doc: Do not use dvr_snat on computes

Change-Id: I1194d9995c16c1e178026986d9465aa09c1529c8
Closes-Bug: #1934666

doc/source/admin/deploy-ovs-ha-dvr.rst

@@ -106,6 +106,11 @@ Network node
       [DEFAULT]
       agent_mode = dvr_snat
 
+.. note::
+
+   ``agent_mode = dvr_snat`` is not supported on compute nodes.
+   For discussion please see: `bug #1934666 <https://bugs.launchpad.net/neutron/+bug/1934666>`_
+
 
 #. Restart the following services:
 

neutron/conf/agent/l3/config.py

@@ -38,6 +38,7 @@ OPTS = [
                       "with DVR.  This mode must be used for an L3 agent "
                       "running on a centralized node (or in single-host "
                       "deployments, e.g. devstack). "
+                      "dvr_snat mode is not supported on a compute host. "
                       "'dvr_no_external' - this mode enables only East/West "
                       "DVR routing functionality for a L3 agent that runs on "
                       "a compute host, the North/South functionality such "