Refer to the exhibit.
AS #1 and AS #2 have multiple EBGP connections with each other. AS #1 wants all return traffic that
is destined to the prefix 10.10.10.1/32 to enter through the router R1 from AS #2. In order to
achieve this routing policy, the AS 1 advertises a lower MED from R1, compared to a higher MED
from R3, to their respective BGP neighbor for the prefix 10.10.10.0/24. Will this measure guarantee
that the routing policy is always in effect?
A.
Yes, because MED plays a deterministic role in return traffic engineering in BGP.
B.
Yes, because a lower MED forces BGP best-path route selection in AS #2 to choose R1 as the best
path for 10.10.10.0/24.
C.
Yes, because a lower MED in AS #2 is the highest BGP attribute in BGP best-path route selection.
D.
No, AS #2 can choose to alter the weight attribute in R2 for BGP neighbor R1, and this weight
value is cascaded across AS #2 for BGP best-path route selection.
E.
No, AS #2 can choose to alter the local preference attribute to overwrite the best-path route
selection over the lower MED advertisement from AS #1. This local preference attribute is cascaded
across AS #2 for the BGP best-path route selection.
BGP Best path selection
1. highest Weight (Cisco-spec) > 2. highest local pref >3.locally generate or redistributed by igp >shortest AS_path >ebgp >ibgp>lowest igp metric >if multipath >which path receive first >Lowest router id>lowest neighbour address
weight>local>AS-path>origin>MEd
But weight is local only. local-pref is advertised with the route through out his own AS. So everyone else knows the preferred path out. No one else knows about the weight on the exit router.