static route tracking using ip sla

8/3/2019 Static Route Tracking Using IP SLA

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Configuring Static Route Tracking using IPSLA (Basic)Saturday, 10 September 2011 00:42

In today's network environment, redundancy is one of the most important aspects, whether its on the LAN side or on the

WAN side. In this topic I will be covering WAN redundancy with multiple WAN links terminating on a single router.

The best and simplest way to achieve WAN redundancy on Cisco devices is to use Reliable Static backup routes with IP SLAtracking.

IP SLAs is a feature included in the Cisco IOS Software that can allow administrators the ability to Analyze IP Service Levels

for IP applications and services. IP SLA's uses active traffic-monitoring technology to monitor continuous traffic on the

network. This is a reliable method in measuring over head network performance. Cisco Routers provide IP SLA Responders

that give accuracy of measured data across a network.

With IP SLAs, routers and switches perform periodic measurements. The number and type of available measurements are

vast and in this article I will be covering just the ICMP ECHO feature. IP SLA in itself is a very big topic to cover J

Let us take an example of a basic redundant WAN link scenario as shown below:

In the above figure the Cisco device is connected to two WAN links ISP1 and ISP2. The most common setup that we use in

day to day life is to have to default routes configured on the Cisco router pointing to the respective next hop IPs as shown

below:

R1(config)# ip route 0.0.0.0 0.0.0.0 2.2.2.2

R1(config)# ip route 0.0.0.0 0.0.0.0 3.3.3.3 10

If you notice the Administrative Distance for the secondary route pointing to ISP2 is increased to 10 so that it becomes the

backup link.

The above configuration with just two floating static routes partially accomplishes our requirement as it will work only in the

scenario where the routers interfaces connected to the WAN link are in up/down or down/down status. But in a lot of

situations we see that even though the links remain up but we are not able to reach the gateway, this usually happens when

the issue is at the ISP side.


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In such scenarios, IP SLAs becomes an engineer's best friend. With around six additional IOS commands we can have a

more reliable automatic failover environment.

Using IP SLA the Cisco IOS gets the ability to use Internet Control Message Protocol (ICMP) pings to identify when a WAN

link goes down at the remote end and hence allows the initiation of a backup connection from an alternative port. The

Reliable Static Routing Backup using Object Tracking feature can ensure reliable backup in the case of several catastrophic

events, such as Internet circuit failure or peer device failure.

IP SLA is configured to ping a target, such as a publicly routable IP address or a target inside the corporate network or your

next-hop IP on the ISP's router. The pings are routed from the primary interface only. Following a sample configuration of IP

SLA to generate icmp ping targeted at the ISP1s next-hop IP.

R1(config)# ip sla 1

R1(config)# icmp-echo 2.2.2.2 source-interface FastEthernet0/0

R1(config)# timeout 1000

R1(config)# threshold 2

R1(config)# frequency 3

R1(config)# ip sla schedule 1 life forever start-time now

Please not that the Cisco IP SLA commands have changed from IOS to IOS to know the exact command for IOS check the

Cisco documentation. The above commands are for IOS 12.4(4)T, 15.(0)1M, and later releases.

The above configuration defines and starts an IP SLA probe.

The ICMP Echo probe sends an ICMP Echo packet to next-hop IP 2.2.2.2 every 3 seconds, as defined by the frequency

parameter.

Timeout sets the amount of time (in milliseconds) for which the Cisco IOS IP SLAs operation waits for a response from its

request packet.

Threshold sets the rising threshold that generates a reaction event and stores history information for the Cisco IOS IP SLAs

operation.

After defining the IP SLA operation our next step is to define an object that tracks the SLA probe. This can be accomplished

by using the IOS Track Object as shown below:

R1(config)# track 1 ip sla 1 reachability

The above command will track the state of the IP SLA operation. If there are no ping responses from the next-hop IP the

track will go down and it will come up when the ip sla operation starts receiving ping response.

To verify the track status use the use the show track command as shown below:

R1# show track

Track 1

IP SLA 1 reachability

Reachability is Down

1 change, last change 00:03:19

Latest operation return code: Unknown

The above output shows that the track status is down. Every IP SLAs operation maintains an operation return-code value.

This return code is interpreted by the tracking process. The return code may return OK, OverThreshold, and several other

return codes.


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Different operations may have different return-code values, so only values common to all operation types are used. The

below table shows the track states as per the IP SLA return code.

Tracking Return Code Track StateReachability OK or over threshold Up

(all other return codes) DownThe Last step in the IP SLA Reliable Static Route configuration is to add the track statement to the default routes pointing to

the ISP routers as shown below:

R1(config)# ip route 0.0.0.0 0.0.0.0 2.2.2.2 track 1

R1(config)# ip route 0.0.0.0 0.0.0.0 3.3.3.3 10

The track number keyword and argument combination specifies that the static route will be installed only if the state of the

configured track object is up. Hence if the track status is down the secondary route will be used to forward all the traffic.

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static route tracking using ip sla

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