Which BGP attribute can modify only an adjacent AS’s optimal entry point into the local AS?

A.
MED

B.
local preference

C.
AS path

D.
community

Explanation:
Multiple Exit Discriminator (MED)

The MUTLI_EXIT_DISC (MED) is an optional non-transitive attribute that provides a mechanism for the network administrator to convey to adjacent autonomous systems to optimal entry point in the local AS; Figure 1-11 illustrates this concept.

The Multiple Exit Discriminator
Here, AS 65200 is setting the MED on its T1 exit point to 100, and the MED on its OC3 exit point to 50, with the intended result that the OC3 connection be preferred. However, the problem with using the MED in this way becomes apparent with this simple example. First, AS 65100 will receive three paths to 10.1.1.0/24, one through AS 65300, and two through AS 65200. The MED of the path through AS 65100 and the paths through AS 65200 will not be compared, since their AS Path is not the same. If AS 65100 has set their BGP local preferences on router A, B, and C, to favor the path through AS 65300, then the MED from AS 65200 will have no impact, per MED is considered after local preference in the BGP decision algorithm.

NOTE
MEDs received from different autonomous systems are not compared as a default behavior, though many implementations provide a mechanism to enable comparing of MEDs between different autonomous systems. Benefits and offshoots of using MEDs, and comparing them between different ASes, will be discussed in later sections.
If the path through AS 65300 did not exist, or was not preferred over the path through AS 65200 for some other reason, the MEDs advertised by routers D and E might have some impact on the best path decision made by AS 65100. However, if AS 65100 sets some BGP metric with a higher degree of preference in the decision algorithm, such as the local preference, to prefer one path over the other, the MED would never be considered.