NetworkSec - Layer 3 - Autonomous System (AS)

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Autonomous System (AS)

AS autonomous system 自治系統

• a network under a single administrative control.
• Assumes the Internet is an arbitrarily interconnected set of AS’s.
  • local traffic: traffic that originates at or terminates on nodes within an AS
  • transit traffic: traffic that passes through an AS.

classify AS’s into 3 types:

• Stub AS:
  • an AS that has only a single connection to one other AS;
  • such an AS will only carry local traffic
  • (small corporation)
• Multihomed AS:
  • an AS that has connections to more than one other AS, but refuses to carry transit traffic
  • (large corporation at the top)
• Transit AS:
  • an AS that has connections to more than one other AS,
  • and is designed to carry both transit and local traffic
  • (backbone providers)
• 在每一個自治系統中，
  • 會有一個 Backbone Area 網路
    • 與外部路由網路互相連接的區段
    • 負責外部路由網路與自治系統內部其他網路的溝通，
    • 也是Transition Area。 
  • 除了Backbone Area外，自治系統中其他的網路區段就是 Non-Backbone Area。
    • 自治系統中所有的Non-Backbone Area都必須連接到Backbone Area上。
• 各個路由器扮演的角色 
  • Backbone Area 中:
    • Autonomous System Boundary Router (ASBR)
      • 自治系統邊界router.
      • 用來連接外部路由網域和自治系統。 
      • OSPF: Backbone Router.
      • IS-IS: L2 Router。
    • Normal Router
      • 提供不同Area之間的連接性。 
  • Non-Backbone Area:
    • Normal Router
      • 用於連接不同的Area，
      • 維護所連接的Area的Link-State路由資料庫，
      • 也負責轉送封包到其他的Area
    • OSPF: Area Border Routers(ABRs)
    • IS-IS: L1/L2 Routers。

自治系統編號 

• 分辨不同的自治系統
  • 透過 自治系統編號(ASN) 來區別。
  • 世界上多個組織可以使用自己私有的自治系統編號，以便於和他們的ISP業者之間透過BGP協定連線，
• 因此，自治系統編號又分為:
  • 私有的自治系統編號,
  • 公有並透過註冊之後的自治系統編號.
• 每個ISP業者必須登記公開至少一個ASN，用於BGP協定。
  • ASN極重要，是等一下用於路由協定設定時辨識自治系統的重要條件, 是識別各個網路的指標。 
• 而如何分辨公開註冊ASN和私有ASN呢？
  • IANA（Internet Assigned Number Authority) 使用16位元的長度來儲存自治系統編號（2的16次方).
    • 64512到65535之間的編號: 保留給私有自治系統所使用，
    • 1到64511之間的號碼: 公開註冊的自治系統編號

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Different Routing Protocols

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Interior vs Exterior Gateway Protocols

Routing protocols can categorized based on the scope of their operation:

• Interior Gateway Protocols (IGP)
  • operate within an autonomous system (AS)
  • used to exchange routing information.
    • Link-state routing protocols: OSPF, IS-IS
    • Distance-vector routing protocols: RIPv1, RIPv2, IGRP, EIGRP,
• Exterior Gateway Protocols (EGP)
  • operate between autonomous systems.
  • Used for exchanging routing information between autonomous systems.
    • BGP (Border Gateway Protocol) ,
    • path vector routing protocol,

example:

• R1 and R2 are in one AS (AS 65002).
• R3 and R4 are in one AS (AS 65003).
• router ISP1 is a router in a separate autonomous system (AS 65001), run by a service provider.
• EGP Border Gateway Protocol is used to exchange routing information between the service provider’s AS and each of the other autonomous systems.

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