Network Control Plane

Routing Across the Internet

Wednesday, March 18, 2026

Discussion

• These smart beds began roasting their owners
  • What went wrong?
  • Works as intended or design flaw?
  • What could we, the consumers, do differently?
• Read more…

Lesson Objectives

1. Autonomous System and its role in network routing; understand boundaries and interior routing
2. Intra-AS: OSPF’s use of LSAs for scalable routing
3. Inter-AS: BGP as the primary protocol; exchanging routing information between different AS
4. Different types of BGP peering relationships, eBGP and iBGP, and their significance in inter-AS communication.

Textbook Readings

• \(5.3\) Intra-AS Routing in the Internet: OSPF (Open Shortest Path First)
• \(5.4\) Routing Among the ISPs: BGP

• Scale + autonomy: billions of endpoints and independent administrations.
• “Shortest path” (OSPF) vs policy-chosen path (BGP)
• OSPF area scope of LSAs & role of ABR vs ASBR
• eBGP vs iBGP: who peers with whom?
• “Hot potato”: locally optimal ≠ globally optimal
• We’ll use small config/packet snippets to ground concepts 🧩
• OSPF ≈ performance within an admin domain
• BGP ≈ policy across domains

Recap

Link State Routing

• Routers flood LSAs
• Global knowledge of network topology
• Dijkstra’s algorithm to compute shortest paths

Distance Vector

• Share information with neighbors only
• Iterative updates!
• Loops possible until convergence

• Which layer?
• What is the main function of this layer?
• What two planes exist at this layer? How are they different?
• What protocols did we learn so far at this layer?

Autonomous Systems (AS)

Why Autonomous Systems?

• How big is the Internet? 🌍
• What if every router shared full routing state? 🤯
• Could we group certain routers?
• And then share only reachability between groups?
• Autonomous Systems (AS) to the rescue!

• Cities = ASes; local roads (intra-AS); highways (inter-AS)
• Cities: local control, highways: agreed-upon rules.
• Highways: only share which cities are reachable, not every street.
• can’t store all destinations; exchanging full tables would swamp links.
• administrative control: each AS sets its own policies.

What is an AS?

• A network (or set) under single administrative control 🏢
• Clear borders at edge routers; has an AS number (ASN)
• Goals: scalability, policy independence, fault containment
• What is your current AS?
• Which AS is the biggest? How many addresses does it cover?

• If students ask: 32-bit ASNs exist; not needed for this lesson’s depth.

Intra-AS vs Inter-AS Responsibilities

• Intra-AS: pick efficient internal paths; optimize performance/availability
• Inter-AS: exchange reachability & enforce policy/economics 💸
• Different metrics, objectives, and protocols by design

• Plant the seed: intra-AS uses cost-like metrics; inter-AS uses attributes & policy.

Intra-AS Routing Protocols

Intra-AS Protocols

• RIP (Distance Vector), OSPF (Link State), IS-IS (similar LS)
• EIGRP (DV-based; historically proprietary)
• OSPF is highly scalable, feature-rich

• Quick taxonomy; don’t deep-dive RIP/EIGRP—just context.

OSPF Core Mechanics

• Flood link-state info (LSAs) to build a shared topology map
• Run Dijkstra to compute shortest paths → forwarding table
• Supports message authentication 🔐

• Stress “same view of the area’s topology” → consistent shortest paths.

OSPF Hierarchy & Areas

• Area 0 (backbone) connects all other areas
• Internal routers: entirely inside one area
• ABR: between areas; summarizes & advertises to backbone
• ASBR: connects to other ASes; injects external routes
• Flooding stays within an area; summaries cross areas

• Key mental model: full knowledge of own area; only “direction/cost” to others.

Inter-AS Routing Protocols

What protocols exist?

• EGP (Exterior Gateway Protocol)
• IDRP (Inter-Domain Routing Protocol)
• NIRA (Next Internet Routing Architecture)
• RIFT (Routing in Fat Trees)
• LISP (Locator/ID Separation Protocol)
• BGP (Border Gateway Protocol)

Why BGP?

