Optional BGP Attributes
Question
Which two BGP attributes are optional? (Choose two.)
Answers
Explanations
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A. B. C. D.BC
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.
Cluster List is Optional non-transitive BGP attribute.
Incorrect Answers:
A: The ORIGIN is a well known mandatory attribute.
D: The AS_PATH is a well-known mandatory attribute.
http://www.informit.com/articles/article.aspx?p=331613&seqNum=5BGP (Border Gateway Protocol) is a widely used routing protocol for internet service providers and large enterprises. BGP uses various attributes to make routing decisions and select the best path for traffic.
Out of the four BGP attributes mentioned in the question, two are optional:
MED (Multi-Exit Discriminator): MED is an optional non-transitive attribute that is used to convey routing preferences from one AS (Autonomous System) to another. The MED is used to indicate the preferred path for inbound traffic when there are multiple exit points from the same AS. When multiple exit points have the same MED, the router with the shortest AS path to the destination is chosen.
Cluster list: The cluster list is an optional non-transitive BGP attribute that is used in confederations to prevent routing loops. It is a list of BGP cluster IDs that the route has passed through. If a router receives an update with its own cluster ID in the cluster list, it will reject the update to prevent loops.
On the other hand, the two remaining attributes, Origin and AS Path, are mandatory BGP attributes that every BGP speaker must include in the updates they send to their peers:
Origin: The origin attribute is a mandatory BGP attribute that specifies the origin of the route. There are three possible values for the origin attribute: IGP (Interior Gateway Protocol), EGP (Exterior Gateway Protocol), and Incomplete. The IGP value indicates that the route was originated within the local AS, while the EGP value indicates that the route was learned from an external AS. The Incomplete value is used when the origin of the route is unknown.
AS Path: The AS path attribute is a mandatory BGP attribute that lists the AS numbers that the route has passed through. The AS path attribute is used to prevent routing loops and to provide information about the path that the traffic will take. BGP uses the AS path attribute to select the best path by comparing the length of the AS path. The shorter AS path is preferred over the longer AS path.
