V_useloopback is set.
slot4 and slot 6 are point-to-point interfaces.

-------------------------------------------------------------------------------
bash# ifconfig slot4 inet 172.16.1.48 10.0.0.20 netmask 255.255.255.255
bash# ifconfig slot6 inet 172.16.1.48 10.0.0.20 netmask 255.255.255.255
bash#
bash# netstat -rn
Routing tables

Internet:
Destination        Gateway            Flags    Refs      Use  Netif Expire
10.0.0.20          link#7             UH          0        0  slot4
127.0.0.1          link#1             UH          0        0    lo0
172.16.1.48        link#7             UHS         1        0    lo0
-------------------------------------------------------------------------------

Loopback route has two references.
Network route leads to slot4.

-------------------------------------------------------------------------------
bash# sysctl net.link.ether.inet.useloopback=0
net.link.ether.inet.useloopback: 1 -> 0

bash# ifconfig slot6 inet 172.16.1.49 10.0.0.20 netmask 255.255.255.255
bash# netstat -rn
Routing tables

Internet:
Destination        Gateway            Flags    Refs      Use  Netif Expire
10.0.0.20          link#7             UH          0        0  slot4
127.0.0.1          link#1             UH          0        0    lo0
172.16.1.48        link#7             UHS         0        0    lo0
-------------------------------------------------------------------------------

V_useloopback is not set.
New 172.16.1.49 loopback route is not installed.
Old 172.16.1.48 loopback route has one reference.

-------------------------------------------------------------------------------
bash# ifconfig slot4 inet 172.16.1.49 10.0.0.20 netmask 255.255.255.255
bash# netstat -rn
Routing tables

Internet:
Destination        Gateway            Flags    Refs      Use  Netif Expire
10.0.0.20          link#9             UH          0        0  slot6
127.0.0.1          link#1             UH          0        0    lo0
-------------------------------------------------------------------------------

New 172.16.1.49 loopback route is not installed.
Old 172.16.1.48 loopback route was deleted.

Network route leads to slot6 as it was replaced during 172.16.1.48
address deletetion.

-------------------------------------------------------------------------------
bash# sysctl net.link.ether.inet.useloopback=1
net.link.ether.inet.useloopback: 0 -> 1

bash# ifconfig slot4 inet 172.16.1.48 10.0.0.20 netmask 255.255.255.255
bash# ifconfig slot6 inet 172.16.1.48 10.0.0.20 netmask 255.255.255.255
bash#
bash# netstat -rn
Routing tables

Internet:
Destination        Gateway            Flags    Refs      Use  Netif Expire
10.0.0.20          link#7             UH          0        0  slot4
127.0.0.1          link#1             UH          0        0    lo0
172.16.1.48        link#7             UHS         1        0    lo0
-------------------------------------------------------------------------------

V_useloopback is set.
Loopback route has two references.

-------------------------------------------------------------------------------
bash# sysctl net.link.ether.inet.useloopback=0
net.link.ether.inet.useloopback: 1 -> 0

bash# ifconfig slot4 down
bash# ifconfig slot6 down
bash#
bash# netstat -rn
Routing tables

Internet:
Destination        Gateway            Flags    Refs      Use  Netif Expire
127.0.0.1          link#1             UH          0        0    lo0
172.16.1.48        link#7             UHS         1        0    lo0

bash# ifconfig slot4 up
bash# ifconfig slot6 up
bash#
bash# netstat -rn
Routing tables

Internet:
Destination        Gateway            Flags    Refs      Use  Netif Expire
10.0.0.20          link#7             UH          0        0  slot4
127.0.0.1          link#1             UH          0        0    lo0
-------------------------------------------------------------------------------

V_useloopback is not set.
Loopback route is deleted after up->down->up cycle.

-------------------------------------------------------------------------------
bash# sysctl net.link.ether.inet.useloopback=1
net.link.ether.inet.useloopback: 0 -> 1

bash# ifconfig slot4 down
bash# ifconfig slot4 up
bash#
bash# netstat -rn
Routing tables

Internet:
Destination        Gateway            Flags    Refs      Use  Netif Expire
10.0.0.20          link#9             UH          0        0  slot6
127.0.0.1          link#1             UH          0        0    lo0
172.16.1.48        link#7             UHS         0        0    lo0
-------------------------------------------------------------------------------

V_useloopback is set.
Loopback route is installed after up->down->up cycle.

-------------------------------------------------------------------------------
bash# ifconfig slot6 inet 172.16.1.49 10.0.0.20 netmask 255.255.255.255
bash#
bash# netstat -rn
Routing tables

Internet:
Destination        Gateway            Flags    Refs      Use  Netif Expire
10.0.0.20          link#7             UH          0        0  slot4
127.0.0.1          link#1             UH          0        0    lo0
172.16.1.48        link#7             UHS         0        0    lo0
172.16.1.49        link#9             UHS         0        0    lo0
-------------------------------------------------------------------------------

New 172.16.1.49 loopback route is installed

-------------------------------------------------------------------------------
bash# ifconfig slot6 inet 172.16.1.48 10.0.0.20 netmask 255.255.255.255
bash# netstat -rn
Routing tables

Internet:
Destination        Gateway            Flags    Refs      Use  Netif Expire
10.0.0.20          link#7             UH          0        0  slot4
127.0.0.1          link#1             UH          0        0    lo0
172.16.1.48        link#7             UHS         1        0    lo0
-------------------------------------------------------------------------------

Old 172.16.1.49 loopback route is deleted.
172.16.1.48 loopback route has two references.