IP Routing 101

In this section, we will answer three fundamental questions:

•What is a router?

•How does a router decide where to forward an IP packet?

•How does a router learn about remote networks?

We will begin with the layered model and then follow an IP packet hop by hop.

To tie what we have learned so far together, let’s examine something we used to call the routing fish problem.

In Part 1, we learned how a router forwards packets.

Now we will look at:

•How routers learn about remote networks

•What routing protocols actually do

•How different routing protocols approach the problem

•How routers choose between competing route sources

•How routing information becomes forwarding information

This optional lecture provides a quick refresher on IPv4 subnetting and CIDR notation. Throughout the course, you have seen familiar prefixes such as /8, /16, and /24 in the slides, examples, and demonstrations. If you are not completely comfortable with what those prefix lengths mean, how they relate to subnet masks, or how to determine the network and host portions of an address, this lecture will help reinforce those fundamentals. A solid understanding of subnetting makes routing tables, route advertisements, longest-prefix matching, and the behavior of routing protocols much easier to follow.

In Part 1, we will use RIP to explore:

• Distance-vector routing

• Hop-count metrics

• Periodic routing updates

• Route convergence

• Loop-prevention techniques

• Basic RIP configuration and verification

In this demonstration, we will:

•Enable RIPv2

•Select participating interfaces/networks

•Apply passive-interface where appropriate

•Verify learned routes

•Observe hop-count metrics

•Watch route changes after a failure

•Inspect RIP updates in Wireshark

In this part, we will explore:

•How OSPF learns network topology

•How OSPF calculates path cost

•How areas improve scalability

•How routers exchange link-state information

•How OSPF reacts to network changes

•Basic configuration and verification

This course focuses on OSPF for IPv4.

In this section, we will explore:

•What BGP is and where it is used

•How Autonomous Systems exchange routes

•How BGP selects paths

•How BGP prevents routing loops

•The difference between eBGP and iBGP

•How policy influences routing decisions

•Basic BGP configuration and verification

BGP is the routing protocol that connects autonomous systems across the Internet.

In this section we will learn to:

•Follow a structured troubleshooting process

•Verify physical and interface connectivity

•Examine the routing control plane

•Verify routing protocol operation

•Confirm routes in the RIB and FIB

•Test the forwarding plane

•Use evidence to isolate the actual failure

•Apply the complete process in a troubleshooting demonstration

Congratulations! You made it all the way! We hope you enjoyed the class!

We covered a lot of material in this course, but the real goal was not to memorize routing commands.

The goal was to understand how routing works well enough that the commands make sense.

You should now be able to look at a routed network and reason through what the router is doing:

• what routes it knows,

• how those routes were learned,

• why one route was selected over another,

• how that decision is translated into forwarding,

• and what to check when traffic does not reach the destination.

RIP gave us a simple distance-vector model.

OSPF showed us link-state routing and topology-based path calculation.

BGP showed us policy-based routing between autonomous systems.

And the final section brought those ideas together into a troubleshooting workflow.