Why does the default gateway matter so much while trying to access other network resources?

Before installing MPLS, we in the branch office used to rely on ASA site-to-site VPN to access HQ network file resources. The branch has its own LAN 192.168.1.x while the HQ 10.10.10.x. The ASA's IP 192.168.1.14 used to be the only default gateway. Now just this morning we installed the MPLS router and its IP 192.168.1.3 becomes the new default gateway with the following routing:

All traffic to 10.10.10.x will be routed via 192.168.1.3
All others (internet) will be routed to ASA 192.168.1.14

Now since I was not able to change the default gateway in time for all clients in the Branch from .14 to .3 and as the result the clients who still used .14 as the default gateway became not able to access the HQ's network resources -- they cannot open network shared file folders even they can ping the server without problem.

This is so confusing... why we can ping the server but cannot open its folder? Just because using the different gateway. Don't forget the site-to-site VPN via the ASA is still functional. Why not working? Please help me understand? Did I miss anything? Thanks.
CastlewoodAsked:
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Bryant SchaperCommented:
So the short answer is that your routing table is wrong. You could use either as a default gateway but they need to know where the other networks are.

Think if the the default gateway as the location to send traffic when the computer doesn't know where to send it. In small offices this is normally Internet traffic so the router has the Internet connection.  

Your MPLS may also broadcast a default route. You can ignore the route and have carrier not send it. But you also need to have both the Asa and the MPLS router know where to send traffic. So in your case if network a is on the MPLS then the Asa needs a route that says a is on the MPLS. The MPLS needs a route that says send 0.0.0.0/0 traffic to the Asa.

Hope that clears it up a bit. But the problem you have is that while you can ping the other device is still not sure how communicate back to you. It's default gateway needs a route to the other network over the MPLS. You can do this on the OS as well, but it is messier.
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JustInCaseCommented:
Default gateway is - as the name says - gateway, gate - portal.
:)
Network traffic in the same network range is accessed directly one host asks for MAC address of other host and then they can communicate. If target IP address is not part of host network, host need some address that is able to forward traffic for other networks (that is default gateway). All traffic that is not in the same IP range as sender IP is sent to default gateway, and from there traffic is routed.
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CastlewoodAuthor Commented:
"So the short answer is that your routing table is wrong. You could use either as a default gateway but they need to know where the other networks are. "

So the wrong routing table is in the HQ? since the traffic goes out via the ASA in Branch reaching to HQ but doesn't know how to get back to the Branch.
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JustInCaseCommented:
So the wrong routing table is in the HQ? since the traffic goes out via the ASA in Branch reaching to HQ but doesn't know how to get back to the Branch.
In that case ping would not work. So, I guess routing is not issue here.

One typical situation where this is happening that you can ping, and cannot do almost anything is problem with MTU. Packets are too big to travel in one piece and virtual reassembly is not allowed.
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