Routing Backbone Slides Internet Architecture
Autonomous Systems & Inter-Domain Routing
CS 500: Advanced Networking
Who Owns the Internet?
Trace a packet from your local university lab to a server in Tokyo.
How many companies touch your data?
Does your ISP pay the next hop, or do they "hand off" for free?
Who is responsible if the packet disappears in the middle?
Global Reachability
The Autonomous System (AS)
"A set of routers under a single technical administration... using an interior gateway protocol and common metrics."
— RFC 1930
AS Numbers (ASN)
Unique 16-bit or 32-bit identifiers assigned by IANA/RIRs.
Internal Unity
A single routing policy for the entire network block.
External Protocol
Uses BGP to exchange reachability with other ASes.
The Two Layers of Routing
IGP
Interior Gateway Protocol
Routing inside an AS.
Examples:
OSPF, IS-IS, EIGRP, RIP
Focus: Shortest path, fast convergence, topological visibility.
EGP
Exterior Gateway Protocol
Routing between ASes.
The Standard:
BGP (v4)
Focus: Policy enforcement, business relationships, scalability.
The Provider Hierarchy
TIER 1
Global Backbones
Full reachability through peering alone. Buy transit from no one.
AT&T, Lumen, Telia, NTT
TIER 2
Regional Providers
Peer with some, buy transit from Tier 1s to reach the "whole" internet.
Comcast, Vodafone, Cox
TIER 3
Local Access
Primarily consumers of transit. Connect the "last mile" to end users.
Local WISPs, Small Telcos
AS Hierarchy Map Worksheet Backbone Mapping Lab
AS-HIE-01 // INTER-DOMAIN ARCHITECTURE
Student:
Date:
PART 01
Classifying the Backbone
Analyze the characteristics below. Determine if the description fits a Tier 1 , Tier 2 , or Tier 3 provider. Justify your choice based on the reachability and economic model.
Provider A: "GlobalReach Systems"
"We maintain a settlement-free peering policy with 12 other global networks. We do not purchase transit from any third party to reach any destination on the IPv4/IPv6 internet."
Tier 1
Tier 2
Tier 3
Analysis / Justification
Provider B: "MetroNet Regional"
"We peer locally at four major IXPs to reduce latency for our customers. However, to reach the global routing table, we maintain transit contracts with NTT and Lumen."
Tier 1
Tier 2
Tier 3
Analysis / Justification
PART 02
IGP vs. EGP Scenarios
Constraint: You are the lead architect for AS 64512. Your network spans three cities. You have two upstream providers (ISP X and ISP Y).
1. Protocol Selection
You need to ensure that routers within your New York office and London office share the fastest internal link state information. Which protocol type do you use, and why? Provide a specific example (e.g., OSPF).
2. Inter-Domain Handshake
You want to announce your IP prefix (192.0.2.0/24) to ISP X. Explain why an IGP cannot be used for this task. What specific information must be exchanged during the BGP session setup?
Confidential // Architectural Review Global Internet Routing // Lesson 1
AS Routing Teacher Guide Instructor Resource // L1
Inter-Domain Foundations
Lesson Plan & Keys
Learning Objectives
Define an Autonomous System (AS) and its role in global routing.
Distinguish between IGP (within an AS) and EGP (between ASes).
Map the hierarchical structure of the internet (Tier 1, 2, and 3 providers).
Explain the economic difference between peering and transit .
Pacing Guide
00-10m The Hook: Tracepath
10-30m Lecture: AS & Hierarchy
30-50m Activity: Hierarchy Map
50-60m Debrief & Discussion
Facilitation Notes
The "Tracepath" Hook
Start by running a traceroute or mtr to a global destination (e.g., www.u-tokyo.ac.jp). Ask students to identify where the "handoffs" occur. Look for hostname changes (e.g., from a university domain to cogentco.com or level3.net). This visualizes the jump between Autonomous Systems.
Key Conceptual Hurdles
Mistake #1
IGP vs EGP confusion
Students often think BGP can replace OSPF. Clarify that BGP doesn't care about the link state inside the AS; it only cares about the path between neighbors.
