Motorola BSR 2000, Configuration And Management Manual

Page: 203 / 650

Release 1.0 Configuring the CMTS
526360-001-00 Rev. B MGBI 6-17
If the DHCP Lease Query attempt fails, packets associated with the CM and its CPE
are discarded. The BSR sends DHCPLEASEQUERY messages to the specified
DHCP server and accepts DHCPACTIVE, DHCPKNOWN and DHCPUNKNOWN
replies from the DHCP server.
The DHCP lease query function requires that the DHCP Server is configured with the
following values for the different DHCP message types:
■
DHCP Lease Query (13)
■
DHCP Known (14)
■
DHCP Unknown (15)
■
DHCP Active (16)
■
DHCP Unimplemented (17)
The following steps demonstrate how the BSR uses the DHCP lease query feature:
1. Cable Subscriber requests and gets an IP address from DHCP server.
2. Cable Subscriber starts to pass traffic through the cable interface.
3. The BSR inspects the cable network traffic to ensure source IP addresses are valid
by doing the following:
•
Verify DHCP server acknowledgement messages to learn if IP packets are
forwarded only once for an IP address.
•
Query the DHCP server to verify if an IP address was legally assigned by
verifying DHCP lease information table. If it is confirmed that static IP
address was assigned by a hacker for a CM, packets are not forwarded
beyond the cable interface.
•
Disallow ARP broadcasts
•
Query the DHCP server to verify that one IP address to MAC address
binding appears for a CM. If there is more than one IP address to MAC
address combination, one IP was assigned by DHCP and the other IP address
is statically (manually) set by a hacker. In this instance, only packets sent
from the legal source learned through DHCP are forwarded.
The DHCP lease query feature can also determine:
■
If the BSR is replaced or inadvertently rebooted.
■
When a CM re-registers and acquires a new lease.
■
When a CM or CPE maintains its lease because it has not expired.