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Chapter 31 Configuring Cisco IOS IP SLAs Operations
Understanding Cisco IOS IP SLAs
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 or a Cisco ME 3400 switch running the metro base image. The
responder does not need to support full IP SLAs functionality.
Figure 31-1
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 31-2
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.
Figure 31-2
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.
121380
T1
Source router
RTT (Round-trip time) = T4 (Time stamp 4) - T1 (Time stamp 1) -
Target router
Responder
=T3-T2
T4
T2
T3
