loops and other problems with default routing

[maiaeutic]

I am somewhat perplexed by the following quote on page 374 of the 5th edition CCNA guide:
You should only use default routing on stub networks -- those with only one exit path out of the network....
If you tried to put a default route on router R3, packets wouldn't be forwarded to the correct networks because they have more than one interface routing to the other routers. You can easily create loops with default routing, so be careful!"

What I am not clear on, is 1. why you can create loops with default routing on non-stub networks, and 2. why the data would fail to meet the intended destination. I don't understand why it matters that there are multiple interfaces, since you are specifying the next hop as part of the default routing command.
How could the packet go out the wrong interface, or even fail to meet a network destination several hops away?

I similiarly fail to grasp how a loop could be created due to default routing. If someone could help me with a conceptualization or example for how each of these scenarios could occur on router R3 in the example in the CCNA guide on pg. 341 in 5th edition (or using some other conceotualization), it would be appreciated.

[lammle]

I created a PPT (office 2007) with an example that I see at customers ALL the time! Although this seems obivous when looking at a drawing, it is so easy to do in the real world, and it happens all the time.
Cheers!
Todd Lammle

[lammle]

I guess it wouldn't take PPT, so I made an acrobat and attached it here...
hope this helps.
Todd

[maiaeutic]

Todd,

Thanks for providing that example, I can see what is going on in that case.
What I still don't understand, is how such a loop could be created if only a single router with mulitple interfaces (such as R3) was configured with a default route (as long as the other routers in the network were configured properly of course). But perhaps that's not what you were asserting in the example.
Also, I am still unclear on the meaning of the statement (pg. 374 5th edition) "packets wouldn't be forwarded to the correct networks because they have more than one interface routing to the other routers." Referring to the example with R1, R2, R3, the Corp router and the 871W. Again, my confusion comes from the fact that you specify the next hop in the default routing command, how would it matter that there is another interface on the router, which necessarily leads to an IP address on a different subnet on the neighboring router.

[lammle]

The key sentence is "if the other routers are all configured correctly".
That is the problem, you have other admins that can and will configure the other router incorrectly. I see it all the time.
My paragraph there assumes multiple admins where you are assuming one admin. I see where I didn't make this clear enough now.
thanks!
Todd

[maiaeutic]

Ok Todd, thanks for the clarification.