Install FRR, addresses and forwarding

root@edge:~# cat /etc/debian_version; dpkg-query -W frr nftables systemd; uname -r
13.7
frr	10.3-3+deb13u1
nftables	1.1.3-1
systemd	257.13-1~deb13u1
7.0.14-linuxkit

• Line 3: FRR 10.3, straight from Debian 13's own repository.
• Line 5: systemd 257: networkd sets the addresses, and services start the router at boot.

root@edge:~# systemctl status frr --no-pager | head -17
● frr.service - FRRouting
     Loaded: loaded (/usr/lib/systemd/system/frr.service; enabled; preset: enabled)
     Active: active (running) since Wed 2026-11-04 22:02:01 UTC; 16s ago
 Invocation: 6cb8de65d13345c89e10cdd043cf15e3
       Docs: https://frrouting.readthedocs.io/en/latest/setup.html
    Process: 135 ExecStart=/usr/lib/frr/frrinit.sh start (code=exited, status=0/SUCCESS)
   Main PID: 146 (watchfrr)
     Status: "FRR Operational"
      Tasks: 8 (limit: 9562)
     Memory: 19M (peak: 35.3M)
        CPU: 130ms
     CGroup: /docker/68dabd70d05c7d5877f2dde6ce7bac3e44adce867fa60f2035d700237fbdaeae/system.slice/frr.service
             ├─146 /usr/lib/frr/watchfrr -d -F traditional zebra mgmtd staticd
             ├─156 /usr/lib/frr/zebra -d -F traditional -A 127.0.0.1 -s 90000000
             ├─161 /usr/lib/frr/mgmtd -d -F traditional -A 127.0.0.1
             └─163 /usr/lib/frr/staticd -d -F traditional -A 127.0.0.1


root@edge:~# grep -E "^[a-z0-9]+d=" /etc/frr/daemons
bgpd=no
ospfd=no
ospf6d=no
ripd=no
ripngd=no
isisd=no
pimd=no
pim6d=no
ldpd=no
nhrpd=no
eigrpd=no
babeld=no
sharpd=no
pbrd=no
bfdd=no
fabricd=no
vrrpd=no
pathd=no

• Line 3: Enabled and running from the moment it's installed…
• Lines 14–17: …but only watchfrr, zebra, mgmtd and staticd. No routing protocol yet.
• Lines 21–38: Every routing daemon is off until you turn it on.

# /etc/systemd/network/10-isp1.network: the link to ISP 1 (primary).
[Match]
Name=eth0

[Network]
Address=192.0.2.1/31
LinkLocalAddressing=no
IPv6AcceptRA=no

• Lines 2–3: Matches the interface by name.
• Lines 6–8: A fixed address on the /31 link to ISP 1, and no IPv6 on this link.

# /etc/systemd/network/11-isp2.network: the link to ISP 2 (backup).
[Match]
Name=eth1

[Network]
Address=192.0.2.3/31
LinkLocalAddressing=no
IPv6AcceptRA=no

# /etc/systemd/network/12-core.network: the link to the office's core router.
[Match]
Name=eth2

[Network]
Address=10.0.0.1/30
LinkLocalAddressing=no
IPv6AcceptRA=no

# /etc/systemd/network/13-lo.network: a loopback address for the router ID.
[Match]
Name=lo

[Network]
Address=10.255.0.1/32

• Line 6: A loopback address, used as the BGP and OSPF router ID.

# /etc/sysctl.d/99-forwarding.conf: this machine is a router.
net.ipv4.ip_forward = 1

root@edge:~# ls /etc/systemd/network/ && systemctl enable --now systemd-networkd 2>&1 | grep -c "Created symlink"
10-isp1.network
11-isp2.network
12-core.network
13-lo.network
5

root@edge:~# networkctl list --no-pager | grep -E "IDX|eth|lo "; ip -br -4 addr
IDX LINK     TYPE     OPERATIONAL SETUP
  1 lo       loopback routable    configured
  4 gretap0  ether    off         unmanaged
  5 erspan0  ether    off         unmanaged
 11 eth2     ether    routable    configured
 12 eth0     ether    routable    configured
 13 eth1     ether    routable    configured
lo               UNKNOWN        127.0.0.1/8 10.255.0.1/32 
eth2@if5311      UP             10.0.0.1/30 
eth0@if5312      UP             192.0.2.1/31 
eth1@if5313      UP             192.0.2.3/31 

root@edge:~# sysctl net.ipv4.ip_forward; sysctl --system | grep ip_forward
net.ipv4.ip_forward = 0
net.ipv4.ip_forward = 1

root@edge:~# ping -c 1 -W 1 192.0.2.0 | tail -2; ping -c 1 -W 1 192.0.2.2 | tail -2; ping -c 1 -W 1 10.0.0.2 | tail -2
1 packets transmitted, 1 received, 0% packet loss, time 0ms
rtt min/avg/max/mdev = 0.071/0.071/0.071/0.000 ms
1 packets transmitted, 1 received, 0% packet loss, time 0ms
rtt min/avg/max/mdev = 0.047/0.047/0.047/0.000 ms
1 packets transmitted, 1 received, 0% packet loss, time 0ms
rtt min/avg/max/mdev = 0.062/0.062/0.062/0.000 ms

• Lines 1–6: Four files, one per interface, and networkd enabled at every boot.
• Lines 9–15: Every link configured: networkd applied the files.
• Lines 16–19: The addresses, including the router ID on the loopback.
• Lines 21–23: Forwarding was off, as on a fresh install; the sysctl.d file turns it on.
• Lines 25–31: Both ISPs and the core router answer.