ATOMIC_AGGREGATE vs AGGREGATOR
在剛開始接觸BGP時，常常會因為英文單字的相似而被混淆，我想是大家共同的感覺，因此我將兩個很接近的屬性列出來加以比較給大家介紹一下。
ATOMIC_AGGREGATE
ATOMIC_AGGREGATE is a well-known discretionary attribute.
When a BGP speaker aggregates several routes for the purpose of advertisement to a particular peer, the AS_PATH of the aggregated route normally includes an AS_SET formed from the set of ASes from which the aggregate was formed. In many cases, the network administrator can determine if the aggregate can safely be advertised without the AS_SET, and without forming route loops.
If an aggregate excludes at least some of the AS numbers present in the AS_PATH of the routes that are aggregated as a result of dropping the AS_SET, the aggregated route, when advertised to the peer, SHOULD include the ATOMIC_AGGREGATE attribute.
ATOMIC_AGGREGATE這個屬性一旦存在(並無內含任何的數值，僅僅為一個指標意義而已)就代表這筆路由中可能發生了AS-Path Information Lost的情況，比如將AS123, AS321的路由合併之後只留下|123|的AS-Path；因此ATOMIC_AGGREGATE跟BGP command中的AS-SET這個參數互斥，只要設定了AS-SET這個參數，AS123, AS321的路由合併之後，AS-Path會變成|{123, 321}|。
AGGREGATOR
AGGREGATOR is an optional transitive attribute, which MAY be included in updates that are formed by aggregation (see Section 9.2.2.2). A BGP speaker that performs route aggregation MAY add the AGGREGATOR attribute, which SHALL contain its own AS number and IP address. The IP address SHOULD be the same as the BGP Identifier of the speaker.
AGGREGATOR這個參數則是包含了路由合併之後的資訊，如AS Number及IP位址。因此有別於所謂的ATOMIC_AGGREGATE。
PS:資料來源 - RFC4271
ATOMIC_AGGREGATE
ATOMIC_AGGREGATE is a well-known discretionary attribute.
When a BGP speaker aggregates several routes for the purpose of advertisement to a particular peer, the AS_PATH of the aggregated route normally includes an AS_SET formed from the set of ASes from which the aggregate was formed. In many cases, the network administrator can determine if the aggregate can safely be advertised without the AS_SET, and without forming route loops.
If an aggregate excludes at least some of the AS numbers present in the AS_PATH of the routes that are aggregated as a result of dropping the AS_SET, the aggregated route, when advertised to the peer, SHOULD include the ATOMIC_AGGREGATE attribute.
ATOMIC_AGGREGATE這個屬性一旦存在(並無內含任何的數值，僅僅為一個指標意義而已)就代表這筆路由中可能發生了AS-Path Information Lost的情況，比如將AS123, AS321的路由合併之後只留下|123|的AS-Path；因此ATOMIC_AGGREGATE跟BGP command中的AS-SET這個參數互斥，只要設定了AS-SET這個參數，AS123, AS321的路由合併之後，AS-Path會變成|{123, 321}|。
AGGREGATOR
AGGREGATOR is an optional transitive attribute, which MAY be included in updates that are formed by aggregation (see Section 9.2.2.2). A BGP speaker that performs route aggregation MAY add the AGGREGATOR attribute, which SHALL contain its own AS number and IP address. The IP address SHOULD be the same as the BGP Identifier of the speaker.
AGGREGATOR這個參數則是包含了路由合併之後的資訊，如AS Number及IP位址。因此有別於所謂的ATOMIC_AGGREGATE。
PS:資料來源 - RFC4271
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