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Cisco Catalyst Blade Switch 3120 for HP Software Configuration Guide
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Chapter 41 Configuring Cisco IOS IP SLAs Operations
Understanding Cisco IOS IP SLAs
IP SLAs Responder and IP SLAs Control Protocol
The IP SLAs responder is a component embedded in the destination Cisco device that allows the system
to anticipate and respond to IP SLAs request packets. The responder provides accurate measurements
without the need for dedicated probes. The responder uses the Cisco IOS IP SLAs Control Protocol to
provide a mechanism through which it can be notified on which port it should listen and respond. Only
a Cisco IOS device can be a source for a destination IP SLAs Responder.
Note
The IP SLAs responder can be a Cisco IOS Layer 2, responder-configurable switch, such as a
Catalyst 2960 or Cisco ME 2400 switch. The responder does not need to support full IP SLAs
functionality.
Figure 41-1
shows where the Cisco IOS IP SLAs responder fits in the IP network. The responder listens
on a specific port for control protocol messages sent by an IP SLAs operation. Upon receipt of the
control message, it enables the specified UDP or TCP port for the specified duration. During this time,
the responder accepts the requests and responds to them. It disables the port after it responds to the IP
SLAs packet, or when the specified time expires. MD5 authentication for control messages is available
for added security.
You do not need to enable the responder on the destination device for all IP SLAs operations. For
example, a responder is not required for services that are already provided by the destination router (such
as Telnet or HTTP). You cannot configure the IP SLAs responder on non-Cisco devices and Cisco IOS
IP SLAs can send operational packets only to services native to those devices.
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 41-2
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.
