Specifications

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Chapter 3 Spectrum Management for the Cisco Cable Modem Termination System
Configuration Examples
• Configure matching SNMP community strings on the Cisco CMTS and DCMTA software tool so
that the DCMTA software can obtain the spectrum management data using SNMP requests. On the
Cisco CMTS, this requires the minimum following CLI commands:
snmp manager
snmp-server engineID local engine-id
snmp-server community community-string RW
snmp-server trap-source interface-providing-access-to-DCMTA-PC
snmp-server enable traps cable hopping
Because the DCMTA software requires SNMP read-write access, Cisco also recommends setting up
an access list that restricts SNMP read-write access to the IP address for the particular PC that is
running the DCMTA software.
• The DCMTA software should be used for live troubleshooting of specific problems in the return
path. Do not use DCMTA for ongoing monitoring because it requests a large volume of data for each
SNMP request, and constant use could affect network bandwidth and the overall performance of the
Cisco CMTS. For best results, the DCMTA software should be launched when needed and then
exited after the return path problem has been resolved.
• Because the DCMTA software uses SNMP polling to obtain the spectrum management data, its
graphical displays show a digital representation that appears more jagged than the analog swept
spectrum displays that are generated by hardware spectrum analyzers.
• When monitoring individual cable modems, the DCMTA graphical displays could show the cable
modem missing for several seconds, even though the cable modem is still connected and online. This
can happen for two reasons:
– The DOCSIS specification allows a CMTS to periodically miss a ranging burst from a cable
modem without affecting the cable modem’s connectivity. If this occurs while the DCMTA
software is monitoring a particular cable modem, the graphical display could show that cable
modem missing for several seconds.
– The DMCTA software uses SNMP requests to obtain its spectrum management data. Because
SNMP is based on the connectionless UDP protocol, it is possible that SNMP packets can be
dropped or lost by the routers and other network devices between the Cisco CMTS and DCMTA
PC. If this happens, the graphical display could show a drop in the spectrum that was described
by the data that was contained in the lost packets.
In both of these cases, the missing spectrum data should reappear quickly, within several seconds.
If it does, and if no further problems occur with that particular cable modem or spectrum, then the
problem is likely due to the transient network problems described above.
Configuration Examples
Upstream Traffic Shaping and Rate Limiting Examples
Verifying Upstream Rate Limiting Example
To determine if upstream rate limiting is configured and activated, enter the show running-config
command and look for the cable interface configuration information. If upstream rate limiting is
configured and enabled, a rate limiting entry displays in the show running-config command output. If
upstream rate limiting is disabled, no cable upstream rate-limit appears in the output.
You can also perform the following tasks to verify that rate limiting is enabled on the upstream channel: