ip netns add a
ip netns add b1
ip netns add b2
ip netns add c
ip link add a0    type dummy
ip link add a-b1  type veth peer name b1-a
ip link add a-b2  type veth peer name b2-a
ip link add b1-b2 type veth peer name b2-b1
ip link add b1-c  type veth peer name c-b1
ip link add b2-c  type veth peer name c-b2
ip link add c0    type dummy
ip link set dev a0    netns a
ip link set dev a-b1  netns a
ip link set dev a-b2  netns a
ip link set dev b1-a  netns b1
ip link set dev b1-b2 netns b1
ip link set dev b1-c  netns b1
ip link set dev b2-a  netns b2
ip link set dev b2-b1 netns b2
ip link set dev b2-c  netns b2
ip link set dev c-b1  netns c
ip link set dev c-b2  netns c
ip link set dev c0    netns c

Terminal a:

ip netns exec a /bin/bash
export PS1="${debian_chroot:+($debian_chroot)}\u@ns-a-:\w\$ "
sysctl -w 'net.ipv6.conf.all.forwarding=1'
ip link set up dev a0
ip link set up dev a-b1
ip link set up dev a-b2
ip addr add fc0a::a/64 dev a0
ip addr add fc01::a/64 dev a-b1
ip addr add fc02::a/64 dev a-b2

Terminal b1:

ip netns exec b1 /bin/bash
export PS1="${debian_chroot:+($debian_chroot)}\u@ns-b1-:\w\$ "
sysctl -w 'net.ipv6.conf.all.forwarding=1'
ip link set up dev b1-a
ip addr add fc01::b1/64 dev b1-a
ip addr add fc03::b1/64 dev b1-b2
ip addr add fc04::b1/64 dev b1-c
ping -qc1 fc01::a

Terminal b2:

ip netns exec b2 /bin/bash
export PS1="${debian_chroot:+($debian_chroot)}\u@ns-b2-:\w\$ "
sysctl -w 'net.ipv6.conf.all.forwarding=1'
ip link set up dev b2-a
ip addr add fc02::b2/64 dev b2-a
ip addr add fc03::b2/64 dev b2-b1
ip addr add fc05::b2/64 dev b2-c
ping -qc1 fc02::a

Terminal c:

ip netns exec c /bin/bash
export PS1="${debian_chroot:+($debian_chroot)}\u@ns-c-:\w\$ "
sysctl -w 'net.ipv6.conf.all.forwarding=1'
ip link set up dev c-b1
ip link set up dev c-b2
ip link set up dev c0
ip addr add fc04::c/64 dev c-b1
ip addr add fc05::c/64 dev c-b2
ip addr add fc0c::c/64 dev c0
ping -qc1 fc04::b1
ping -qc1 fc05::b2

ip netns add a
ip netns add b1
ip netns add b2
ip netns add c1
ip netns add c2
ip netns add d
ip link add a-b1 type veth peer name b1-a
ip link add a-b2 type veth peer name b2-a
ip link add b1-c1 type veth peer name c1-b1
ip link add b1-c2 type veth peer name c2-b1
ip link add b2-c1 type veth peer name c1-b2
ip link add b2-c2 type veth peer name c2-b2
ip link add c2-d type veth peer name d-c2
ip link add c1-d type veth peer name d-c1
ip link set dev a-b1 netns a
ip link set dev a-b2 netns a
ip link set dev b1-a netns b1
ip link set dev b1-c1 netns b1
ip link set dev b1-c2 netns b1
ip link set dev b2-a netns b2
ip link set dev b2-c1 netns b2
ip link set dev b2-c2 netns b2
ip link set dev c1-b1 netns c1
ip link set dev c1-b2 netns c1
ip link set dev c1-d netns c1
ip link set dev c2-b1 netns c2
ip link set dev c2-b2 netns c2
ip link set dev c2-d netns c2
ip link set dev d-c1 netns d
ip link set dev d-c2 netns d

Terminal a:

ip netns exec a /bin/bash
ip link set up dev a-b1
ip addr add fc0a:b1::a/64 dev a-b1

Terminal b1:

ip netns exec b1 /bin/bash
ip link set up dev b1-a
ip addr add fc0a:b1::b1/64 dev b1-a
ping fc0a:b1::a