How to restart networking in qemu?

ip link show shows the following:

5: virbr0: <NO-CARRIER,BROADCAST,MULTICAST,UP> mtu 1500 qdisc noqueue state DOWN mode DEFAULT group default qlen 1000    link/ether 52:54:00:fb:a6:78 brd ff:ff:ff:ff:ff:ff15: vnet9: <BROADCAST,MULTICAST> mtu 1500 qdisc noqueue state DOWN mode DEFAULT group default qlen 1000    link/ether fe:54:00:ba:d7:98 brd ff:ff:ff:ff:ff:ff

Question

1. Doing sudo ip link set vnet9 up makes its state UNKNOWN . Why ?
2. What commands can I bring up networking without restarting the vm