BGP attributes are categorized into four distinct types based on how they are handled by a BGP speaker:Well-known mandatory,Well-known discretionary,Optional transitive, andOptional non-transitive. Understanding these categories is essential for traffic engineering and ensuring consistent policy across an Autonomous System.

According to Juniper Networks technical documentation, theCommunityattribute is classified as anoptional transitiveattribute. The term "optional" implies that a BGP implementation is not required to support or recognize the attribute. However, because it is "transitive," if a Juniper router receives an update containing a community tag that it does not recognize or has no specific policy for, it must accept the attribute and pass it along to other BGP peers unchanged. This ensures that community-based policies can be signaled across intermediate ASes that may not be configured to act upon those specific tags.

In contrast:

Origin (Option A)andAS Path (Option B)arewell-known mandatoryattributes. Every BGP update must include these, and every BGP-compliant router must recognize them.

MED (Option D)(Multi-Exit Discriminator) is anoptional non-transitiveattribute. If a router receives a MED and advertises that route to an EBGP peer, the MED is typically stripped away (unless specific configurations like path-selection cisco-non-deterministic are used), as it is intended only to influence the immediate neighboring AS.

The Community attribute (defined in RFC 1997) is a powerful tool in Junos OS, often used for tagging routes to trigger specific routing policies, such as setting local preference or identifying the geographic origin of a prefix. By being transitive, it allows for sophisticated administrative control across complex multi-provider environments.