Which BGP Attributes Affect Return Traffic Preference?
Question
You are attempting to influence return traffic into your autonomous system by preferring one router as an entry point.
Which two BGP attributes will affect this behavior? (Choose two.)
Answers
Explanations
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When an autonomous system (AS) advertises its prefixes to other ASes via Border Gateway Protocol (BGP), it can use BGP attributes to influence the routing of traffic towards its network. In this scenario, the goal is to influence return traffic into the AS by preferring one router as the entry point.
Two BGP attributes that can affect this behavior are:
- Local Preference: This attribute is used to indicate the preferred path for incoming traffic into the AS. When a BGP router receives multiple paths to the same destination prefix, it will compare the local preference value of each path and choose the one with the highest value. The local preference attribute is only used within the AS and is not propagated to other ASes.
By setting a higher local preference value on the router that you want to prefer as the entry point, you can influence incoming traffic to use that router to reach your network. This can be useful if you have multiple border routers advertising the same prefixes.
- AS Path: The AS path attribute is a sequence of AS numbers that describes the path that a prefix has traversed to reach the current AS. When a BGP router receives multiple paths to the same prefix with different AS paths, it will prefer the path with the shortest AS path length.
By manipulating the AS path attribute, you can influence incoming traffic to prefer a particular path to reach your network. For example, you could prepend your AS number multiple times to the AS path of prefixes advertised by a specific router to make the path appear longer and less attractive to incoming traffic.
The other two options listed in the question, Originator ID and Multi-Exit Discriminator (MED), are not directly related to influencing incoming traffic into an AS.
Originator ID is used to prevent routing loops in BGP by identifying the router that originated a particular route update. It is not used to influence traffic routing.
MED is used to indicate the preferred path for outgoing traffic from one AS to another AS when multiple exit points exist. It is not used to influence incoming traffic routing.
