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