Cisco IOS includes a mechanism to monitor link attributes and ensure they stay
within the parameters of a Service Level Agreement (SLA). An SLA is a contract
between a service provider and its customer which outlines minimum benchmarks
the service provider must maintain. In the data networking world, such
benchmarks can include minimum throughput, delay, jitter, or other attributes.
An IOS router can be configured with an IP SLA monitor to constantly evaluate
these properties of a link or path and ensure that a service provider is fulfilling
its obligation.
However, SLA monitors can also be useful within an enterprise.
Consider the following topology:
The 192.168.0.0/24 subnet is connected to the rest of the network by
R1 and R2,and web access to the Internet is accomplished through a
proxy (172.16.55.87) located elsewhere on the network. R1 and R2
do not share a common path to the proxy, and one might lose
connectivity to the proxy while the other does not.
The VRRP deployment facing 192.168.0.0/24 complicates this,
as web access will be lost if the master router can no longer forward
HTTP requests to the proxy. R1 has been configured with a VRRP
priority of 110, while R2 has the default priority of 100.
Fortunately, we can configure an IP SLA monitor on R1 to
check for HTTP connectivity to the web proxy. The monitor can
then be referenced by the VRRP configuration to lower the
router's VRRP priority when the monitor fails. First we have to
configure an HTTP IP SLA monitor:
R1(config)# ip sla 1
R1(config-ip-sla)# http get http://172.16.55.87/
R1(config-ip-sla-http)# frequency 60
R1(config-ip-sla-http)# timeout 5000
The above configuration creates a monitor which sends an HTTP GET
request to the specified URL every 60 seconds and checks for a valid
response. The timeout has been set for 5000 milliseconds, or
5 seconds. Note that this configuration only checks for HTTP
connectivity to the proxy itself; however, if we wanted to be especially
thorough, IOS also provides an option to check for HTTP connectivity
to external sites through the proxy:
R1(config-ip-sla)# http get http://external-site/ proxy
http://172.16.55.87/ name-server 172.16.44.10
Next, we schedule the monitor to run. In this case, we want the
monitor to run continuously beginning right now.
R1(config)# ip sla schedule 1 start-time now life
forever
The monitor will now start. After a few minutes, we can view the
IP SLA statistics to verify it is successful:
R1# show ip sla statistics
Round Trip Time (RTT) for Index 1
Latest RTT: 72 milliseconds
Latest operation start time: *00:42:17.839 UTC Fri Mar 1 2002
Latest operation return code: OK
Latest DNS RTT: 0 ms
Latest TCP Connection RTT: 20 ms
Latest HTTP Transaction RTT: 52 ms
Number of successes: 3
Number of failures: 0
Operation time to live: Forever
Now that our SLA monitor is up and running, we need to create a
tracked object pointing to it. Depending on the IOS version in use, an
IP SLA monitor is referenced as a Response Time Reporter (RTR)
with rtr (prior to 12.4(20)T), or with ip sla (12.4(20)T and later). The
legacy RTR syntax is used here:
R1(config)# track 1 rtr 1 state
R1(config-track)# exit
The tracked object serves as a wrapper for the IP SLA monitor so that
it can be referenced from VRRP, which is the last portion to configure:
R1(config)# interface f0/1
R1(config-if)# vrrp 1 track 1 decrement 20
R1(config-if)# ^Z
R1# show vrrp
FastEthernet0/1 - Group 1
State is Master
Virtual IP address is 192.168.0.1
Virtual MAC address is 0000.5e00.0101
Advertisement interval is 1.000 sec
Preemption enabled
Priority is 110
Track object 1 state Up decrement 20
Master Router is 192.168.0.2 (local), priority is 110
Master Advertisement interval is 1.000 sec
Master Down interval is 3.570 sec
Now, if the SLA monitor fails to receive an HTTP response from the
proxy, the tracked object changes state to "down," and the the VRRP
process on 192.168.0.2 will decrement its priority by 20. With R2
configured with the default priority of 100, it should take over as the
master VRRP router. We can disconnect R1 from the proxy to observe
an IP SLA monitor failure and subsequent VRRP failover:
R1# show ip sla statistics
Round Trip Time (RTT) for Index 1
Latest RTT: 0 milliseconds
Latest operation start time: *01:00:17.839 UTC Fri Mar 1 2002
Latest operation return code: Socket receive error
Latest DNS RTT: 0 ms
Latest TCP Connection RTT: 0 ms
Latest HTTP Transaction RTT: 0 ms
Number of successes: 20
Number of failures: 1
Operation time to live: Forever
R1#
*Mar 1 01:00:25.179: %VRRP-6-STATECHANGE: Fa0/1 Grp 1
state Master -> Backup
R2# show vrrp
FastEthernet0/1 - Group 1
State is Master
Virtual IP address is 192.168.0.1
Virtual MAC address is 0000.5e00.0101
Advertisement interval is 1.000 sec
Preemption enabled
Priority is 100
Master Router is 192.168.0.3 (local), priority is 100
Master Advertisement interval is 1.000 sec
Master Down interval is 3.609 sec
Of course, if the connection to the proxy is restored, and the IP SLA
monitor recovers, R1 restores its VRRP priority to 110, and takes
over to once again become the master router.
Best Regards,
Deepak Arora
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