I use local interfaces in the file
/etc/network/interfaces
# The loopback network interface
auto lo
iface lo inet loopback
# Dummy's
auto dummy0
iface dummy0 inet manual
iface dummy0 inet6 manual
pre-up ip link add dummy0 type dummy
auto dummy1
iface dummy1 inet manual
iface dummy1 inet6 manual
pre-up ip link add dummy1 type dummy
On FRR I configure the inferface dummy0 like this :
!
interface dummy0
description *** loopback0 ***
ip address W.X.Y.Z/32
ipv6 nd suppress-ra
!
The loopback (here the dummy) interfaces are used to mount my internal BGP sessions and access to the router from the WAN (public ip).