With IPHost Network Monitor you can run simple snmp requests against a Cisco device in your network.
ccl2vpnPWMAC
Cisco Cable L2VPN PWMAC
1.3.6.1.4.1.9.9.700.1.3.1.2
This object identifies the Cable Modem MAC address.
Back to CISCO-CABLE-L2VPN-MIB MIB page.
IPHost Network monitor allows you to monitor ccl2vpnPWMAC on Cisco device via the SNMP protocol. Download IPHost Network Monitor (500 monitors for 30 days, 50 monitors free forever) to start monitoring Cisco switches right now.