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DHCP Options and Selective Traffic Processing Overview

Using selective traffic processing is helpful in network environments where DHCP clients access services that are provided by multiple vendors and by multiple DHCP servers. For example, a DHCP client might gain Internet access from a particular DHCP server provided by one vendor, and access an IPTV service from a different DHCP server owned by a second vendor. Using the option-specific information in the DHCP client packets enables DHCP relay agent to differentiate between the two servers and to take the correct action for the subscriber.

You might also use selective processing to distinguish between services to different DHCP subscribers on the same interface. For example, a household might include two IP devices that obtain their IP addresses from the service provider’s DHCP server. The service provider might want to bind one of the devices to the incoming interface, sharing that address with other households. At the same time the service provider might want the second device to have its own filter and CoS capabilities. For this second device, the service provider can use selective processing to create a dynamic IP demux interface.

You can configure selective processing support globally or for a named group of interfaces. You can also configure the support for the extended DHCP relay agent on a per logical system and per routing instance basis.

To configure selective processing, you specify the DHCP or DHCPv6 option attribute that identifies the traffic, the match criteria used to filter the traffic, and the action to perform with the filtered traffic.

• DHCPv4 option 60 (Vendor Class Identifier)
• DHCPv4 option 77 (User Class Identifier)
• DHCPv6 option 15 (User Class Option)
• DHCPv6 option 16 (Vendor Class Option)

You can configure exact match or partial match criteria to filter client traffic, and specify either the ascii option (to define a nonempty ASCII string of 1 through 255 alphanumeric characters) or the hexadecimal option (to define a hexadecimal string of 1 through 255 hexadecimal characters [0 through 9, a through f, and A through F]).

Best Practice: Because of the format of DHCP option 77 and DHCPv6 option 16, we recommend you configure hexadecimal matching only with these two options instead of ASCII matching.

You can configure an unlimited number of match strings. If you configure a string as both exact match (equals) and a partial match (starts-with) criteria, the exact match takes precedence. Wildcard characters are not supported in exact match or partial match strings.

Use the following match criteria to filter client traffic:

• equals—Your specified string is an exact match to the option string in client traffic.
• starts-with—Your specified string is a subset of the option string in client traffic, starting with the left-most character. For example, your configuration of the string “test” is a subset of “test123” in the client’s option string, and matches the starts-with criteria.
• default-action—The option string in client traffic does not satisfy any match criteria, or no match criteria are configured.

  Note: The default-action is optional. If the match criteria are not satisfied or not configured and there is no default-action configured, DHCP relay processes the traffic in the normal manner.

• drop—Discard the traffic.
• forward-only—Forward the traffic, without creating a new subscriber session.

  Note: When you use the forward-only action, the only configured overrides operation supported is the trust-option-82 option. DHCP relay agent ignores all other overrides options that are configured.

• local-server-group—Forward the traffic to the specified group of DHCP local servers that provides the requested client service. This option is not supported for DHCPv6 relay agent.
• relay-server-group—Forward the traffic to the specified group of DHCP servers that provides the requested client service.