backoff values, note the following:
•The cable modem reconnection time after a power outage is related to the following factors:
–DHCP, TOD, and TFTP server capacity. These 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 cable modem reconnection and reduces server load.
–Backoffs that are too small result in cable modems 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 cable modem performance (brand to brand) in cable modem restart time.
•All cable modems should recover in approximately 0 to 10 minutes after all services are restored (Cisco uBR7200 series, RF transport, DHCP/TFTP/TOD servers). If cable modems take longer than 10 minutes to recover, there could be a problem in the cable modem, in the headend settings, or in the DOCSIS provisioning servers.
Note Upstream segments serving a relatively large number of cable modems (for example, more than 1600 or so) may suffer recovery times longer than
10 minutes.
Cisco IOS Multiservice Applications Configuration Guide
