Configuring Cisco IP SLA and Object Tracking

Cases for use

1. You would like to modify your network routing in response to a change of conditions either in your network or outside your network.
2. You have an interest in determining performance characteristics for latency/bandwidth across your network to provide metrics over time.
3. A combination of both of the above.

Restrictions

Using 3750 Switches as an example, if you’re running IPBase, you’ll only be able to configure IP SLA Responders. Full IP SLA features are available in advanced IOS images.
Check your platform and feature set of IOS for your ability to run IP SLA features.

Use Case 1

You have two egress routes from different ISP’s available from a single Layer 3 Device. you wish to modify the default route from your Layer 3 device depending on
upstream connectivity to an address you define on each link.

Scenario

12.2(55) IPServices images on 3750 MLS Platforms are being used.

Two IP Next hops for routing your data plane traffic are :

Link 1 = 1.1.1.1
Link 2 = 2.2.2.2

The device on the primary ISP’s network you’re tracking to determine routing preference is :

1.1.10.10

Configuration

Create a simple icmp-echo SLA instance number 10 to check availability on your preferred upstream link.

ip sla 10
 icmp-echo 1.1.10.10
 timeout 500
 frequency 3
ip sla schedule 1 life forever start-time now

Create a tracked object number 99 checking your SLA probe 10 for reachability.

track 99 ip sla 10 reachability

Configure a default route to the preferred provider with a tracking object associated with it.

ip route 0.0.0.0 0.0.0.0 1.1.1.1 track 99

Add a new default route with a higher metric than your existing default route (3). This will only be installed into the routing table if the tracked route is removed due to the primary link’s own tracked object being down.

ip route 0.0.0.0 0.0.0.0 2.2.2.2 3

So the result is the static configuration is the secondary route which could be described as a catch all after the dynamics of the primary route fail.
Again to summarise, the primary route and, the SLA probe and tracking is where all the logic and dynamics are configured.

 

Use Case 2

You’re using a First Hop Redundancy Protocol, in this case, HSRP, configured between three Layer 3 devices.
You wish to modify which of the HSRP Devices is forwarding traffic depending on the status of an object you define on each Layer 3 Device.

Scenario

12.2(55) IPServices images on 3750 MLS Platforms are being used.

HSRP Router 1 = VLAN10 10.1.10.2
HSRP Router 2 = VLAN10 10.1.10.3
HSRP Router 3 = VLAN10 10.1.10.4

HSRP Virtual IP = 10.1.10.1

The object’s you’re tracking

HSRP Router 1 – Upstream Device = 8.8.8.8
HSRP Router 2 – Upstream Interface = GigabitEthernet 0/1
HSRP Router 3 – Nothing

Configuration

Create IP SLA and Tracked Objects

Router 1

ip sla 10
 icmp-echo 8.8.8.8
 timeout 500
 frequency 3
ip sla schedule 1 life forever start-time now
track 99 ip sla 10 reachability

Configure HSRP

Router 1

interface vlan 10
standby 10 ip 10.1.10.1
standby 10 preempt
standby 10 priority 110
standby 10 track 99 30

Router 2

interface vlan 10
standby 10 ip 10.1.10.1
standby 10 preempt
standby 10 track GigabitEthernet 0/1 20

Router 3

interface vlan 10
standby 10 ip 10.1.10.1
standby 10 priority 90

This configuration shows that Router 1 is the preferred HSRP gateway with Priority 110, then Router 2, then Router 3 with Priority 90. Default HSRP Priority is 100 (Router 2).

Notice you don’t have to create a tracked object for Router 2 as the HSRP track command can monitor interfaces local to the device as part of the HSRP configuration. 

You also don’t need pre-emption on Router 3 because It’ll never find itself with a higher priority than the other two routers unless they have failed and decremented their priority because of a failure in the objects that they are tracking. 

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