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Cisco Dual Wan Fail-Over using SLA Tracking July 3, 2009

Posted by daakeung in Cisco, IT.
Tags: , , , , ,
Maintaining a high availability connection is critical in any modern network infrastructure.  Some would have access to the internet via multiple ISPs.
In most cases where the ISPs would not peer via any routing protocol, customers are left to setup fail-over with static routes.
There are two types of failures:
  1. Physical : the router would detect one of its interface not connected, invalidated any next hop routes through it. In turn move to another valid route with a higher metric.
  2. Logical : the router has it’s interface online, but somewhere along the path it has to take to reach the destination is unavailable.
Basic router commands with higher metric will not fail-over with a logical failure.
Using SLA Tracking, we can accomplish this.
Consider the following diagram and configuration:
Cisco Dual WAN

Cisco Dual WAN

Cisco IOS used: c181x-adventerprisek9-mz.124-24.T

! Identify the SLA.
ip sla 10

! Set up the IP address to ping, in this case yahoo.com ip.

! Set how long in milliseconds to wait for a reply.
timeout 1000

! Repeat Rate.
frequency 3

! Start SLA 10 from now to forever.
ip sla schedule 10 life forever start-time now

! Setup track 10 to sla 10 for reachabilty.
track 10 ip sla 10 reachability

! Setup route for testing reach ability to the internet.
ip route

! Setup primary link with track 10 parameter.
ip route track 10

! Setup fail-over link with higher metric.
ip route 20
Depending on your configuration, you will need to setup NAT on both interfaces.