Local preference defaults to a value of 100, and unlike administrative weight, is sent in route updates to other Border Gateway Protocol routers within the same Autonomous System. Local preference is also set by the network administrator in the router configuration. Since BGP communication occurs only between peer routers, TCP is used as the transport protocol to add a measure of reliability to route update delivery. When configuring a Border Gateway router, it is necessary to define an IP address for every peer that is to receive route updates. It is recommended that you configure a loopback address on the destination router for this purpose.
Outbound traffic is simpler to control, so let's look at that first.
The easiest way to split outbound traffic between two links is not to use BGP at all, but to define two equal cost default routes for the router connecting your site to the Internet. The router will then balan ce outbound traffic to the Internet across the two links regardless of ultimate destination. The downside is if the ultimate destination on the Internet is closer to one ISP than the other, you have no control over which link the outbound traffic will travel over. The second option that uses BGP to share route information among entities requires a lot of router memory, since it has to hold tens of thousands of entries in its routing table. When implementing BGP, there are a few choices that can be made with respect to the number and nature of the routing updates sent to the organization by the ISPs.