Need help making sure I understand some IP Routing stuff

jsb515

I'm reading Todd Lammle's book on CCENT and studying IP Routing since I found this to be a weak area for me. Chapter 6 on page 385 his talking about stub network is networking with only one exit path out of the network and then makes a reference to page 355 saying Router 1, 2, and the 871w are stub networks. But on Router 1 I see s0/0/0 and s0/0/1 being used so wouldn't that be two paths out and not one? on Router 2 I need S0/2/0 being used and on the 871W I see Fa0/1 is the only route being connected to Router 3.

xXErebuS

jsb515 wrote: »

I'm reading Todd Lammle's book on CCENT and studying IP Routing since I found this to be a weak area for me. Chapter 6 on page 385 his talking about stub network is networking with only one exit path out of the network and then makes a reference to page 355 saying Router 1, 2, and the 871w are stub networks. But on Router 1 I see s0/0/0 and s0/0/1 being used so wouldn't that be two paths out and not one? on Router 2 I need S0/2/0 being used and on the 871W I see Fa0/1 is the only route being connected to Router 3.

There are different types of stub areas; and that it only has one exit path is a bad explanation and a wrong explanation. I prefer to think of a stub area in lamens terms as an area that has less routes to reduce overhead in that area.

boredgamelad

I just looked up the diagram and I understand your confusion. But "paths out" in this case doesn't refer to the physical number of connections, but the logical. Physically speaking, yes, packets could theoretically travel over s0/0/0 or s0/0/1. But in order to reach other networks, all packets are going to go through the Corp router.

Look at it this way: You are in Los Angeles and you want to fly to New York City. Three planes are leaving from LAX and have a layover in Chicago. No matter which plane you get on, your path from LAX to NYC is going to look like this:

LAX > CHI > NYC

(Of course this isn't a perfect analogy since in a real airport you've got hundreds of planes leaving to hundreds of destinations. If it works better for you, imagine instead of LAX that you're leaving from a small town regional airport in rural Illinois.)

jsb515

Todd Lammle emailed me back and answered my question. Here is his response. Thanks also for helping with your responses.

This is true because even though R1 and R2 have multiple paths out of the router towards the Corp, they are not separate links to different network, they all go to the Corp network – one way in or out – which is the definition of stub. Just because you load balance or have redundant links, the LAN’s behind each network is a stub.
Take care!
Todd