fastethernet4 shutdown required when cable modem fails

[asienaert]

Hi,

Let me start with saying that I'm not a network expert and definitely not a Cisco expert, so all help is appreciated.

We have an 881W router connected to our cable modem and use it for both internet and VoIP applications.

The modem has a dynamic IP address and therefore the router is configured as a DHCP client.

However, the last couple of weeks the cable modem often loses its connection to the ISP for a very short time (seconds...).

I know that this is an ISP problem and we are off course discussing this with the responsible people their.

However, even when the connection is up again we can no longer connect to the internet.

The only thing we found that helps us reestablishing the connection is shutting down the fastethernet4 (WAN) interface.

I have 2 questions:

1) What can be the reason for this? The modem's IP address DOES NOT change.

Is it normal that the connection is not automatically reestablished?

2) Is there a way to configure the router such that a manual intervention (restarting the WAN interface) is not necessary?

Thanks and kind regards,

Alan

[Richard Burts]

Alan

I would say that the behavior that you describe is not normal. In general if an interface (especially an Ethernet interface) goes down and comes back up, then I would expect for the connection to resume.

From your description I would assume that something that was negotiated between the router and the cable modem becomes invalidated when the connection goes down and comes back up and you need to get the router to initiate negotiations again. Your post is clear that the IP address of the modem does not change. Is it possible that the address assigned to the router interface changes?

Is there anything configured on the router interface other than DHCP? Perhaps you can post the interface configuration?

In addition to the possibility that an IP address might be changing I can think of several other possible issues including:

- possibly the MAC address mapping in the ARP table needs to be refreshed (perhaps the entry gets removed when connectivity is lost), This could be an issue on the router side of the connection or could be on the cable modem.

- possibly the bindings in the DHCP table of the cable modem need to be refreshed.

I am not sure if there is something you could configure using IP SLA that might help with re=initializing the interface.

HTH

Rick

[asienaert]

Thanks Rick for your swift response.

I have attached my configuration file.

I will take some time to look at your suggestions and see what I can do (as I already said, I just have some basic network knowledge)

Regards,

Alan

[Petar Milanov · Accepted solution]

hi Alan,

you can do tracking to the modem's WAN IP address by this way:

!

ip sla 1

icmp-echo x.x.x.x

timeout 1000

frequency 5

ip sla schedule 1 start-time now life forever

!

track 1 ip sla 1 reachability

!

Now you can tell the Cisco Event Manager to restart fa 4 when track goes to down:

!

event manager applet track-FA4

event track 1 state down

action 1.0 cli command "conf t"

action 1.1 cli command "int fa 4"

action 1.2 cli command "shutdown"

action 1.3 cli command "no shutdown"

action 1.4 cli command "end"

!

I think this should resolve your issue. If it is not working (because every situation is different) you can try the Event Manager to work when tracking is going to UP state. If you do not lose connectivity to your WAN IP address, you can ping some public IP address like some DNS server or whatever you decide.

Hope I helped

Best regards,

Tihomir Yosifov

IT Support

[asienaert]

Thanks for the sample code: it does the job

I simulated a modem disconnect by cycling the modem power and everythings keeps working: VoIP, VPN, etc...

Pingen the WAN IP address did not work but I did what you suggested and I'm now pingen the dns server.

However, I had to make a small addition: I had to set the threshold value.

ip sla 1
icmp-echo 195.130.130.130
timeout 500
threshold 300
frequency 5
ip sla schedule 1 life forever start-time now

Thanks again,

Alain

[Petar Milanov]

You are very welcome