Noobie Link Monitor and SD-WAN questions

On a client's T35 running 12.5.7 U3, I am setting up SD-WAN for the first time and doing the link monitor setup first. The help article here monitor/link_monitor_about.html states, "Specify a different Link Monitor host for each external interface."

I want to know if their VoIP phone provider's circuit (Fonality/Netfortris) should work best over Spectrum or over their new AT&T connection, so I want to use their VoIP phone provider's FQDN as the target on both of the interfaces. Will that cause problems? Is that what I should be doing to monitor each provider's quality?

On External-1-Spectrum, I have targets of and with pinging to both. On External-2-ATT, I have targets of and one of AT&T's DNS servers with pinging to both.

What do you think?

Gregg Hill


  • Answering my own question, having the same target on both link monitoring interfaces does exactly what I allows comparing the performance on the two different circuits going to the same target. I changed from FQDN for the Fonality targets to the IP of that FQDN, just to keep DNS lookups out of the picture

    I just looked in the web UI at the SD-WAN loss, latency, and jitter graphs. It is MUCH better on AT&T than Spectrum, especially for jitter.

    Gregg Hill

  • Now my questions (or need to verify that I understand it all) are about how Multi-WAN, Link Monitoring, and SD-WAN all work together. I have 55 policies on the client's T35. One of them is for two VLANs for guest and employee wireless devices to use.

    This is my setup:

    I have Multi-WAN set with External-1-Spectrum as primary and External-2-ATT as secondary, set to Failover and immediate failback.

    I have Link Monitoring for both ISPs with the client's Fonality server as the one to ping monitor for loss, latency, and jitter so that I can compare the circuits, plus Spectrum and AT&T DNS servers as the secondary ping monitors respectively.

    In SD-WAN, I have an External-1-Primary action with Spectrum as primary, and I have an External-2-Primary with AT&T as primary.

    I am using their guest/employee device VLANs as a test, and I have that policy using the External-2-Primary SD-WAN action. Traffic from those two VLANs goes out the AT&T connection as I expected.

    Now my first question about this setup is, for all of the other policies that do NOT have a specific SD-WAN action applied, do Link Monitor and Multi-WAN take over here in case the External-1-Spectrum connection goes down? I suspect that because no SD-WAN action is applied that would override the Multi-WAN settings, that Link Monitoring would cause the connection to fail over to the External-2-ATT connection. That is my desired result, i.e., that if Spectrum goes down, EVERYTHING goes to AT&T, then when Spectrum comes back, it all fails back to Spectrum or AT&T according to the SD-WAN actions applied.

    Can anyone verify that is what will/should happen?

    Gregg Hill

  • Another question: I have the recommended minimum of two ping targets for each interface in Link Monitoring. My assumption is that BOTH of External-1-TWC's targets must fail the ping response three time at 5-seconds intervals in order for failover to trip, and that if either interface's targets still get a response from at least one of their targets, then that interface is considered to be up still. That is what would make sense to me.

    Gregg Hill

  • Q: do Link Monitor and Multi-WAN take over here in case the External-1-Spectrum connection goes down?
    A: that is how I understand it to work

    If one has 2 link monitor targets for an external interface, it makes no sense to me that if 1 gets no reply and the other does, to consider the link down.
    This should be clarified in the docs, but isn't.

  • Answering my own "...for all of the other policies that do NOT have a specific SD-WAN action applied, do Link Monitor and Multi-WAN take over here in case the External-1-Spectrum connection goes down?" question, a few minutes after I posted that question, the client's Spectrum connection was detected as down and everything failed over to the ATT secondary line. How's THAT for timing a question? IT all failed back to Spectrum within a minute of the failover.

    I thought I was losing my mind (and I was right, but that's beside the point). So, yes, if one does not have an SD-WAN action defined on a policy, it will get the global Multi-WAN action applied, which in my case, was to failover to the secondary circuit. The timing of it threw me off completely.

    Gregg Hill

  • Thank you , Bruce. At least I did grasp it correctly. It has been a rough few days.

    Gregg Hill

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