Troubleshooting guide

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Cisco Broadband Local Integrated Services Solution Troubleshooting Guide
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Chapter 5 Troubleshooting DOCSIS Networks
Troubleshooting Slow Peformance
<snip>
. . . . . . .
89 3304 81013 40 0.00% 0.00% 0.00% 0 PIM Process
90 12 769 15 0.00% 0.00% 0.00% 0 CEF Scanner
92 0 385 0 0.00% 0.00% 0.00% 0 DHCPD Timer
93 40 13058 3 0.00% 0.00% 0.00% 0 DHCPD Database
In the example show above, the current CPU load on the CMTS is 45%/21%. This means that the total
CPU utilization is at 45% of the capacity of the system. In addition 21% of the CPU is being used to
service interrupts. This second figure typically equates to the portion of the CPU being used to route
packets and switch traffic through the CMTS.
If the five minutes CPU utilization is consistently above 80% during the peak usage time in your system
then end users may start to experience slower performance and increased latency. If the five minutes
CPU utilization is constantly above 95% during the peak usage time in your network then you need to
take urgent action to ensure that the CMTS will remain in a stable state.
Common strategies for reducing high CPU utilization on your CMTS include:
• Upgrading to release 12.1(9)EC or later, activating the global configuration command ip cef, and
making sure that no interfaces on the CMTS have the command no ip route-cache configured. This
typically leads to a 10 to 15 percent reduction in traffic related CPU utilization. Make sure that all
of these steps are taken in conjunction.
• Making sure that SNMP management stations are not being too aggressive in polling the CMTS.
This leads to a high CPU utilization in the IP SNMP process.
• Not running the show tech command several times in succession. This leads to an artificially high
CPU utilization in the Virtual Exec Process.
• Making sure that no debugs are running on the CMTS.
For more information about High CPU utilization on Cisco Routers, including Cisco CMTS products,
please refer to Troubleshooting High CPU Utilization on Cisco Routers.
Under Powered or Misconfigured CPE Equipment
In many cases, the cause of slow access to a cable network is a problem in the end user's CPE equipment.
If only one or a handful of users are experiencing slow throughput, and the rest of the user population
are experiencing no problem, then this is a strong indication that there may be a unique problem within
that user's environment.
• Under powered or overloaded CPE - If the end users complaining of difficulties are using antiquated
CPE equipment, or equipment that may not be powerful enough to run their chosen operating system
or Internet access software, then clearly, this end user will have difficulties. The only resolution if
this is the case is for the end user to upgrade their CPE equipment.
• Firewall or performance measurement software - If the end user is running any firewall, network
performance measurement or other similar software, then it would be a good troubleshooting step
to have the user turn this software off to see if it has any effect on performance. Quite often these
kinds of software can have a negative impact on performance.
• Misconfigured TCP/IP settings - Most service providers require that end users have their CPE
equipment acquire an IP address, network mask, default gateway and DNS servers via DHCP. Check
to make sure that any end users experiencing problems have their CPE devices configured to use
DHCP to acquire all of these parameters.
If an end user claims to have none of the problems listed above then you should confirm that the end user
is not exceeding their maximum download or upload rate as per the sections above.