select <Not Specified >
In the Policy Name field, select the DHCP policy that is appropriate for the
client−class, such as policyCableModem. To leave the policy name unspecified,
select <Not Specified >
6.
As a final step in this dialog box, add a comment or keyword in the User Defined
String field. You can use this to index, sort, or search for the client−classes.
7.
Click Apply to continue adding client−classes in the same way, or OK to finish. To
remove a client−class from the DHCP Server Properties dialog box, select it, then
click the Remove button.
8.
Fourth, Associating a Selection Tag with a Scope.
In the Server Manager window, double−click the scope for which you want to apply
selection tags for client−classes.
1.
Click the Selection Tags tab from the Scope Properties window.2.
Click the Edit Tags... button. This opens the Choose Scope Selection Tags dialog
box.
3.
Select the check boxes for one or more of the scope selection tags defined for the
server.
4.
Click OK.5.
Click OK in the Scope Properties dialog box.6.
Reload the DHCP server.7.
Repeat these steps for each additional scope.8.
Q. How to Calculate the Hexadecimal Value for DHCP Option 2 (time
offset)
If a cable modem were being used in a region that was GMT − 4 hours. In this case the
negative value changes the procedure a little bit. The appropriate value would be calculated as
follows: (Notice that 1hr = (60 minutes / hour) * (60 seconds / minute) = 3600 sec).
The number of seconds equivalent to − 4 hours = − 4 hours * (3600 second/hr) = −
14400 seconds.
1.
In order to convert − 14400 to an unsigned 32 bit value we need to perform the
following operation. (2^32 means 2 to the power of 32 = 4294967296). Then 2^32 −
14400 = 4294967296 − 14400 = 4294952896. We had to use this step because option
2 is 32 bits long.
2.
Using a scientific calculator or a tool such as the calculator application included with
Microsoft Windows we convert 4294952896 to a hexadecimal value. This turns out
to be FFFFC7C0.
3.
The value placed in the dhcp pool configuration now becomes option 2 hex
FFFF.C7C0.
4.
For more detailed information on how to do this read the document How to Calculate the
Hexadecimal Value for DHCP Option 2 (time offset).
Q. How does the CMTS know the difference between PCs and cable
modems?
In the past the we used to let Cisco Network Registrar (CNR) figure it out using DHCP
option 82. The CMTS inserts option 82 into the DHCP discover packet. The cable modems
Mac address is stuffed into this upstream discover packet and forwarded to the DHCP
server. The DHCP server looks for a match of the "remote id" and "Mac address" that
Cisco − CNR and DHCP FAQs for Cable Environment
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