Cisco 1841 - Unable to browse internet after 12 minutes

[solaikumar k]

Hello Folks,

We have Cisco 1841 router which is used to connect our isolated LAN for internet access . We have configured the device without any issues and kept that in network .

Topology is | Private LAN <--> Router <-->ISP |

But after few minutes (12-13 mins max) we are observing below

1) Cannot ping the default gateway who is neighbor to our WAN interface.

2) That causes all the traffic to stop and we had to bounce the WAN interface to make it work . again it works for 13 mins max.

3) Did some debug , but no luck

4) No logs were recorded at the router during the issue.

5) Changed to another brand new router , but the issue remains same.

Any suggestions from your end ?

Configuration snap

---------Truncated--------------

interface FastEthernet0/0
description WAN-INF
no ip address
ip nat outside
ip virtual-reassembly
duplex auto
speed auto
!
interface FastEthernet0/0.1234
description WAN
encapsulation dot1Q 1234
ip address 5.5.5.2 255.255.255.252
ip nat outside
ip virtual-reassembly

!

interface FastEthernet0/1
description **** Private LAN ****
ip address 192.168.0.254 255.255.255.0
ip access-group nat in
ip nat inside
ip virtual-reassembly
duplex auto
speed auto
!

ip nat inside source list nat interface FastEthernet0/0.1234 overload
ip route 0.0.0.0 0.0.0.0 5.5.5.1
!
ip access-list extended nat
permit ip 192.168.0.0 0.0.0.255 any

-------------------End------------------------------------------------------------------

[Mark Malone]

hi do you see any issues under the wan interface can you post the show int f0/0

This isn't the fix  but it may stop it from failing run an ip sla across the f0/0 to some upstream route or even the gateway to try and keep constant traffic going across it

If you connect directly to the modem in front of you do you see the same issue as it could be the ISP device  considering you have already swapped out your router , have you checked with the ISP providing the circuit do they see same issue or is this purely on your side

[solaikumar k]

Hi Mark, No interface issues as far as i have observed. even we have changed the port to some other free one. but no luck.

BTW I may not be able to connect to modem , hope my previous comment might answer your question.

[johnlloyd_13]

hi,

is ISP circuit terminated on a L2 switch?

can you try to terminate ISP circuit directly to 1841 F0/0, re-configure below and observe again?

no interface FastEthernet0/0.1234

no ip access-list extended nat

no ip nat inside source list nat interface FastEthernet0/0.1234 overload

interface FastEthernet0/0
ip address 5.5.5.2 255.255.255.252
ip nat outside

access-list 10 permit 192.168.0.0 0.0.0.255

ip nat inside source list 10 interface FastEthernet0/0 overload

[solaikumar k]

Dear John,

Yes, the link is terminated at the switch . but only ISP has access.

Furthermore we have already tried assigning Ip address on the interface instead of SUB interface. but no luck due to tagging issues.

Thanks for response...!

[solaikumar k]

The issue is resolved after modifying one value on the sub interface ... Please guess... I didn't expect this would have been an issue , but now it worked.

[johnlloyd_13]

Was it the 'native' keyword?

[solaikumar k]

No Jhon, The default ARP timeout of 4 Hrs has been changed to 30 Seconds. It worked. 

[Richard Burts]

I am glad that you got this problem worked out. This is not an unusual problem. The ARP timeout is relatively long at 4 hours and the timeout in the switch address table is usually pretty short. This may result in the ARP table having a MAC address but that address is not known in the switch address table. Making the ARP timeout shot is a way to resolve this problem.

HTH

Rick