TL&DR: FRRouting advertises the IP prefix on the lo loopback interface with zero cost.

Let’s start with the background story. When we added FRRouting containers support to netlab, someone decided to use lo0 as the loopback interface name. That device doesn’t exist in a typical Linux container, but it’s not hard to add it:

$ ip link add lo0 type dummy
$ ip link set dev lo0 up

However, when I tried to use OSPF with SR/MPLS, FRRouting complained that lo0 is not a loopback interface. Technically, that’s true; let’s take a look at the ip link printout:

$ ip link
1: lo: <LOOPBACK,UP,LOWER_UP> mtu 65536 qdisc noqueue state UNKNOWN mode DEFAULT group default qlen 1000
    link/loopback 00:00:00:00:00:00 brd 00:00:00:00:00:00
2: lo0: <BROADCAST,NOARP,UP,LOWER_UP> mtu 1500 qdisc noqueue state UNKNOWN mode DEFAULT group default qlen 1000
    link/ether 36:8e:4a:ad:3c:85 brd ff:ff:ff:ff:ff:ff
4433: eth0@if4434: <BROADCAST,MULTICAST,UP,LOWER_UP> mtu 1500 qdisc noqueue state UP mode DEFAULT group default
    link/ether 02:42:c0:a8:79:67 brd ff:ff:ff:ff:ff:ff link-netnsid 0
4441: eth1@if4442: <BROADCAST,MULTICAST,UP,LOWER_UP> mtu 1500 qdisc noqueue state UP mode DEFAULT group default
    link/ether aa:c1:ab:3f:97:50 brd ff:ff:ff:ff:ff:ff link-netnsid 1

Not a big deal. It was time to fix that discrepancy and use lo as the loopback interface on FRRouting (and bring the configuration templates in line with the Cumulus Linux ones). I was ready to run the integration tests a few minor edits later, but the OSPF tests kept failing.

As I started investigating the root cause of the failure, I found an amazing detail: the default OSPF cost is ten on the lo0 interface and zero on the lo interface:¹

x2# show ip ospf int
eth1 is up
  Internet Address 10.1.0.6/30, Broadcast 10.1.0.7, Area 0.0.0.0
  Router ID 10.0.0.3, Network Type POINTOPOINT, Cost: 10
lo is up
  Internet Address 10.0.0.3/32, Broadcast 10.0.0.3, Area 0.0.0.0
  Router ID 10.0.0.3, Network Type LOOPBACK, Cost: 0
lo0 is up
  This interface is UNNUMBERED, Area 0.0.0.0
  Router ID 10.0.0.3, Network Type BROADCAST, Cost: 10

There’s nothing wrong with what FRRouting does; RFC 2328 allows OSPF cost on stub networks to be zero. It’s just different from what we’ve seen in other OSPF implementations.