Chapter 5: IP Services – Implementing the Cisco IOS IP Service Level Agreement (IP SLA) Feature

The Cisco IOS IP SLA feature, formerly known as the Service Assurance Agent (SAA), and prior to that simply the Response Time Reporter (RTR) feature, is designed to provide a means of actively probing a network to gather performance information from it. Whereas most of the tools described in the following sections are designed to monitor and collect information, IP SLA is based on the concept of generating traffic at a specified interval, with specifically configured options, and measuring the results. It is built around a source-responder model, where one device (the source) generates traffic and either waits for a response from another device (the responder) or another device configured as a responder captures the sender’s traffic and does something with it. This model provides the ability to analyze actual network performance over time, under very specific conditions, to measure performance, avert outages, evaluate quality of service (QoS) performance, identify problems, verify SLAs, and reduce network outages. The IP SLA feature is extensively documented at http://www.cisco.com/go/ipsla .

The IP SLA feature allows measuring the following parameters in network performance:
■ Delay (one-way and round-trip)
■ Jitter (directional)
■ Packet loss (directional)
■ Packet sequencing
■ Path (per hop)
■ Connectivity (through the UDP Echo, ICMP Echo, ICMP Path Echo, and TCP
Connect functions)
■ Server or website download time
■ Voice-quality metrics (MOS)

Implementing the IP SLA feature requires these steps:
Step 1. Configure the SLA operation type, including any required options.
Step 2. Configure any desired threshold conditions.
Step 3. Configure the responder(s), if appropriate.
Step 4.  Schedule or start the operation and monitor the results for a sufficient period of time to meet your requirements.
Step 5.  Review and interpret the results. You can use the Cisco IOS CLI or an SNMP manager to do this.

After IP SLA monitors have been configured, they cannot be edited or modified. You must delete an existing IP SLA monitor to reconfigure any of its options. Also, when you delete an IP SLA monitor to reconfigure it, the associated schedule for that IP SLA monitor is deleted, too. IP SLAs can use MD5 authentication. These are configured using theip sla key-chain command.

Below example shows a basic IP SLA configuration with the UDP Echo function. On the responding router, the only required command is global configip sla monitor responder . On the originating router, the configuration shown in the example sets the source router to send UDP echo packets every 5 seconds for one day to 200.1.200.9 on port 1330.

Example IP SLA Basic Configuration

SLAdemo#config term
SLAdemo(config)#ip sla monitor 1
SLAdemo(config-sla-monitor)#type udpEcho dest-ipaddr 200.1.200.9 dest-port 1330
SLAdemo(config-sla-monitor)#frequency 5
SLAdemo(config-sla-monitor)#exit
SLAdemo(config)#ip sla monitor schedule 1 life 86400 start-time now

A number of show commands come in handy in verifying IP SLA performance. On the source router, the most useful commands are show ip sla monitor statistics and show ip sla monitor configuration . Here’s a sample of the show ip sla monitor statistics command for the sending router in the configuration in the Example:

SLAdemo#show ip sla monitor statistics
Round trip time (RTT) Index 1
      Latest RTT: 26 ms
Latest operation start time: 19:42:44.799 EDT Tue Jun 9 2009
Latest operation return code: OK
Number of successes: 228
Number of failures: 0
Operation time to live: 78863 sec

 

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