Mistake #2
Tier 1 definition
Clarify that Tier 1s are defined by not paying for transit . They reach the entire internet via peering. If you pay someone else for reachability, you are Tier 2 or 3.
Answer Key: Hierarchy Map Worksheet
PROVIDER A: GLOBALREACH SYSTEMS
Answer: Tier 1
Justification: The provider explicitly states they do not purchase transit from anyone. This "full reachability via peering" is the defining characteristic of a Tier 1 provider.
PROVIDER B: METRONET REGIONAL
Answer: Tier 2
Justification: They peer at IXPs (Tier 2 behavior) but still pay for transit (NTT/Lumen) to reach the global internet. Tier 3s usually have little to no peering and buy transit from local/regional Tier 2s.
Routing Logic Scenarios
1. Protocol Selection
"Use an IGP (OSPF/IS-IS) . IGPs are optimized for internal topology discovery and fast convergence within a single administrative domain. BGP is too slow for internal link fluctuations."
BGP Attributes Slides The Decision Process
BGP Attributes & Path Selection Logic
L2: Policy-Based Routing
> show ip bgp 1.1.1.0/24
> BGP routing table entry for 1.1.1.0/24, version 12
> Paths: (2 available, best #1, table default)
Why Choose a Slower Path?
In IP routing, we usually want the "shortest" path.
"BGP is not about distance. It's about business."
• Path A: 3 hops, costs $1,000/mo
• Path B: 12 hops, costs $0/mo (Peering)
A router will choose Path B every single time.
BGP TABLE v4.1.2
*> 10.0.0.0/8 [LP: 200] [AS: 123 456]
* 10.0.0.0/8 [LP: 100] [AS: 999]
_BEST PATH SELECTED
The "Ingredients" of a Route
AS_PATH
A list of all ASNs a route has passed through. Primary loop prevention mechanism.
Type: Well-known Mandatory
NEXT_HOP
The IP address of the next AS boundary router. If unreachable, the route is invalid.
Type: Well-known Mandatory
LOCAL_PREF
Indicates preference for an exit point. Not shared between ASes. High value wins.
Type: Well-known Discretionary
The Tie-Breaker Hierarchy
01 Weight (Cisco Specific)
02 Local Preference
03 Self-Originated
04 Shortest AS_PATH
05 Origin Type (I < E < ?)
06 MED (Multi-Exit Disc.)
07 eBGP over iBGP
Influence: Inbound vs. Outbound
LOCAL_PREF
Influences Outgoing traffic. You tell your own AS which way to leave.
"This is how I want to talk to you."
MED (Multi-Exit)
Influences Incoming traffic. You suggest to a neighbor which of your ports to use.
"This is how I want you to talk to me."
Path Selection Logic Lab Path Selection Workshop
BGP-LOG-02 // DETERMINISTIC DECISION LOGIC
OPERATOR:
TIMESTAMP:
Network Context
You are managing AS 65000 . You have received three different advertisements for the prefix 203.0.113.0/24. Your goal is to apply the BGP Best Path Algorithm to determine which path will be installed in the routing table.
Attribute Path A (ISP 1) Path B (ISP 2) Path C (IXP) Next Hop 1.1.1.1 (Valid) 2.2.2.2 (Valid) 3.3.3.3 (Valid) Local Pref 100 200 100 AS_PATH 10 20 30 40 50 60 Origin IGP (i) IGP (i) Incomplete (?) MED 0 10 50
Phase 1: Elimination
Compare Path A and Path B using the Local Preference attribute. Which path is preferred at this stage, and why?
Phase 2: The Final Winner
Compare the winning path from Phase 1 against Path C. Consider the AS_PATH length and Origin type. Which path is the final "Best Path"? Document the exact step in the decision process that determined the winner.
Phase 3: Policy Modification
Management wants to ensure traffic to 203.0.113.0/24 always flows through the IXP (Path C) because it is settlement-free (no cost).
