implementation of some typical bgp cases (by quagga)
reference: https://www.cisco.com/c/en/us/support/docs/ip/border-gateway-protocol-bgp/26634-bgp-toc.html#weight
Note: Don't forget to change the absolute paths in init_mininet.py bgpd.conf and zebra.conf
Note: Don't forget to give permission for the user to create and modify files