Cisco Broadband Cable Command Reference Guide

Chapter 2 Cisco Cable Modem Termination System Commands
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Cisco Broadband Cable Command Reference Guide
cable source-verify
Note The leasetimer option is active only if you have also specified the cable source-verify dhcp command
for the cable interface. If the dhcp option is not used, the leasetimer option has no effect. In addition,
the leasetimer option can be configured only on an interface, not a subinterface. Applying it to a master
interface automatically applies it to all subinterfaces.
The leasetimer option allows you to configure how often the timer checks the lease times, so as to
specify the maximum amount of time a CPE device can use an IP address that was previously assigned
by the DHCP server but whose lease time has since expired. The time period can range from 1 minute
to 240 minutes (4 hours), with a grace period of 2 minutes to allow a PC enough time to make a DHCP
request to renew the IP address. To turn off the timer, so that the CMTS no longer checks the lease times,
issue the cable source-verify command without the dhcp option, or turn off the feature entirely with the
no cable source-verify command.
Tip In some circumstances, spoofing can still occur even after the cable source-verify command is used, due
to the behavior of the ARP protocol. For additional security, consider blocking ARP requests to the CMs
using the no cable arp command. For more details, see the Cisco Tech Note at the following URL:
http://www.cisco.com/warp/public/109/source_verify.html
Examples The following example shows how to turn on CM upstream verification and configures the Cisco CMTS
router to send DHCP LEASEQUERIES to verify unknown source IP addresses in upstream data packets:
router# configure terminal
router#(config) interface c4/0
router(config-if)# cable source-verify dhcp
router(config-if)#
The following example shows how to enable the leasetimer feature so that every two hours, the CMTS
checks the IP addresses in the CPE database for that particular interface for expired lease times:
router# configure terminal
router#(config) interface c1/0
router(config-if)# cable source-verify dhcp
router(config-if)# cable source-verify leasetimer 120
The following example shows the error message that is displayed if you try to configure the leasetimer
option on a subinterface, instead of an interface:
router# configure terminal
router#(config) interface c1/0.1
router(config-subif)# cable source-verify dhcp
router(config-subif)# cable source-verify leasetimer 120
% Invalid input detected at '^' marker.
Router(config-subif)#
Related Commands Command Description
cable arp Enables or disables the use of the ARP protocol for CMs and their
CPE devices.
cable helper-address Specifies a destination IP address for User Datagram Protocol (UDP)
broadcast (DHCP) packets.
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