Challenge: You are NOT allowed to touch Path C's configuration. You must only change attributes on Path B to make it less desirable than C. List two specific attribute changes you could make to Path B to achieve this.
1
2
LOGS: SUCCESSFUL STATUS: SIMULATION_ACTIVE
© INTERNET BACKBONE OPERATIONS 2026
Path Selection Key The Answer Matrix
L2 // PATH SELECTION LOGIC KEY
Instructor Key Only
BGP-KEY-002
PHASE 1: Elimination (A vs B)
Preferred Path: Path B
Reasoning: BGP compares Local Preference before it looks at AS_PATH. Path B has a Local Preference of 200 , while Path A has the default of 100 . In BGP logic, a higher Local Preference wins instantly. Path A is eliminated at Step 2 of the decision process.
PHASE 2: Final Selection (B vs C)
Final Winner: Path B
Detailed Logic: Step 2 determines the winner again. Even though Path C has a shorter AS_PATH (1 hop vs Path B's 2 hops), the Local Preference tie-breaker is checked before AS_PATH length.
PHASE 3: Policy Modification (ISP Intervention)
Required outcome: Make Path C the winner by only modifying Path B.
OPTION 01: LOCAL_PREF
Set Path B Local Pref to < 100
Setting Path B to 50 would cause Path C (100) to win at Step 2 of the decision process.
OPTION 02: AS_PATH PREPENDING
Prepend AS 65000 multiple times to Path B
If we first equalize Local Pref (both 100), we can make Path B's path length longer than C's by adding our own ASN to the path multiple times.
Pedagogical Tip
Many students intuitively think "Shortest Path First." Emphasize that BGP is designed to ignore distance if it conflicts with financial or administrative policy. This is why "Shortest AS_PATH" is so far down the list (Step 4).
Peering Economics Slides Market Dynamics
Peering Economics & Internet Exchange Points
L3: The Business of Connectivity
The Two Financial Pillars
Paid Transit
A customer-provider relationship. You pay to reach everywhere else.
Service Level Agreements (SLAs)
Guaranteed reachability
High recurring cost
Free Peering
A settlement-free agreement. You exchange traffic for free with a peer.
Zero monthly port fees
Lower latency (direct path)
No SLA / "Best Effort"
The Watering Hole: IXPs
Internet Exchange Points are physical locations where networks congregate to connect.
Layer 2 Fabric
Usually a large ethernet switch fabric (VLANs per peer).
Route Servers
Facilitate multi-lateral peering (connect once, peer with many).
Economic Incentive
"Keep local traffic local."
Why send a packet across the ocean to a Tier 1 provider just to have it come back to your neighbor across the street?
Peering Equilibrium
PEERING TRAFFIC (80%)
TRANSIT (20%)
"A healthy ISP aims for 80% peering. Every byte sent via peering is a byte not paid for on a transit bill."
Benefit
Save Millions in Opex
Experience
Lower Latency, Happier Users
The "Restrictive" Peering Policy
Some providers (Tier 1s) refuse to peer with others for free.
Ratio Analysis
If ISP A sends 10 Gbps and ISP B only sends 1 Gbps back, ISP B is essentially getting "free transit."
"You are too small."
"If I peer with you for free, I lose the transit revenue you would have paid me."
"The ratio is uneven."
"Your users download more than they upload. I am doing more work (carrying more bits) than you."
ISP Negotiation Game The Peering Table
Simulation // Transit vs. Peering Negotiation
IXP-SIM-03
Role Assignment
Tier 1 Global (Transit Seller)
Tier 2 Regional (The Buyer)
Content Network (The Source)
The Objective
Reduce your network's operating costs while maintaining 100% global reachability. You must negotiate with the other parties at this IXP to decide whether to form a Peering Agreement (free) or a Transit Agreement (paid).
