Try it yourself

# Two routers, two links. Run with the static or BGP configs:
#   DESIGN=static docker compose -f lab-compose.yaml up -d --build
#   DESIGN=bgp    docker compose -f lab-compose.yaml up -d --build
name: why-lab
x-router: &router
  build: .
  image: ttr-frr-why:10.2.1
  privileged: true            # iptables (to fail a link quietly) and ip link
services:
  r1:
    <<: *router
    hostname: r1
    volumes: ["./daemons:/etc/frr/daemons:ro", "./r1-${DESIGN:-static}.conf:/etc/frr/frr.conf:ro"]
    networks: { linka: { interface_name: eth0 }, linkb: { interface_name: eth1 } }
  r2:
    <<: *router
    hostname: r2
    volumes: ["./daemons:/etc/frr/daemons:ro", "./r2-${DESIGN:-static}.conf:/etc/frr/frr.conf:ro"]
    networks: { linka: { interface_name: eth0 }, linkb: { interface_name: eth1 } }
networks:
  linka: { enable_ipv4: false, enable_ipv6: true, ipam: { config: [{ subnet: "fd00:aa::/64" }] } }
  linkb: { enable_ipv4: false, enable_ipv6: true, ipam: { config: [{ subnet: "fd00:bb::/64" }] } }

# FRR 10.2.1 plus the tools to fail a link quietly and measure the outage.
FROM quay.io/frrouting/frr:10.2.1
RUN apk add --no-cache iptables iputils-ping \
 && mkdir -p /var/log/frr && chown frr:frr /var/log/frr \
 && echo "service integrated-vtysh-config" > /etc/frr/vtysh.conf

bgpd=yes
ospfd=no
ospf6d=no
ripd=no
ripngd=no
isisd=no
pimd=no
ldpd=no
nhrpd=no
eigrpd=no
babeld=no
sharpd=no
pbrd=no
bfdd=yes
fabricd=no
vrrpd=no
pathd=no
staticd=yes
vtysh_enable=yes
zebra_options="  -A 127.0.0.1 -s 90000000"
bgpd_options="   -A 127.0.0.1"
staticd_options="-A 127.0.0.1"
bfdd_options="   -A 127.0.0.1"

frr version 10.2.1
frr defaults traditional
hostname r2
log file /var/log/frr/frr.log informational
service integrated-vtysh-config
!
interface eth0
 description link A
 ip address 192.0.2.1/31
exit
!
interface eth1
 description link B
 ip address 192.0.2.3/31
exit
!
interface lo
 ip address 198.51.100.2/32
exit
!
! The "simple" design: reach r1 over link A, with a floating backup over link B
! (administrative distance 200, so it's only used if the first route disappears).
ip route 198.51.100.1/32 192.0.2.0
ip route 198.51.100.1/32 192.0.2.2 200

frr version 10.2.1
frr defaults traditional
hostname r2
log file /var/log/frr/frr.log informational
service integrated-vtysh-config
!
interface eth0
 description link A
 ip address 192.0.2.1/31
exit
!
interface eth1
 description link B
 ip address 192.0.2.3/31
exit
!
interface lo
 ip address 198.51.100.2/32
exit
!
! The routing-protocol design: a BGP session over each link, each announcing
! r2's loopback. Whichever session is alive carries the traffic.
router bgp 64501
 bgp router-id 198.51.100.2
 bgp log-neighbor-changes
 neighbor 192.0.2.0 remote-as 64500
 neighbor 192.0.2.0 description r1 via link A
 neighbor 192.0.2.2 remote-as 64500
 neighbor 192.0.2.2 description r1 via link B
 !
 address-family ipv4 unicast
  network 198.51.100.2/32
  neighbor 192.0.2.0 route-map ALLOW in
  neighbor 192.0.2.0 route-map ALLOW out
  neighbor 192.0.2.2 route-map ALLOW in
  neighbor 192.0.2.2 route-map ALLOW out
  maximum-paths 1
 exit-address-family
exit
!
! A two-router lab: accept and send everything. Real edges filter (see the BGP lessons).
route-map ALLOW permit 10
exit