Hello, and welcome back to our weekly tutorial video series. This video will be the first of several in which we will deal with IP routing. We begin with the reasons why we need routing, followed by configuring static routing on the Cisco routers in our lab setup.

New tutorial videos are posted every Monday, so keep checking back!

If you have any questions, or would like to suggest topics for future videos, please leave them in the comments section below.

Further reading:

CCNA Cheat-Sheet Study Guide: IP Routing: http://resources.intenseschool.com/ccna-cheat-sheet-study-guide-ip-routing/

Video Transcript:

Welcome back to this video series to prepare you for the configuration aspect of the CCNA R&S exam.

For the next couple of videos in this series, we will be discussing IP routing. In this video, we will begin with general routing concepts and look at static routing on Cisco routers.

Let’s use this diagram to explain the need for routing. In this diagram, HOST A and HOST B are on the same subnet of 192.168.10.0/24. HOST C is on another subnet of 192.168.20.0/24.

If HOST A wants to communicate with HOST B, it will send an ARP request for the IP address 192.168.10.2. This is so that it can get the layer 2 address for that IP address. This ARP request is broadcast so all hosts on that subnet should receive the request. On receiving the ARP request, HOST B replies telling HOST A it can be reached on a particular MAC address if its an Ethernet network; and so, they can now communicate.

Now, let’s say HOST A wants to communicate with HOST C. Even if it were to send an ARP request for HOST C’s IP address, that request will never get to HOST C because they are in different subnets i.e. broadcast domain is different. Therefore, there needs to be a device in between both subnets that can forward packets, that is route packets, between hosts in different subnets.

IP routing on Cisco routers can either be static or dynamic. Static means we manually define the destination along with how to get to that destination. Dynamic means we use a routing protocol to do the job for us. Smaller networks usually make use of static routing while the larger networks use dynamic routing protocols or a combination of both.

We can view the routing table on Cisco IOS routers using the “show ip route” command. By default, we will only see directly connected routes i.e. networks that the router’s interfaces are part of. That’s what C stands for.

Now let’s do a bit of an exercise. I will add another router to the topology and connect it to the ISP router. I will use an address of 192.0.2.0/30 between them so 192.0.2.1 for ISP router and 192.0.2.2 for BRANCH_RTR. Like I already explained, if we try to ping this new address from the OFFICE_RTR, it will fail because this router does not know how to reach that address i.e. it does not have a route for that destination in its routing table.

We can configure a static route on this router for that particular destination. We use the “ip route” command to configure static routes. So ip route 192.0.2.0 255.255.255.252 and then we need to specify the nexthop for that route or the interface through which to send traffic for that destination through. You can also use both.

The nexthop address refers to the address to which traffic for a particular destination should be sent. The device referenced by the nexthop address may own the destination route (like in our case) or that device may also point to another device for that destination.

We will use the nexthop address of 41.1.1.1 which is the IP address of the ISP router. Let’s view the routing table again. As you can see, a static route, depicted by S is now in the routing table.

By default, static routes have an administrative distance of 1 meaning that if there is a static route for a non-directly-connected network in the routing table, it will be preferred over other routes from other sources such as routing tables. We can configure the Administrative distance for static routes though especially when they are used as floating routes.

A simplified analogy of what happens when we try to ping 192.0.2.2 again from this router is this: it will consult its routing table for how to forward the traffic. It will discover that traffic should be forwarded to 41.1.1.1. If it has the MAC address of 41.1.1.1, it will send the ping packet to that address; else it will send an ARP request for 41.1.1.1.

When ISP router receives that packet, it checks its routing table for how to forward the traffic. It will discover that the destination IP is on one of its connected networks. If it has the MAC address of that IP address, it will forward it to the destination; else it will ARP for the layer 2 address for that IP address and then forward the packet when it receives the MAC address.

Notice that our ping was not successful. We will find out why in the next video.

This brings us to the end of the first video on IP routing. In this video, we have discussed the need for routing and also seen how to configure static routes. I hope you have found this video insightful and I look forward to the next video in the series.