Quarterly Network Stats
Metric Your Value Market Average Outbound Traffic Volume 450 Gbps 300 Gbps Inbound Traffic Volume 120 Gbps 300 Gbps Current Transit Cost (per Mbps) $0.85 $0.70
1. Traffic Ratio Analysis
Calculate your Traffic Ratio (Outbound vs Inbound). If you were a Tier 1 provider, would you peer with this network for free? Why or why not?
2. The Negotiation Script
Write a three-sentence pitch to a potential peer at this IXP. Your goal is to convince them that peering with you is mutually beneficial, despite any ratio imbalance.
3. The Final Agreement
After discussing with your peers, document your final connectivity strategy. Which networks did you peer with? Where are you still paying for transit? What is your estimated cost reduction?
Peered With:
Transit Partner:
Outcome Justification:
Simulation Module // Internal Use Only Lesson 3: Economics of the Backbone
IXP Facilitation Notes Instructor Resource // L3
Negotiating the Core
Simulation Guide
Simulation Context
This simulation places students in the shoes of network architects at an Internet Exchange Point (IXP). They must balance cost (transit fees) against reliability (peer count). The core conflict stems from the Traffic Ratio : who is doing the most work carrying the bits?
The Hidden Math
In the worksheet, the student has a 450 Gbps Outbound / 120 Gbps Inbound ratio (3.75:1). This is heavily asymmetric. Most Tier 1 networks require a ratio closer to 2:1 or even 1.5:1 to peer for free. Students will find that Tier 1s will likely reject their peering request.
Key Terms
Settlement-Free: Traffic exchange with no money changing hands.
Paid Peering: A middle ground where a small fee is paid, but it's cheaper than transit.
De-peering: The "nuclear option" of disconnecting a peer during a dispute.
Negotiation Facilitation
1
Divide the Class
Assign students to 3-person groups. Group roles: Entity A (Provider) , Entity B (The Student ISP) , IXP Mediator .
2
The "Pitch" Phase (15 mins)
Students must present their ratio-justification pitch. If Entity A (Tier 1) says "No", Entity B must offer something else: "We have high-value users," "We are the only way to reach this niche market," or "We will pay a small port fee."
3
Debrief Discussion
Ask the class: "Is the internet a public utility or a private business?" This gets into the heart of why peering is often contentious and political.
Deep Inquiry Questions
"What happens when a Tier 1 de-peers another Tier 1?"
Context: This has happened (e.g., Cogent vs. Level 3). It creates a partition in the internet where users on one ISP cannot reach users on the other.
"Why do Content Networks (Google/Netflix) peer so aggressively?"
Context: They want to be as close to the user as possible to reduce latency and improve video quality, saving transit costs for both themselves and the access ISP.
Routing Hijack Slides Systemic Fragility
BGP Hijacking & Route Leaks
L4: Vulnerabilities of the Backbone
Protocol by Handshake
BGP was designed in an era of implicit trust .
The Fundamental Flaw:
"By default, a router believes any route advertisement it receives from a neighbor is authentic."
There is no built-in mechanism in vanilla BGP to verify that an AS actually owns the IP space it is claiming.
MALICIOUS_ADVERTISEMENT.LOG
Received UPDATE from AS 666
Network: 8.8.8.0/24 (Google DNS)
AS_PATH: 666
!! ROUTE ACCEPTED !!
Threat Taxonomy
Prefix Hijacking
An AS advertises an IP prefix it does not own, either by accident or malice.
Techniques
Exact Match (Announcing /24)
More Specific (Announcing /25)
AS Path Impersonation
Route Leak
An AS incorrectly announces routes learned from one provider to another provider.
Impact
Traffic redirection through unfit links
Massive congestion/outages
Inadvertent Man-in-the-Middle
Anatomy of an Outage
2008 YouTube Hijack
Pakistan Telecom (AS 17557) attempted to block YouTube domestically using a "null route" for its subnet.
> YouTube Origin: 208.65.152.0/22
> PT Advertisement: 208.65.153.0/24
// Result: "More Specific" rule meant the whole world sent YouTube traffic to Pakistan.
2 HOURS
GLOBAL DARKNESS
PCCW (Upstream) accepted the route and propagated it globally in minutes.
