AS Autonomous System

Introduction

Administrative Distance (AD) is a trust score (0-255) that routers use to pick the best path to a network. Lower AD = More reliable route!

Default AD Values (Simplified):

  • Connected Interface (Directly linked): 0
  • Static Route (Manually set): 1
  • External BGP (tech4gsm.com): 20
  • EIGRP: 90
  • OSPF: 110
  • RIP: 120
  • External EIGRP: 170
  • Internal BGP: 200
  • Unknown Route: 255 (Never used)

Example:
If a router gets the same route via OSPF (AD 110) and EIGRP (AD 90), EIGRP’s route wins because of its lower AD.

How Routers Choose the Best Path:

  1. Check AD First: Lower AD = Better.
  2. Same AD? Compare Metrics: Lower metric (like bandwidth/cost) wins.
  3. Same AD + Metric? Load Balancing: Traffic splits across routes (equal or unequal).

What is an Autonomous System (AS)?
An Autonomous System (AS) is a network group managed by one organization, using a 16-bit unique ID (1-65535).

Public AS Numbers (1-64511):

  • Used for multi-homing (connecting to multiple ISPs).
  • Example: A global business needing unique BGP routing.

Private AS Numbers (64512-65535):

  • Used for single-ISP connections (like small businesses).
  • Saves public AS numbers.

Who Assigns AS Numbers?

  • IANA (Internet Assigned Numbers Authority) allocates blocks to regional registries (e.g., ARIN, RIPE).
  • Registries assign AS numbers to organizations.

FAQs (2025 Updates)

Q1: Why is AD important?
A: AD prevents routing loops by ensuring only the most trusted protocol’s route is used.

Q2: Can I change the default AD?
A: Yes! But tweak carefully—wrong values can break routing.

Q3: What’s the difference between Public and Private AS?
A: Public AS is for multi-ISP setups; Private is for single-ISP networks.

Q4: Will we run out of AS numbers?
A: As of 2025, over 50,000 AS numbers are active, but IANA manages distribution to avoid exhaustion.

Q5: Is BGP better than OSPF?
A: BGP (AD 20/200) excels for large networks (like the internet), while OSPF (AD 110) is faster for smaller networks.

Key Takeaways:

  • AD decides route trustworthiness; lower is better.
  • AS simplifies large-network management with a unique ID.
  • Public vs. Private AS depends on your ISP setup.

Stay updated with 2025 routing trends for efficient network management! 🌐🔧