does not have a route to that destination from any non-BGP source, the withdrawn route will be removed from the main routing table. As long as there is tiebreaker the route selection process moves to the next step. The local preference, weight, and other criteria can be manipulated by local configuration and software capabilities. Such manipulation, although commonly used, is outside the scope of the standard. For example, the community attribute (see below) is not directly used by the BGP selection process. The BGP neighbor process can have a rule to set local preference or another factor based on a manually programmed rule to set the attribute if the community value matches some pattern-matching criterion. If the route was learned from an external peer the per-neighbor BGP process computes a local preference value from local policy rules and then compares the local preference of all routes from the neighbor. Communities BGP communities are attribute tags that can be applied to incoming or outgoing prefixes to achieve some common goal. While it is common to say that BGP allows an administrator to set policies on how prefixes are handled by ISPs, this is generally not possible, strictly speaking. For instance, BGP natively has no concept to allow one AS to tell another AS to restrict advertisement of a prefix to only North American peering customers. Instead, an ISP generally publishes a list of well-known or proprietary communities with a description for each one, which essentially becomes an agreement of how prefixes are to be treated. Examples of common communities include: local preference adjustments, geographic peer type restrictions denial-of-service attack identification AS prepending options. An ISP might state that any routes received from customers with following examples: To Customers North America (East Coast) 3491:100 To Customers North America (West Coast) 3491:200 The customer simply adjusts their configuration to include the correct community or communities for each route, and the ISP is responsible for controlling who the prefix is advertised to. The end user has no technical ability to enforce correct actions being taken by the ISP, though problems in this area are generally rare and accidental. It is a common tactic for end customers to use BGP communities (usually ASN:70,80,90,100) to control the local preference the ISP assigns to advertised routes instead of using MED (the effect is similar). The community attribute is transitive, but communities applied by the customer very rarely