IP Routing

Common View of the Telco Network

Common View of the IP Network (Layer 3)

What This Course Is About

Forwarding vs. Routing

What You Should Take Away

Anthropology of Routing

Routing can happen at any level

IP is a Network Layer Protocol

All Hail the IP Datagram!

IP Hour Glass

Routers Talking to Routers

Architecture of Dynamic Routing

"Topology information is flooded within..."

The Gang of Four

Autonomous Routing Domains (ARDs)

Autonomous System (AS) Numbers

Autonomous Routing Domains Don’t Always Need BGP or an ASN

ASNs Can Be “Shared” (RFC 2270)


How Many ASNs?

How many prefixes?

A Bit of OGI’s AS Neighborhood


Partial View of cs.wisc.edu Neighborhood

Policy : Transit vs. Nontransit

Customers and Providers

The “Peering” Relationship

Peering Provides Shortcuts

Peering Wars

Policy-Based vs. Distance-Based Routing?

Why not minimize “AS hop count”?


Four Types of BGP Messages

BGP Attributes

Attributes are Used to Select Best Routes

BGP Route Processing

Route Selection Summary

ASPATH Attribute

Shorter Doesn’t Always Mean Shorter

BGP Routing Tables

AS Graphs Depend on Point of View

AS Graphs Can Be Fun

AS Graphs Do Not Show “Topology”!

So Many Choices


Implementing Backup Links with Local Preference (Outbound Traffic)

Multihomed Backups
(Outbound Traffic)

Shedding Inbound Traffic with ASPATH Prepending

… But Padding Does Not Always Work

COMMUNITY Attribute to the Rescue!

Don’t celebrate just yet…

Customer installs a “backup link” …

Disaster Strikes!

The primary link is repaired, and something odd occurs…

WAIT! It Gets Better…


What the heck is going on?

Are we too complacent?

Worrisome trends…

Let’s look at “te” communities…

Some AS286 Communities

Need “Semantics of Interdomain Routing”