In the network as shown in the figure, which of the following descriptions about R4 preferred routing is correct?

Question

Options

• AIt is impossible to determine, MED cannot be used as a condition for BGP route selection between ASs.
• BPrefer the route delivered by R3, because the larger the MED value, the better.
• CHard [balancing?] is performed, if the MED value received from R2/R3 are equal to the default value.
• DPrefer the route delivered by R2, because the smaller the MED value, the better.

• A
  9% (2)
• B
  5% (1)
• C
  14% (3)
• D
  73% (16)

Explanation

MED (Multi-Exit Discriminator) prefers lower values, so R4 selects the route from R2 because it carries a smaller MED. MED is a BGP path attribute used to signal to neighboring ASes which entry point is preferred - it functions like a metric where a lower number means a more preferred path.

Why the distractors are wrong:

• A is incorrect because MED is absolutely a valid BGP path selection attribute - it appears in the standard BGP decision process and is used specifically to influence inter-AS routing.
• B inverts the rule - a larger MED indicates a less-preferred path, not a better one.
• C is incorrect because BGP does not perform load balancing simply because two routes share the same MED value; it continues down the decision algorithm using subsequent tiebreakers (e.g., IGP cost, router ID).

Memory tip: Think of MED like a road toll - the lower the toll (MED), the more traffic you want to attract. Routers "prefer the cheaper road."

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