Which of the following attributes can be used to prevent route advertisement loops in a...

To prevent route advertisement loops in a network using route reflectors, specific attributes are employed.

The key attributes used for loop prevention in BGP are:

MED (Multi-Exit Discriminator): Used to influence the incoming traffic from neighboring ASes. It is not used for loop prevention.

ORIGIN: Indicates the origin of the route but does not directly prevent loops.

CLUSTER_LIST: Used by route reflectors to prevent loops. When a route reflector propagates a route, it adds its cluster ID to the CLUSTER_LIST. If a route with its own cluster ID is received, it is rejected to prevent loops.

AS_PATH: Records the ASes a route has traversed, preventing loops by rejecting routes containing the local AS number.

Therefore, the CLUSTER_LIST attribute is specifically used by route reflectors to prevent routing loops by keeping track of the clusters a route has passed through.

References:

Nokia Border Gateway Protocol Fundamentals documentation, Section 6.5 on Attributes for Loop Prevention in Route Reflectors.