BGP
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定義
ボーダーゲートウェイプロトコル（Border Gateway Protocol）。自律システム間で到達可能性情報を交換するルーティングプロトコルであり、インターネット上でデータがどのように転送されるかを事実上決定します。「インターネットの郵便サービス」とも呼ばれています。
How BGP Works
BGP is a path-vector protocol — rather than tracking link metrics, each router advertises complete AS paths to destinations. When a router receives multiple advertisements for the same prefix, it selects the best path using a deterministic decision process: local preference, AS path length, origin type, MED, and several tiebreakers.
BGP sessions between routers are called peering sessions. Two routers that peer with each other exchange routing information over a persistent TCP connection on port 179. iBGP sessions connect routers within the same Autonomous System (AS)A large network or group of networks under a single administrative policy, identified by a unique Autonomous System Number (ASN). The internet is composed of tens of thousands of interconnected autonomous systems., while eBGP sessions connect routers in different autonomous systems.
BGP and Internet Stability
BGP is both the backbone and an occasional liability of the global internet. The protocol has no built-in authentication, making it vulnerable to route hijacking — where one AS maliciously or accidentally advertises prefixes belonging to another. RPKI (Resource Public Key Infrastructure) is the emerging standard to cryptographically validate route origins and prevent hijacking.
A BGP route leak in 2010 caused a significant portion of global internet traffic to briefly route through China. In 2019, a misconfigured small ISP caused widespread outages affecting major CDNs and cloud providers.
Checking BGP Routes
Use WHOIS Lookup to look up the ASN and BGP prefix associated with any IP address. For deeper analysis, public BGP looking glass servers and route collectors like RIPE RIS allow you to inspect live BGP tables from vantage points worldwide. Understanding Routing TableA data structure stored in a router that lists known network destinations and the next hop or interface for reaching each one. Routing tables are populated by static configuration or dynamic routing protocols like BGP and OSPF. structure helps interpret BGP path selection decisions.