DHCP Initial Logon Transaction
The following is a detailed description of the attributes extracted from the DHCP initial logon transaction:
For cable environments—The subscriber ID is the modem MAC address, which you extract from option 82 (Remote-ID sub-option of the DHCP Relay Agent Information Option). Therefore, for a successful logon operation, it is required that option 82 contains the modem MAC address in the DHCP initial logon transaction. If option 82 is missing, it is not possible to perform a logon operation. Furthermore, the value of option 82 is compared with the haddr field to identify modem transactions and not login the modem IP address to the Subscriber Manager.
For non-cable DHCP environments—The LEG supports using other DHCP options for the subscriber ID. If the DHCP option does not exist in the packet, it is possible to use the IP address as a fallback. In this case, the subscriber ID is in the format IP_a.b.c.d.
The chain of decisions regarding the subscriber-ID is as follows:
a. Use the configured DHCP option as the subscriber-ID if it exists.
b. Otherwise, if the fallback to IP is enabled, use the IP address.
c. Otherwise, attempt to extend the lease based solely on the IP address. (This will only work if the IP address is in the database).
Each subscriber might have multiple IP addresses, depending on the number of CPE devices connected to the modem. A logon operation is triggered for each assigned IP address in the DHCP message.
If the transaction correlates to a CPE device, the assigned IP address for that CPE device is added to the Subscriber Manager database. The IP address of the modem is not added to the Subscriber Manager database. If the transaction correlates to a modem device, no IP mappings are added to the Subscriber Manager database, but a logon operation is performed anyway to update package information.
If the transaction correlates to a CPE device, the assigned IP is added to the Subscriber Manager database with a lease time taken from option 51 (lease time option).
Note that option 51 must contain the lease time; otherwise, no logon operation is performed.
The policy information is assigned according to configurable options in the DHCP message. The LEG includes a component that converts the package information data from the DHCP packet to a subscriber package ID. If the packet does not contain package information, it is possible to log in the subscriber with a default package, or log in the subscriber with no package information at all.
After extracting the above information, the LEG performs a logon operation to the Subscriber Manager.
DHCP Release Transaction
The DHCP release transaction is handled differently to the other DHCP transactions. If the transaction correlates to a CPE device, the LEG performs an Subscriber Manager logout operation with the IP address of the CPE, which appears as a released IP address in the packet itself.
Note A logout operation is also performed when the lease time of the subscriber is expired, and the Subscriber Manager is configured to perform auto logouts. Release transactions also trigger logout operations, but do not replace the auto logout mechanism of the Subscriber Manager.
Cisco SCE-Sniffer DHCP LEG and SNMP-BG
Effective with Cisco SCE Release 4.1.0, the Cisco SCE-Sniffer DHCP LEG can work with SNMP-BG wherein the Cisco SCE-Sniffer DHCP LEG introduces subscribers to the Cisco Service Control Subscriber Manager and the SNMP-BG process associates the subscriber to a Vlink ID.
This is achieved by having the subscriber giaddr to be the same as the IP address of the CMTS device so that the SNMP-BG process places the subscriber in the primary queue of the respective CMTS to retrieve the Vlink ID.
To associate the subscriber giaddr value as the CMTS IP address, configure the giaddr_ip parameter.
For more details on configuring the Cisco SCE-Sniffer LEG, see the “Configuring the General Settings” section.