Published on 14 Sep 2015
The DHCP service in Broadband Command Center has three objects: Client Classes, Device Classifiers, and Classified Network Settings. These three objects can sometimes seem to be accomplishing the same goals. The below definitions outline when it is appropriate to use each of these.
1. Client Class
Client Classes are traditionally used to ensure that a set of network options are made available to a group of devices. A Client Class allows you to group a set of devices together for configuration.
These groups are based on a single criterion of the device — typically the device’s MAC address or a MAC address-based criterion such as Remote ID (information sent to devices behind the cable modem in a cable network). For example, a Client Class may refer to a set of devices associated with a particular set of customers. A Client Class may also contribute DHCP option data.
2. Device Classifier
Device Classifiers enable you to classify devices based on some intrinsic property of the device. For example, you may group DOCSIS devices together as a Device Classifier. Device Classifiers do not directly contribute DHCP option data.
Usually there are few Device Classifiers in a system because there are typically many different “types” of devices.
3. Classified Network Setting
Classified Network Settings are a way to further specialize the Client Class based on the type of device. A Classified Network Setting may be used in conjunction with a Client Class to explain how a particular set of devices should act.
Inside the Client Class, a Classified Network Setting can be used to determine what provisioning information each group of devices (Device Classifier) receives. For instance, you could create a Client Class for all provisioned devices and decide to separate out DOCSIS devices from PacketCable devices by grouping all DOCSIS devices under one Device Classifier and all PacketCable devices under another.
From there, establishing Classified Network Settings will enable you to determine what provisioning information each Device Classifier receives. In this example, DOCSIS devices will receive one Classified Network Setting, while PacketCable devices receive a different Classified Network Setting, and all other devices receive a third setting. Classified Network Settings may contribute DHCP option data.
How it Works
A Client Class can match incoming devices in two ways. It either matches certain values from the incoming DHCP message or it is controlled by external lookups.
Matching Values from Incoming DHCP Message
In this scenario, the Remote ID (DHCPv4 option 82 sub-option 2) or the Circuit ID (DHCPv6 option 18 in the Relay-Forward portion of the incoming message) part of an incoming DHCP message may be used to match devices with Client Classes. These message components correspond to the most common methods that operators use to identify individual subscribers.
Alternatively, Client Class membership may be controlled by external lookups. The DHCP service may ask an LDAP or SQL database about the subscriber and that database will contain a list of Client Classes that the device belongs to.
A Device Classifier matches incoming devices if it matches certain values from the incoming DHCP message, for example, DHCPv4 option 60 (Vendor Class Identifier). In the DOCSIS world, this contains an identifier starting with “DOCSIS” for the cable modem, “PKTC” for PacketCable-compliant MTAs, and a different identifier for other types of devices.
Classified Network Setting
A Classified Network Setting is a specialization of a Client Class based upon the Device Classifiers that match the device. For example, a Client Class may be based on the subscriber ID but it can be further specialized as a Classified Network Setting by giving different information to the cable modem. You may use Classified Network Settings to give different service class setting to different sets of cable modems.
There may be one additional Classified Network Setting that is not based on a Device Classifier and is therefore only used if no other Classified Network Setting matches the device. For example, you could have a Classified Network Setting for DOCSIS, another for PacketCable, and a third for everything else.
Further reading: Confused about DHCP options? Read more about DHCP Options in Plain English here.