IP Routing
Course Description
IP routing is how routers and multilayer switches forward IP packets from one network to another. This course covers what happens in between the routing protocols: how a router picks a route with administrative distance and longest prefix match, how static and floating static routes work, and what the IP, TCP, UDP, and ICMP headers look like. We then move on to the tools you use to control routing: route-maps, prefix-lists, distribute-lists, policy-based routing, and redistribution between routing protocols. We finish with GRE tunnels, VRFs, IP SLA, BFD, and other topics that make your routing more reliable.
Course Highlights
In this course you will learn:
- How routers forward IP packets, step by step, from host to host.
- How routers select a route with administrative distance, metrics, and longest prefix match.
- How to configure static routes, floating static routes, and router on a stick.
- What the fields in the IPv4, TCP, UDP, and ICMP headers are used for.
- How to filter and manipulate routes with route-maps, prefix-lists, and distribute-lists.
- How to overrule the routing table with policy-based routing (PBR).
- How to redistribute between routing protocols, and how to fix the sub-optimal routing and loops it can cause.
- How to use GRE tunnels, VRF lite, and LISP.
- How to make routing more reliable with IP SLA, BFD, and fast reroute.
Presented to you by instructor Rene Molenaar, CCIE #41726
Course at a glance
- 57 lessons in 8 units.
- Format: self-paced lessons with Cisco IOS configurations, packet captures, and video.
- Last updated: October 2026.
Requirements
- Basic knowledge of IP addressing and subnetting.
- For the advanced units: a good understanding of routing protocols like OSPF and EIGRP.
Course Schedule
What is IP routing?
IP routing is the forwarding of IP packets from one network to another. A switch forwards Ethernet frames based on MAC addresses within a network; a router looks at the destination IP address of a packet and decides which interface and next hop to use to get it closer to its destination. Every router on the path makes this decision on its own, based on its routing table. Along the way, the IP addresses in the packet stay the same, while the Ethernet header is replaced at every hop.
A router learns routes in three ways: directly connected networks, static routes that you configure, and dynamic routing protocols like OSPF, EIGRP, and BGP. When it learns the same prefix from multiple sources, administrative distance decides which one goes into the routing table. When a packet matches multiple routes in the routing table, the router uses the most specific one: the longest prefix match. On top of that, you can overrule or influence these decisions with tools like route-maps, policy-based routing, and redistribution.
Who needs to understand IP routing? Anyone who works with networks. Every host needs a default gateway, every router needs a routing table, and every routing protocol is built on the concepts in this course. When something doesn’t work, knowing how routers make their forwarding decisions is what helps you find the problem.
IP Routing Learning Roadmap
This roadmap follows the course units. Each topic has a short explanation and links to the lessons where we cover it in detail.
Routing Fundamentals
A host sends packets for other networks to its default gateway, and from there, routers forward them hop by hop. In this unit you’ll see IP routing step by step, and configure static routes and floating static routes. When a router learns the same prefix from multiple sources, it prefers the lowest administrative distance: 0 for directly connected, 1 for static, 90 for EIGRP, 110 for OSPF, and 120 for RIP. To forward a packet, it then uses the most specific matching route, the longest prefix match. You’ll also configure router on a stick to route between VLANs with a single router interface.
Classful Routing, Summarization and Special Cases
Classful routing protocols don’t send a subnet mask in their updates, and classful routing behaves differently when there is no exact match, which is why you should understand both before you work with older networks. Route summarization combines networks into a single route to keep routing tables small. This part of the unit also covers some special cases: proxy ARP, IP unnumbered interfaces that borrow an address from another interface, and directed broadcasts.
Packet Headers and Troubleshooting
To troubleshoot routing, it helps to know what is inside a packet. The IPv4 header is 20 to 60 bytes and carries the addresses, TTL, and fragmentation fields that routers look at. TCP and UDP add their own headers, and ICMP carries the messages that ping and traceroute use. A router can also send an ICMP redirect to point a host at a better next hop. When a ping fails, you troubleshoot it by pinging one step closer until it succeeds; other problems, like MTU issues or asymmetric routing, need a closer look.
