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turotu's avatar
turotu
New Contributor
7 years ago

No ARP response from CTMS/Cox gateway - Intermittent connectivity

Recently I started getting periods of no internet connection. After troubleshooting a bit it looks like my router doesn't get a response (most of the time) when sending ARP requests to Cox to get the mac address of the cox gateway/router. 

Normally, ARP would work like this:

  1. My router -> Cox: Give me mac address for ip address 72.216.100.1
  2. Cox -> My router: ip address 72.216.100.1 has mac address d4:2c:44:7d:24:19

However, in my case Cox doesn't seem to respond back. In certain cases this will leave the router with no mac address for the cox gateway and complete loss of internet occurs.

This looks like this:

and ping will return "Destination Host Unreachable". 

Here is the packet capture between me and Cox. You can see my router asking for the address of the cox gateway (72.216.100.1), but it doesn't (typically) get a response. Note that other responses are coming through fine from Cox and the connection itself seems fine with no packet loss observed.

The reason I get some connectivity back in the ping plotter image, is that Cox is broadcasting a lot of ARP requests. Sometimes those ARP broadcast will contain the the cox gateway IP (72.216.100.1) and hence my router can indirectly get the mac address. However, it seems like since those are inferred they get purged relatively fast from the ARP cache (or some other reason) and then internet connectivity is lost again. 

So far the workaround for this is to add a static entry in the ARP table (e.g. 72.216.100.1 has mac address d4:2c:44:7d:24:19), so my router doesn't have to ask Cox for the mac address. And with that I have no connectivity issues. (though this is probably fragile and can break if that ip address gets associated with different hardware, or if I get a new ip/gateway I might have to add a new static route).

Anybody else seen this? Is this normal behavior (not responding to ARP requests)?

  • Rob_H_'s avatar
    Rob_H_
    Contributor II
    7 years ago

    The fact that the loss of connectivity doesn't occur with the static ARP table entry in your edgerouter means that we could rule out the RF side of the CMTS.  The question, as you demonstrated, seems to be what sometimes happens to your broadcast ARP request from your edgerouter when the packet moves beyond the RF side of the CMTS presumably to a switch before it reaches the default gateway?  Please come back and share the resolution.