What kind of BGP attribute is the Multi_Exit_Disc?

The Multi-Exit Discriminator (MED) is classified as an optional non-transitive BGP attribute. It is "optional" because a BGP implementation is not required to recognize or act upon it. It is "non-transitive" because the attribute is only relevant between two adjacent autonomous systems (ASes). When a BGP speaker receives an update with a MED attribute from an external BGP (eBGP) peer, it uses it for its path selection process but does not propagate the attribute to any other AS. This prevents the preferences of one AS from influencing routing decisions in other, unrelated ASes.

A. Optional transitive attribute. Incorrect. MED is non-transitive. It is designed to be advertised to a neighboring AS only and must not be propagated further.
C. Well-known mandatory attribute. Incorrect. Well-known mandatory attributes, such as ASPATH, must be recognized by all BGP implementations and included in every UPDATE message. MED is neither.
D. Well-known discretionary attribute. Incorrect. MED is an optional attribute, not a well-known one. Well-known attributes must be recognized by all BGP implementations, which is not a requirement for MED.

1. RFC 4271: A Border Gateway Protocol 4 (BGP-4), Y. Rekhter, T. Li, S. Hares, January 2006.

Section 5.1.4, MULTIEXITDISC: "This is an optional, non-transitive BGP attribute that is intended to be used on external (inter-AS) links to discriminate among multiple exit or entry points to the same neighboring AS."

2. Nokia Service Routing Architect (SRA) Self-Study Guide: BGP, Nokia, 2018.

Module 3: BGP Path Attributes, Section on Attribute Types: The guide explicitly categorizes BGP attributes. In the table of attributes, MULTIEXITDISC (MED) is listed with the type "Optional Non-transitive (Type Code 4)".

3. Computer Networking: A Top-Down Approach (7th Edition), J. Kurose, K. Ross, Pearson, 2017.

Chapter 5.3.3, BGP Path Attributes and BGP Routes: The text describes the MED attribute as being used to influence a neighbor's forwarding decision and notes that it is not propagated beyond the receiving AS, which is the definition of a non-transitive attribute. It is also classified as optional.