Cable Modem CPE and every second ping dropping when using manual ip config

todd.fletcher · ‎11-14-2001

We currently have a problem with Cable Modem CPE devices attached via Cable Modem to a cisco uBR7246VXR CMTS.

The problem is that the CPE devices when manually configured with ip address information and then pinged from the CMTS will work fine, but then pinged from the switch that is one step further back which the CMTS is directly connected to, results in every second ping being dropped.

If the CPE device uses our DHCP server to get the EXACT same ip address information then everything works fine. There are no IP address conflicts occuring and no duplicate routes as far as I can tell, nor are we using ip source-verify on the CMTS. Any help greatly appreciated.

MickPhelps · ‎11-14-2001

Have you tried doing an extended ping from the 7246 using the interface that connects to the switch as the source address of the ping?

I would do a debug on the 7246, assuming that the current utilization isn't too high, and verify that the 7246 is getting the pings, where its sending them, and how many responses are getting back. (use an access-list with your debug so you don't melt anything)

Mick.

todd.fletcher · ‎11-28-2001

Further investigation has revealed that this only happens when the CPE has one IP address, then they either manually or using DHCP get another IP address. The CMTS then has two ARP table entries for the one MAC address, each with a different IP. The CMTS will then do some form of round robin I presume, resulting in every second ping getting dropped because only one of the IP addresses is actually valid. Is there a solution to this ? obviously if I set the arp timeout to something really really low like 5 seconds then it would work as the old entry would get removed.

MickPhelps · ‎11-29-2001

That's pretty bizarre. Why would a router care if there was the same MAC address for multiple IPs. That would mean that a router running NAT on the other side of this would break.

What testing did you do and how did you reach this conclusion? Any chance this is an IOS bug? Can you verify the "round robin" by using a sniffer?

Mick.