Cisco IE 4000, Software Configuration Manual

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Configuring Cisco IOS IP SLAs Operations
Information About Configuring Cisco IOS IP SLAs Operations
Response Time Computation for IP SLAs
Switches and routers can take tens of milliseconds to process incoming packets due to other high priority processes.
This delay affects the response times because the test-packet reply might be in a queue while waiting to be processed.
In this situation, the response times would not accurately represent true network delays. IP SLAs minimizes these
processing delays on the source device as well as on the target device (if the responder is being used) to determine true
round-trip times. IP SLAs test packets use time stamping to minimize the processing delays.
When the IP SLAs responder is enabled, it allows the target device to take time stamps when the packet arrives on the
interface at interrupt level and again just as it is leaving, eliminating the processing time. This time stamping is made with
a granularity of sub-milliseconds (ms).
Figure 107 on page 962 demonstrates how the responder works. Four time stamps are taken to make the calculation for
round-trip time. At the target router, with the responder functionality enabled, time stamp 2 (TS2) is subtracted from time
stamp 3 (TS3) to produce the time spent processing the test packet as represented by delta. This delta value is then
subtracted from the overall round-trip time. Notice that the same principle is applied by IP SLAs on the source router
where the incoming time stamp 4 (TS4) is also taken at the interrupt level to allow for greater accuracy.
Figure 107 Cisco IOS IP SLAs Responder Time Stamping
An additional benefit of the two time stamps at the target device is the ability to track one-way delay, jitter, and directional
packet loss. Because much network behavior is asynchronous, it is critical to have these statistics. However, to capture
one-way delay measurements, you must configure both the source router and target router with Network Time Protocol
(NTP) so that the source and target are synchronized to the same clock source. One-way jitter measurements do not
require clock synchronization.
IP SLAs Operation Scheduling
When you configure an IP SLAs operation, you must schedule the operation to begin capturing statistics and collecting
error information. You can schedule an operation to start immediately or to start at a certain month, day, and hour. You
can use the pending option to set the operation to start at a later time. The pending option is an internal state of the
operation that is visible through SNMP. The pending state is also used when an operation is a reaction (threshold)
operation waiting to be triggered. You can schedule a single IP SLAs operation or a group of operations at one time.
You can schedule several IP SLAs operations on a switch running the IP services image by using a single command
through the Cisco IOS CLI or the CISCO RTTMON-MIB. Scheduling the operations to run at evenly distributed times
allows you to control the amount of IP SLAs monitoring traffic. This distribution of IP SLAs operations helps minimize the
CPU utilization and thus improves network scalability.
IP SLAs Operation Threshold Monitoring
To support successful service level agreement monitoring, you must have mechanisms that notify you immediately of any
possible violation. IP SLAs can send SNMP traps that are triggered by events such as these:

Connection loss

Timeout

Round-trip time threshold
121380
T1
Source router
RTT (Round-trip time) = T4 (Time stamp 4) - T1 (Time stamp 1) -
Target router
Responder
=T3-T2
T4
T2
T3