Comprehensive and Detailed Step-by-Step Explanation:
1. BGP Attributes Overview:
BGP uses attributes to describe the properties of routes. These attributes are classified into the following categories:
Well-Known Mandatory: Attributes that must be recognized and included in every BGP update. Examples: AS_PATH, ORIGIN, NEXT_HOP.
Well-Known Discretionary: Attributes that must be recognized but are not required in every BGP update. Examples: LOCAL_PREF, ATOMIC_AGGREGATE.
Optional Transitive: Attributes that may or may not be recognized, but must be passed along to other BGP peers. Examples: COMMUNITY.
Optional Non-Transitive: Attributes that may or may not be recognized, and are not passed along to other BGP peers. Examples: MED (Multi-Exit Discriminator).
2. Analysis of Each Option:
Option A: Atomic_Aggregate
Correct.
This is a well-known discretionary attribute. It indicates that the advertised route is an aggregate route that summarizes multiple specific routes.
Option B: Local-Preference
Correct.
This is a well-known discretionary attribute. It is used within an AS to prioritize routes, with higher values being preferred.
Option C: Community
Incorrect.
The COMMUNITY attribute is an optional transitive attribute. It is used to tag routes with specific labels to apply routing policies.
Option D: MED (Multi-Exit Discriminator)
Incorrect.
The MED attribute is an optional non-transitive attribute. It is used to influence the choice of the preferred path when there are multiple entry points into an AS.
3. Summary:
Well-Known Discretionary Attributes: Atomic_Aggregate and Local-Preference.
Huawei HCIA-Datacom Study Guide, Chapter "BGP Route Attributes."
RFC 4271: Border Gateway Protocol 4 (BGP-4).