Routing Policies and Policy-Based Routing
Routing protocols advertise everything by default, so you need tools to select and filter prefixes. Prefix-lists match on both the network address and the prefix length, distribute-lists apply them to a routing protocol, and route-maps add if-then logic to match and set values. Access-lists use wildcard bits to match ranges of addresses, and an offset-list adds to the metric of selected routes. Policy-based routing uses a route-map to forward selected traffic to a different next hop than the routing table would. uRPF uses the routing table the other way around: it checks the source address of packets to stop spoofing.
Redistribution
When a network runs more than one routing protocol, for example after a merger or during a migration, redistribution exchanges routes between them. Each protocol has its own metric, so you have to set a seed metric for the routes you redistribute. You’ll configure this between OSPF and RIP, EIGRP and RIP, and EIGRP and OSPF. With multiple redistribution points, routes can be redistributed back into the protocol they came from, which causes sub-optimal routing and routing loops. You’ll learn how to find these problems, which are always related to metrics or administrative distance, and how to prevent them with route tagging.
Tunneling and Virtualization
A GRE tunnel puts packets into other packets, so two routers can act as if they are directly connected and run a routing protocol over the tunnel. If a router learns the tunnel destination through the tunnel itself, you get a recursive routing error. A tunnel key lets a router tell apart multiple tunnels with the same source and destination. VRFs are like VLANs for routers: each VRF has its own routing table, and EVN makes VRF lite easier to configure. LISP separates the identity of a host (EID) from its location (RLOC).
Reliability and Performance Optimization
A static route stays in the routing table as long as the interface is up, even when the next hop is unreachable. IP SLA with object tracking fixes this for static routes and PBR. BFD detects link failures in milliseconds and tells routing protocols like OSPF, EIGRP, and BGP about it, and loop-free alternate fast reroute installs a backup path before it’s needed. You’ll configure EIGRP LFA and OSPF remote LFA. We also look at CEF polarization, and at OER and PfR, which select the best exit path based on the current delay, response time, and link utilization instead of static metrics.
Frequently Asked Questions
Inside networks, OSPF is the most common routing protocol, because it’s an open standard that every vendor supports. Between networks on the Internet, BGP is the only option. EIGRP is popular in Cisco networks, IS-IS in service provider backbones, and RIP is rarely used anymore. If you’re new to routing, start with static routes and OSPF.
A switch forwards Ethernet frames within a network, based on the destination MAC address. A router forwards IP packets between networks, based on the destination IP address. Switching happens at layer 2 of the OSI model and routing at layer 3; a multilayer switch can do both. Learn the difference between routers and switches.
The three types are static, default, and dynamic routing. With static routing, you configure each route by hand, and the router doesn’t adapt when the network changes. A default route is a special static route that matches every destination, often used to send traffic to the Internet. With dynamic routing, protocols like OSPF and EIGRP learn routes from neighbors and find a new path when a link fails; most networks use dynamic routing inside and a static default route toward the Internet. Learn how to configure a static route.
On Cisco routers, IP routing is enabled by default. On a multilayer switch, you enable it with the ip routing global configuration command so the switch can route between VLANs. On a Windows PC, ipconfig /all shows ‘IP Routing Enabled: No’, which is normal: your PC sends traffic for other networks to its default gateway. Learn what a default gateway is.
IP routing is the process of forwarding IP packets from one network to another. Each router looks at the destination IP address of a packet, finds the best match in its routing table, and forwards the packet to the next hop. This repeats on every router until the packet reaches its destination. Read how IP routing works step by step.
A routing table is the list of networks a router knows and how to reach them. Each entry has a prefix, the next hop or outgoing interface, and where the route came from: directly connected, static, or a routing protocol like OSPF. When a packet arrives, the router looks up its destination and uses the most specific match, which is called the longest prefix match. Learn how longest prefix match works.
Administrative distance is how a router decides which source to trust when it learns the same prefix from multiple routing protocols or static routes. The lower the value, the better: directly connected is 0, static is 1, EIGRP is 90, OSPF is 110, and RIP is 120. Learn about administrative distance.
It’s the command for a default route on Cisco routers. The network 0.0.0.0 with subnet mask 0.0.0.0 matches every IP address, so the router uses this route for any destination it doesn’t have a more specific route for. You add the next hop or outgoing interface at the end, for example the IP address of your ISP’s router. Learn how to configure a static route.