Why does it matter?
Interception
Traffic is routed through a malicious AS, allowing for passive eavesdropping or active modification before being sent to the real destination.
Blackholing
Traffic is intentionally redirected to a "null" interface or a destination that cannot handle the load, causing a massive denial of service.
Impersonation
Malicious actors announce a bank's prefix and host a clone site, harvesting credentials from unsuspecting redirected users.
BGP Incident Forensics Case Study Incident Forensics
FILE: BGP-HIJACK-2008-YT
Priority Alpha
Event Timeline
18:47 UTC
Pakistan Telecom (AS 17557) starts announcing 208.65.153.0/24 via BGP.
18:50 UTC
PCCW (AS 3491) accepts the announcement and propagates it to global Tier 1s.
19:00 UTC
YouTube engineers detect total traffic loss for the 153.0/24 subnet.
Subject
AS 17557 (Pakistan Telecom)
Impact
Global redirection of traffic for Google/YouTube services.
1
The "Longest Prefix" Rule
YouTube was announcing 208.65.152.0/22. Pakistan Telecom announced 208.65.153.0/24. Explain why routers across the entire world chose the Pakistan route even though YouTube was also announcing the space.
2
The Role of the Upstream
PCCW was the upstream provider for Pakistan Telecom. Why did PCCW accept a route for a YouTube subnet from a regional ISP in Pakistan? What technical control (or lack thereof) allowed this to happen?
3
Counter-Announcements
To fix the issue, YouTube eventually started announcing 208.65.153.0/24 themselves. Discuss the "BGP war" that followed. Why did this work, and what are the risks of this "fight fire with fire" strategy?
VERIFIED BY BACKBONE SEC OPS
© INCIDENT RESPONSE UNIT 2026
Incident Analysis Teacher Guide Instructor Resource // L4
Incident Analysis Guide
Lesson 4 Keys
Pedagogical Goal
Students should transition from seeing routing as a "set and forget" technical task to understanding it as a high-stakes, trust-based environment where a single configuration error can have global geopolitical and economic consequences.
Case Study Answer Key
1 Longest Prefix Matching
"In BGP, the Longest Prefix Match always wins before any other attribute (like AS_PATH length) is even considered. Since a /24 is more specific than a /22, routers see the /24 as a more precise 'map' for that specific subnet. They will follow the /24 even if it leads to a much longer or slower path."
2 The Upstream Failure
"The upstream provider (PCCW) failed to implement Route Filtering . They should have had a 'prefix list' that only allowed Pakistan Telecom to announce IP space they actually owned. Because PCCW's filters were too permissive, they blindly accepted the malicious/incorrect route and used their own Tier 1 status to tell the rest of the world it was legitimate."
3 BGP Counter-Measures
"YouTube's counter-announcement of an identical /24 created a 'tie' in prefix length. This forced routers to use the next step in the BGP decision process (AS_PATH length). Since YouTube's path was shorter than the path through Pakistan, traffic eventually returned to the real servers. However, this 'de-aggregation' (breaking a /22 into /24s) bloats the global routing table, which is a major long-term scalability concern."
Instructor Warning: More Specifics
A common student error is thinking that BGP attributes (like MED or Local Pref) can stop a hijack. Clarify: Attributes are only compared if the prefix length is identical . If someone announces a more specific route, the attributes are ignored. This is the single most important lesson from the YouTube incident.
BGP Security Slides Authenticated Routing
Securing the Global Backbone
L5: RPKI, ROAs, and BGPsec
What is RPKI?
Resource Public Key Infrastructure
A specialized PKI framework designed to secure the internet's routing infrastructure by tying IP prefixes and ASNs to a cryptographic trust anchor .
The Core Idea:
"I don't trust you because you're my neighbor. I trust you because you have a valid digital certificate from the Regional Internet Registry (RIR)."
How it works
RIRs issue certificates to AS owners.
Owners create ROAs (Route Origin Authorizations).
Routers download and validate these records.
