Need help with gateway of last resort

[c.ray.web]

I have been placed into a network role for a customer while my expertise is much more desktop / server oriented.  I do have my CCENT and have been working on the CCNA so I am not completely lost, but I cant figure this out and the deadline is approaching.

This network consists of one L2 switch & three L3 switches.  Only one L3 has routing enabled. The one that has routing enabled is directly connected to the other three switches (and all of the VLANs they have on them).  

The L3 switch that has routing enabled has a default route of 0.0.0.0 /0 that points to our firewall / internet.  The other two L3's and the one L2 have a default gateway set that points to the L3 with routing. 

How is it the L3 switch with routing enabled "knows" which switch to send packets destined for the internet too?  What am I missing that would help explain this?

And the whole point of the post is to help answer this question:

This organization is moving ISP's shortly, the new demarc will be in a different building (it will go into where one of the L3's is at).  I need to make sure I A) grasp the whole logical design here so that I can B) plan for this ISP switch and make sure the network will function on Monday when I am done.  

I can post configs if it helps.

[John Blakley]

How is it the L3 switch with routing enabled "knows" which switch to send packets destined for the internet too?

The default route is used when the local router doesn't have a route that it knows about in it's table. For example, let's say you have a user who wants to go to a website located at 5.5.5.5. The user doesn't know how to get to that subnet, so it sends it to its default gateway. The default gateway for the user (the L3 switch with routing enabled) doesn't know how to get to 5.5.5.0/24, so it sends to its default route (the firewall).

If the L3 is the only routing switch, the other L3 and L2 switches are in L2 mode I'm assuming. That means that the hosts are using the L3 switch (more than likely) for their default gateway for their respective vlan.

HTH,

John