NetworkPuzzles

When engineers first learn IP SLA, they often start with ICMP echo, because it is simple and useful for checking basic reachability. But in production networks, just knowing that a device is reachable is often not enough.

A server may still answer ping, while the actual service users need is already broken.

That is where DNS and HTTP GET operations become useful. They allow the router to test not only whether a host is reachable, but whether a specific service is actually responding.

In this post, I want to show a very simple example of both:

• DNS IP SLA
• HTTP GET IP SLA

and explain where each one is useful.

A ping only tells us: “Can I reach the remote IP?” That is helpful, but it does not tell us: whether the DNS server is answering queries, whether the web server is serving content, whether the application is usable for the user.

That is why IP SLA supports more advanced operations.

DNS IP SLA

DNS IP SLA is used when we want to verify that a DNS server can answer a name resolution request.

For example, if users say: “Internet is not working”. sometimes the real problem is not routing and not connectivity, but simply that DNS is broken.

In this example, router R1 sends a DNS request to 1.1.1.1 every 8 seconds.

ip sla 2
 dns 1.1.1.1 name www.cisco.com
 frequency 8
ip sla schedule 2 life forever start-time now

Here The router behaves like a DNS client, it asks DNS server 1.1.1.1 to resolve http://www.cisco.com, repeats this test every 8 seconds and it continues forever.

So instead of only checking whether 1.1.1.1 is reachable, we are checking whether it is actually providing DNS service

A DNS server may still respond to ping, but: UDP/53 may be blocked or DNS process may have failed or recursion may be broken or the server may be overloaded. In that case: ICMP succeeds but DNS SLA fails.

Sometimes the ISP path is technically up, but users still cannot browse because DNS resolution is failing. In that case, tracking DNS can be better than tracking only ping.

HTTP GET IP SLA

HTTP GET IP SLA is used when we want to verify that a web service is actually responding. This is different from just pinging a server or checking whether TCP port 80 is open. With HTTP GET, the router behaves like a simple web client and sends an actual HTTP GET request.

In this example, router sends an HTTP GET request to 192.168.34.4 every 60 seconds.

ip sla 5
 http get http://192.168.34.4
 frequency 60
ip sla schedule 5 life forever start-time now

Here the router acts like a web client, it requests a webpage from 192.168.34.4, repeats this every 60 seconds and it continues forever.

A server may still be alive and reachable by ping, but web service may have crashed, or application may not be responding or HTTP process may be down.