How we can use BGP Protocol to Connect The Branches of the companies together?

If you're an Internet service provider (ISP), running BGP in your network is almost a must. I've seen consumer-focused ISPs that tried to get around this recommendation and used BGP solely to peer with their upstream ISPs, but they eventually had to bite the bullet and deploy BGP to increase the stability of their network, provide end-to-end quality-of-service or penetrate enterprise markets. Enterprise-focused ISPs have to run BGP from the start to support their multi-homed customers).

it can be used on the premetere external routers CE and your ISP then you can reditribut BGP routes into your IGP like OSPF, but you have take care in case if you have dual ISP case in this case you have use all controllers and precaution in the router configuration to prevent inter ISP traffic pass through your links.

for more information please read the following documents,and you are welcome any time.

https://supportforums.cisco.com/document//bgp-pe-ce-routing-protocol-overview-mpls-vpns-part-ii

http://www.ssi.gouv.fr/uploads///BGP_configuration_best_practices.pdf