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Restrictions on SLAs
This section lists the restrictions on SLAs.
The following are restrictions on IP SLAs network performance measurement:
The switch does not support VoIP service levels using the gatekeeper registration delay operations measurements.
Only a Cisco IOS device can be a source for a destination IP SLAs responder.
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.
Cisco IP SLAs is a part of
Cisco IOS software that allows Cisco customers to analyze IP service levels for
IP applications and services by using active traffic monitoring—the generation
of traffic in a continuous, reliable, and predictable manner—for measuring
network performance. With Cisco IOS IP SLAs, service provider customers can
measure and provide service level agreements, and enterprise customers can
verify service levels, verify outsourced service level agreements, and
understand network performance. Cisco IOS IP SLAs can perform network
assessments, verify quality of service (QoS), ease the deployment of new
services, and assist with network troubleshooting.
Cisco IOS IP SLAs send data
across the network to measure performance between multiple network locations or
across multiple network paths. They simulate network data and IP services and
collect network performance information in real time. Cisco IOS IP SLAs
generate and analyze traffic either between Cisco IOS devices or from a Cisco
IOS device to a remote IP device such as a network application server.
Measurements provided by the various Cisco IOS IP SLA operations can be used
for troubleshooting, for problem analysis, and for designing network
Depending on the specific
Cisco IOS IP SLA operations, various network performance statistics are
monitored within the Cisco device and stored in both command-line interface
(CLI) and Simple Network Management Protocol (SNMP) MIBs. IP SLA packets have
configurable IP and application layer options such as source and destination IP
address, User Datagram Protocol (UDP)/TCP port numbers, a type of service (ToS)
byte (including Differentiated Services Code Point [DSCP] and IP Prefix bits),
Virtual Private Network (VPN) routing/forwarding instance (VRF), and URL web
Because Cisco IP SLAs are Layer 2 transport
independent, you can configure end-to-end operations over disparate networks to
best reflect the metrics that an end user is likely to experience. IP SLAs
collect and analyze the
following performance metrics:
Delay (both round-trip and
Packet loss (directional)
Packet sequencing (packet
Path (per hop)
Server or website download
Because Cisco IOS IP SLAs is
SNMP-accessible, it can also be used by performance-monitoring applications
like Cisco Prime Internetwork Performance Monitor (IPM) and other third-party
Cisco partner performance management products.
Using IP SLAs can provide the
monitoring, measurement, and verification.
Measurement of jitter,
latency, or packet loss in the network.
Continuous, reliable, and
IP service network health
assessment to verify that the existing QoS is sufficient for new IP services.
availability monitoring for proactive verification and connectivity testing of
network resources (for example, shows the network availability of an NFS server
used to store business critical data from a remote site).
troubleshooting by providing consistent, reliable measurement that immediately
identifies problems and saves troubleshooting time.
Multiprotocol Label Switching
(MPLS) performance monitoring and network verification (if the
switch supports MPLS).
Network Performance Measurement with Cisco IOS IP SLAs
You can use IP SLAs to
monitor the performance between any area in the network—core, distribution, and
edge—without deploying a physical probe. It uses generated traffic to measure
network performance between two networking devices.
Figure 1. Cisco IOS IP SLAs
Operation. The following
figure shows how IP SLAs begin when the source device sends a generated packet
to the destination device. After the destination device receives the packet,
depending on the type of IP SLAs operation, it responds with time-stamp
information for the source to make the calculation on performance metrics. An
IP SLAs operation performs a network measurement from the source device to a
destination in the network using a specific protocol such as UDP.
The IP SLA responder is a
component embedded in the destination Cisco device that allows the system to
anticipate and respond to IP SLA request packets. The responder provides
accurate measurements without the need for dedicated probes. The responder uses
the Cisco IOS IP SLA Control Protocol to provide a mechanism through which it
can be notified on which port it should listen and respond.
The IP SLA responder can be a
Cisco IOS Layer 2, responder-configurable
switch. The responder does not need to
support full IP SLA functionality.
The following figure shows
where the Cisco IOS IP SLA responder fits in the IP network. The responder
listens on a specific port for control protocol messages sent by an IP SLA
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 SLA packet, or when the specified time expires. MD5 authentication for
control messages is available for added security.
Figure 2. Cisco IOS IP
You do not need to enable the
responder on the destination device for all IP SLA 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.
Switches, controllers, 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 minimize 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 SLA test packets use time stamping to minimize the processing delays.
When the IP SLA 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 3. Cisco IOS IP SLA Responder Time Stamping. The following figure 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.
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.
This section does not include configuration information for all available operations as the configuration information details are included in the Cisco IOS IP SLAs Configuration Guide. It does include several operations as examples, including configuring the responder, configuring a UDP jitter operation, which requires a responder, and configuring an ICMP echo operation, which does not require a responder. For details about configuring other operations, see the Cisco IOS IP SLAs Configuration Guide.
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