I've had an interesting side effect when playing around with DNAC application policies.
I built my underlay with ISIS as recommended. I then discover my devices via loopbacks advertised in the underlay.
All has been working fine generally. So I decided to see what Application policies would do to get an idea of the config deployed. Bear in mind there is no data traffic running in this test network other than some basic pings from a laptop to the internet.
I left the Application policy as configured by default in DNAC. All "Business relevant" applications were left as set out the box. My idea was to see what is deployed and from that maybe tweak to suit my needs.
So I called the Application Profile "Test-QoS" and attached it to the default CVD_QUEUEING_PROFILE. I then deployed to all devices in my test site.
The first thing that happened is all my ISIS neighbours dropped and DNAC no longer could reach my devices. ISIS is used to advertise the loopbacks that DNAC manages via so no ISIS neighbours no loopback routes and no DNAC reachability.
Fortunately I have OOB management via all the management interfaces. So I jumped on and removed the QoS policies applied to the underlay and immediately my ISIS neighbours re-formed, routes were advertised in the underlay and DNAC re-established connectivity.
So as a further test I selected a single Edge device and re-deployed the policy. Same again, ISIS neighbours dropped. I have two /30 links from Edge to Border so I removed the DNAC policy from one interface and the ISIS neighbour re-established.
As a further test I configured OSPF on the /30 links. OSPF neighbours were formed immediately across both /30's. One /30 had the DNAC policy and one didn't. The one that had the policy formed an OSPF relationship but not an ISIS relationship. The interface that didn't have the DNAC policy formed both OSPF and ISIS neighbour relationships.
So it seems clear that something in the DNAC policy drops ISIS traffic.
I'm going to play around with the Application and queuing policies to see what resolves the situation. However, is there something I've done wrong here in building my underlay, discovering via loopbacks advertised in the underlay and then applying an out of the box policy from DNAC? I wouldn't expect the out the box policy to have any impact on routing protocols.