Redundant Internet on a Cisco Router – Should You Add a Specific Route for the IP Tracked by IP SLA??
When configuring an IP SLA (Service Level Agreement) probe to ping a remote server, such as Google’s DNS at 8.8.8.8, the primary goal is often to track the state of an Internet Service Provider (ISP). However, a common question arises: How does the router know which interface to use to send these pings?
If you’re monitoring ISP1, it stands to reason that the pings should only be sent through the interface connected to ISP1. But how can we ensure this happens consistently?
Well, the answer is actually quite simple. All you need to do is configure a specific route—or, ideally, a route-map for local policy routing—for the address you’re tracking via IP SLA. This is the only reliable method to ensure that the pings are sent through the correct interface, corresponding to your primary ISP.
One misconception is that configuring the source interface or source IP for your IP SLA monitor will affect routing decisions. In reality, this configuration only defines the source address for the ICMP pings but does not dictate how those packets are forwarded. The router’s routing table ultimately makes the forwarding decision for those packets.
If you only configure a source interface or source IP for your IP SLA probe without considering the routing table, the ICMP packets might be forwarded out of a different interface than intended. This can lead to unpredictable failover behavior.
The main focus of this video is to demonstrate what is likely to happen if a route for the IP tracked by IP SLA is not configured in a dual ISP setup, and how this could impact the reliability of failover.
If you’re just looking for a straightforward answer, it’s simple: the answer is yes, you do need this route. But for those of you interested in learning why, in a bit more depth, this video is for you.
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