Route Origin Authorization (ROA)
Prefix
203.0.113.0/24
ASN
65001
Max Length
/24
Router Logic:
"If someone announces 203.0.113.0/24 AND they are NOT AS 65001, the route is Invalid. Drop the traffic."
This stops basic prefix hijacking cold.
Route Origin Validation (ROV)
VALID
A ROA exists and the announcement matches both the ASN and the prefix length.
INVALID
A ROA exists but the ASN or prefix length does not match. Highly suspicious.
NOT FOUND
No ROA exists for this prefix. Default behavior is to accept (for now).
Beyond RPKI: BGPsec
RPKI only secures the Origin (the end of the path).
What if a malicious actor adds themselves to the middle of a legitimate path?
AS_PATH Validation
BGPsec cryptographically signs the entire AS_PATH at every hop.
The "BGPsec" Challenge
Massive computational overhead to sign/verify every update.
Requires 100% global adoption to be fully effective.
RPKI Validation Lab Origin Validation Lab
MODULE: RPKI-SEC-05 // ROA IMPLEMENTATION
ENGINEER:
Step 01
The Cryptographic Registry
You are the administrator for AS 64500 . You have configured your RPKI validator to sync with the Regional Internet Registry. Below are the current ROAs (Route Origin Authorizations) in your local cache.
Trust Anchor Prefix Valid ASN Max Length ARIN-TA 192.0.2.0/24 AS 64500 /24 RIPE-TA 198.51.100.0/22 AS 64500 /24
Step 02
Route Validation Exercise
Analyze the incoming BGP UPDATE messages below. Determine the Validation State (Valid, Invalid, or Not Found) based on the ROAs above.
ADVERTISEMENT A
NETWORK: 192.0.2.0/24
ORIGIN: AS 64500
Validation State:
Valid
Invalid
Not Found
ADVERTISEMENT B
NETWORK: 198.51.100.0/25
ORIGIN: AS 64500
Validation State:
Valid
Invalid
Not Found
Scenario C Analysis
You receive an advertisement for 192.0.2.0/24 from AS 99999 .
1. What is the validation state?hijack or a route leak ? Justify your answer.
Implementation Challenge: Maximum Length
In Scenario B, the advertisement was for a /25. However, the ROA max-length was /24.
Explain why setting a Max Length that is too high (e.g., /32) can actually make your network more vulnerable to specific types of hijacking.
SECURE BACKBONE PROTOCOLS // 2026 L5: AUTHENTICATION LAB
Security Implementation Key Security Key Matrix
L5 // Origin Validation Reference
Confidential Instructor Key
SEC-KEY-005
Step 02: Validation Results
Scenario A
State: VALID
Reasoning: The announcement matches the ROA exactly. Prefix: 192.0.2.0/24 matches 192.0.2.0/24. ASN: 64500 matches 64500. Length: /24 matches max-length /24.
Scenario B
State: INVALID
Reasoning: While the ASN (64500) matches, the prefix length /25 is greater than the ROA's max-length of /24 . This is a "Max Length Invalid" error.
Scenario C Analysis
1. State: INVALID (ASN Mismatch) The ROA says only AS 64500 can announce this prefix, but AS 99999 is doing it.
2. Outcome: Traffic is Dropped If the policy is "Reject Invalids", the router will not install the route in the RIB, and the traffic will be blackholed (protecting the legitimate owner from interception).
3. Diagnosis: Likely a Hijack Route leaks usually involve the correct ASN but incorrect propagation. An ASN mismatch on a protected prefix is the hallmark of a prefix hijack.
Max Length Challenge
The "Forged-Origin" Problem: If you set a Max Length of /32, you allow anyone to announce any sub-portion of your prefix as long as they spoof your ASN . By setting a strict Max Length (e.g., /24), you force an attacker to announce at least a /24, making it harder for them to use "Longest Prefix Matching" to beat your own legitimate announcements.
Encourage students to check real-world RPKI status on tools like 'RPKI Dash' or 'Cloudflare Radar'. Ver: 1.0.1
