rmmrkr == maas01 (DOWN)
7htx7y == maas02 (UP)

This time I instrumented the get_best_subnet_for_ip() method:

2021-11-05T14:54:48.521389+00:00 maas01 maas.rpc.rackcontrollers: [info] Existing rack controller 'maas02' running version 2.8.7-8611-g.f2514168f-0ubuntu1~18.04.1 has connected to region 'maas01'.
2021-11-05T14:54:48.708314+00:00 maas01 maas.rpc.rackcontrollers: [info] Existing rack controller 'maas02' running version 2.8.7-8611-g.f2514168f-0ubuntu1~18.04.1 has connected to region 'maas01'.
2021-11-05T14:54:52.263227+00:00 maas01 maas.node: [info] vmtest: Status transition from NEW to COMMISSIONING
2021-11-05T14:54:52.407983+00:00 maas01 maas.subnet: [info] DEBUG ip: 172.16.99.1
2021-11-05T14:54:52.408188+00:00 maas01 maas.subnet: [info] DEBUG subnets: <RawQuerySet: #012        SELECT DISTINCT#012            subnet.*,#012            masklen(subnet.cidr) "prefixlen",#012            vlan.dhcp_on "dhcp_on"#012        FROM maasserver_subnet AS subnet#012        INNER JOIN maasserver_vlan AS vlan#012            ON subnet.vlan_id = vlan.id#012        WHERE#012            172.16.99.1 << subnet.cidr /* Specified IP is inside range */#012        ORDER BY#012            /* Pick subnet that is on a VLAN that is managed over a subnet#012               that is not managed on a VLAN. */#012            dhcp_on DESC,#012            /* If there are multiple subnets we want to pick the most specific#012               one that the IP address falls within. */#012            prefixlen DESC#012        LIMIT 1#012        >
2021-11-05T14:54:52.409355+00:00 maas01 maas.subnet: [info] DEBUG ip: 172.16.99.1
2021-11-05T14:54:52.409514+00:00 maas01 maas.subnet: [info] DEBUG subnets: <RawQuerySet: #012        SELECT DISTINCT#012            subnet.*,#012            masklen(subnet.cidr) "prefixlen",#012            vlan.dhcp_on "dhcp_on"#012        FROM maasserver_subnet AS subnet#012        INNER JOIN maasserver_vlan AS vlan#012            ON subnet.vlan_id = vlan.id#012        WHERE#012            172.16.99.1 << subnet.cidr /* Specified IP is inside range */#012        ORDER BY#012            /* Pick subnet that is on a VLAN that is managed over a subnet#012               that is not managed on a VLAN. */#012            dhcp_on DESC,#012            /* If there are multiple subnets we want to pick the most specific#012               one that the IP address falls within. */#012            prefixlen DESC#012        LIMIT 1#012        >
2021-11-05T14:54:52.414451+00:00 maas01 maas.node: [info] DEBUG layer2 get_usable_rack_controllers racks: [<RackController: rmmrkr (maas01)>]
2021-11-05T14:54:52.415302+00:00 maas01 maas.subnet: [info] DEBUG ip: 172.16.99.1
2021-11-05T14:54:52.415510+00:00 maas01 maas.subnet: [info] DEBUG subnets: <RawQuerySet: #012        SELECT DISTINCT#012            subnet.*,#012            masklen(subnet.cidr) "prefixlen",#012            vlan.dhcp_on "dhcp_on"#012        FROM maasserver_subnet AS subnet#012        INNER JOIN maasserver_vlan AS vlan#012            ON subnet.vlan_id = vlan.id#012        WHERE#012            172.16.99.1 << subnet.cidr /* Specified IP is inside range */#012        ORDER BY#012            /* Pick subnet that is on a VLAN that is managed over a subnet#012               that is not managed on a VLAN. */#012            dhcp_on DESC,#012            /* If there are multiple subnets we want to pick the most specific#012               one that the IP address falls within. */#012            prefixlen DESC#012        LIMIT 1#012        >
2021-11-05T14:54:52.417916+00:00 maas01 maas.subnet: [info] DEBUG ip: 172.16.99.1
2021-11-05T14:54:52.418194+00:00 maas01 maas.subnet: [info] DEBUG subnets: <RawQuerySet: #012        SELECT DISTINCT#012            subnet.*,#012            masklen(subnet.cidr) "prefixlen",#012            vlan.dhcp_on "dhcp_on"#012        FROM maasserver_subnet AS subnet#012        INNER JOIN maasserver_vlan AS vlan#012            ON subnet.vlan_id = vlan.id#012        WHERE#012            172.16.99.1 << subnet.cidr /* Specified IP is inside range */#012        ORDER BY#012            /* Pick subnet that is on a VLAN that is managed over a subnet#012               that is not managed on a VLAN. */#012            dhcp_on DESC,#012            /* If there are multiple subnets we want to pick the most specific#012               one that the IP address falls within. */#012            prefixlen DESC#012        LIMIT 1#012        >
2021-11-05T14:54:52.423817+00:00 maas01 maas.node: [info] DEBUG layer2 get_usable_rack_controllers racks: []
2021-11-05T14:54:52.423931+00:00 maas01 maas.node: [info] DEBUG client_idents else: []
2021-11-05T14:54:52.426361+00:00 maas01 maas.node: [info] DEBUG fallback_idents: ['rmmrkr']
2021-11-05T14:54:52.427948+00:00 maas01 maas.node: [info] vmtest: Aborting COMMISSIONING and reverted to NEW. Unable to power control the node. Please check power credentials.
2021-11-05T14:54:52.434744+00:00 maas01 maas.node: [info] vmtest: Status transition from COMMISSIONING to NEW
2021-11-05T14:54:52.477856+00:00 maas01 maas.node: [error] vmtest: Could not start node for commissioning: Unable to connect to any rack controller rmmrkr; no connections available.