Troubleshooting internet routing issue

[gshamlian]

A brief description of our setup:  We use an MPLS circuit for our site's primary connection using a Cisco 1921 router.  A Cisco 3550 switch connects to that.  For the backup circuit we use a DSL modem which connects to a Cisco 831 router which connects to the Cisco 3550 and automatically kicks on when the MPLS circuit drops for whatever reason.

Recently, the MPLS dropped at one of our sites, the backup circuit kicked on as it should but would only route LAN traffic; however, internet traffic seemed to get blocked.  If I ran a traceroute from the switch, it would hit the 831's inside interface, then appear to stop.  If I did the same traceroute from the 831, it would hit the internet site successfully, so it appears to me that something is blocking internet traffic on the 831 between the inside interface and outside interface.  As I am new to routing and especially routing troubleshooting, I'm not really sure what steps to take to try and find the issue.  We have other sites that use the same exact setup and don't have an issue, so the problem seems isolated to this site at this time.

Any advice?  I'll be glad to give any more information that I can.

Thanks in advance

[siddhartham]

What is the gateway on your client devices, is it the 3550 switch or the 1921 router? If 3550 is the gateway, can you post the config of your 3550, and Cisco 831 router- I am asuming you configured NAT on the 831 router.

Thanks,

Siddhartha

[gshamlian]

client devices use the switch as a gateway, correct.

I've attached the modified configs and added ***notes*** on them to hopefully help explain how we have them setup and where connections are.

I didn't set it up, but i'm almost positive that NAT is configured through our firewall in the data center.  All traffic goes through the data center first.

[siddhartham]

According to your config the internet traffic from clients or the switch doesn't go through the datacenter.For example if you ping 4.2.2.2 from the 3550 switch- acc to the route on the switch (ip route 0.0.0.0 0.0.0.0 10.xx.xx.12 254  ***route to 831***) it will be forwarded to the 831 router and from there it will match (ip route 0.0.0.0 0.0.0.0 192.168.0.1  ***route through DSL modem***) and go to DSL modem not to the datacenter (which is on Ethernet0) but you do have nat configured on the 831 router so that shouldn't be an issue.

Can you post the sh ip route out puts from both of those devices..

Siddhartha