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I have a cisco 1921 router that recently started acting up. I haven't changed anything on the router in many months because everything was running smoothly, until a week ago.

Configuration: The setup is pretty basic. The router has one port attached to the ISP over ethernet with a 8 port EHWIC ethernet switch which are used as access ports for workstations. The router is using DHCP, CBAC, NAT, and ACLs. There is only one vlan, one public IP, and one private network. The vlan interface is the gateway for private network. CBAC is used to inspect traffic from the private network to the internet. No services are allowed from the outside.

Symptoms: After the router starts up, everyone can access the internet as usual. Eventually, the router stops all traffic. During this blocking state, no workstation can ping the gateway. I am able to access the console and it works fine. From the console, I am able to ping all the IPs for all router interfaces, and pings work through CBAC, but can not ping out of any interface to any workstation or ISP gateway. Restarting the router fixes the problem for a short period of time. I am able to replicate the problem by doing a speed test or download a large file. After about 50ish mb transfer, the blocking state happens.

Troubleshooting steps:

Disabled CBAC and ACLs. Verified no memory errors. Verified there is plenty of free memory. Debugged NAT, no errors. Upgraded IOS to latest (15.4.3) Any help would be appreciated. Thanks.

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  • Have you tried to enable logs and see what's there?
    – Navern
    Apr 26, 2015 at 22:52
  • Does the time between failure vary, or is it constant? I once had a 1921 have a problem exactly 4 hours after reboot which helped narrow it down to an arp problem. Might be worth posting a cleansed version of your config.
    – ChadH360
    Apr 26, 2015 at 23:27
  • Hardware problem, or something trigger a bad state in the router. A packet capture on the inside interface ?
    – yagmoth555
    Apr 27, 2015 at 1:17
  • ARP and CAM table utilization would be my guess as well. Routers can be used as small switches, but they aren't architected that way. Do you just have one client workstation on each port of the EHWIC?
    – Alo
    Apr 27, 2015 at 4:16
  • @Navern, the logs don't provide many details. Only information about boot up and no errors.
    – jmardis
    Apr 27, 2015 at 11:12

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