• Path-vector: “Here I am, who I can reach, and how” 🧭
• Prioritizes policy & business relationships over pure performance
• The only inter-AS routing protocol in practice

BGP Basics

• Peers exchange reachability info (prefixes) + attributes
• Uses TCP (port 179) for reliable transport

Textbook 5.2

eBGP

• Peers between different autonomous systems
• Only advertises the single best path per prefix to neighbors
• Path-vector: advertises prefixes (NLRI) with attributes (AS-PATH, NEXT-HOP)
• Loop prevention: reject routes that contain your own ASN in AS-PATH
• Policy-driven: import/export filters decide what to learn/advertise; avoid unintended transit
• Edge learns from neighbors via eBGP, then distributes internally via iBGP

iBGP

• distribute external routes internally
• spread exits - reachability!
• does not change AS-PATH

• Policy caution: advertising a route can make you a transit path.

What exactly is being advertised?

• Prefix (NLRI): destination network (CIDR)
• AS-PATH: sequence of ASNs traversed to reach the prefix
• NEXT-HOP: IP of the border router to reach the next AS

Textbook 5.2

• Keep center stage on AS-PATH & NEXT-HOP; mention others as policy levers.

• Connect to real-world economics: peering vs transit relationships.

Populating Forwarding Tables: Hot Potato

• Exit the AS at the closest egress router 🥔🔥
• Minimizes internal cost; may increase total path length
• Chosen via attribute ordering + local policy (e.g., LOCAL_PREF, AS-PATH)

Textbook 5.2

• Contrast with “cold potato” (carry further internally)—policy/business choice.

Intra-AS vs Inter-AS

• Policy: core in inter-AS; limited inside an AS
• Scale: OSPF hierarchy within AS; BGP summarization between ASes
• Performance: intra-AS optimizes metrics; inter-AS prioritizes policy

• Summarize all three axes: policy, scale, performance.

Review

OSPF LSAs always flood across the entire AS (all areas).

1. True
2. False

• Answer: False. Most LSAs are scoped to an area; ABRs summarize across areas.

Which statement about iBGP is correct?

1. It carries routes between different ASes
2. It modifies AS-PATH to add the local AS
3. It distributes external reachability within an AS
4. It replaces OSPF for intra-AS forwarding

• Answer: It distributes external reachability within an AS. iBGP does not change AS-PATH and does not replace OSPF’s role in forwarding.

The main motivation for “hot potato” routing is to:

1. Minimize end-to-end Internet latency
2. Minimize internal AS cost to the nearest exit
3. Satisfy legal requirements
4. Evenly balance load across all links

• Answer: Minimize internal AS cost to the nearest exit.

Match the protocol model:

1. OSPF — distance vector; BGP — link state
2. OSPF — link state; BGP — path vector
3. OSPF — flooding forbidden; BGP — flooding required
4. OSPF — policy-based; BGP — performance-based

• Answer: OSPF — link state; BGP — path vector.

Why might an AS choose not to advertise a learned route to a neighbor?

1. It breaks the TCP three-way handshake
2. Privacy/trust/policy concerns or to avoid becoming transit
3. iBGP forbids external advertisement
4. OSPF area mismatch

• Answer: Privacy/trust/policy concerns or to avoid becoming a transit path.

Which BGP attribute tells a router where to send packets to reach the next AS?

1. AS-PATH
2. NEXT-HOP
3. LOCAL_PREF
4. MED

• Answer: NEXT-HOP. AS-PATH lists ASNs; LOCAL_PREF/MED are policy signals.

Key Takeaways

• The Internet scales by grouping routers into ASes with local control
• OSPF (intra-AS): link-state + Dijkstra; LSAs scoped by area; ABR/ASBR roles; Area 0 backbone
• BGP (inter-AS): path-vector + policy; eBGP across ASes, iBGP inside an AS
• Key attributes: AS-PATH and NEXT-HOP; selective advertisement is normal
• Hot potato: exit at nearest egress to minimize internal cost—even if globally longer

Can BGP be hijacked?

• Pakistan Telecom (2008)
• Amazon Route53 (2018)
• KLAYswap (2022)
• Russia (2023)
• U.S. research & education regional network (2024)
• Cloudflare (2025)

• What went wrong?
• Was it accidental or malicious?
• What was the intended effect?
• What was the impact? Who suffered?
• Who benefited?