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Understanding Dynamic Reconfiguration of Extended DHCP Local Server Clients

Dynamic reconfiguration of clients enables the extended DHCP local server to initiate a client update without waiting for the client to initiate a request.

Default Client/Server Interaction

Typically the DHCP client initiates all of the basic DHCP client/server interactions. The DHCP server sends information to a client only in response to a request from that client. In subscriber management scenarios, this behavior does not enable a client to be quickly updated with its network address and configuration in the event of server changes.

For example, suppose a service provider restructured its addressing scheme or changed the server IP addresses that it provided to clients. Without dynamic reconfiguration, the service provider typically clears the DHCP server binding table, but cannot inform the DHCP clients that their bindings have been cleared. Consequently, the DHCP client operates as though its IP address is still valid, but it is now unable to communicate over the access network, resulting in an outage. The DHCP local server has to wait for the client to send a message to renew its lease or rebind to the server. In response, the server sends a NAK message to the client to force it to begin the DHCP connection process again. Alternatively, the provider can wait for customers to make a service call about the network failures and then instruct them to power cycle their customer premises equipment to reinitiate the connection. Neither of these actions is timely or convenient for customers.

Dynamic Client/Server Interaction for DHCPv4

Dynamic reconfiguration for DHCPv4 is available through a partial implementation of RFC 3203, DHCP Reconfigure Extension for DHCPv4. It enables the DHCPv4 local server to send a message to the client to force reconfiguration.

The server sends a forcerenew message to a DHCPv4 client, initiating a message exchange. In response, DHCPv4 clients that support the forcerenew message then send a lease renewal message to the server. The server rejects the lease renewal request and sends a NAK to the client, causing the client to reinitiate the DHCP connection. A successful reconnection results in the reconfiguration of the DHCP client. Only the exchange of forcerenew, renew, and NAK messages is supported from RFC 3202.

When the local server state machine starts the reconfiguration process on a bound client, the client transitions to the reconfiguring state and the local server sends a forcerenew message to the client. Because the client was in the bound state before entering the reconfiguring state, all subscriber services, such as forwarding and statistics, continue to work. Client statistics are not maintained in the interval between a successful reconfiguration and the subsequent client binding. When the server responds to the client renewal request with a NAK, the client entry is removed from the binding table and final statistics are reported. New statistics are collected when the client sends a discover message to establish a new session.

Dynamic Configuration Options

• To enable dynamic reconfiguration with default reconfiguration values for all DHCP clients, include the reconfigure statement at the [edit system services dhcp-local-server] hierarchy level for DHCPv4 clients. In a routing instance, include the reconfigure statement at the [edit routing-instances routing-instance-name system services dhcp-local-server] hierarchy level.
• Alternatively, to enable dynamic reconfiguration for only the DHCP clients serviced by a specified group of interfaces, include the reconfigure statement at the [edit system services dhcp-local-server group group-name] hierarchy level for DHCPv4 clients. In a routing instance, include the reconfigure statement at the [edit routing-instances routing-instance-name system services dhcp-local-server group group-name] hierarchy level

You can optionally modify the behavior of the reconfiguration process by including the appropriate statements at the [edit system services dhcp-local-server reconfigure] hierarchy level for all DHCPv4 clients. To override this global configuration for only the DHCP clients serviced by a specified group of interfaces, you can include the statements with different values at the [edit system services dhcp-local-server group group-name reconfigure] hierarchy level for DHCPv4 clients.

In a routing instance, you can optionally modify the behavior of the reconfiguration process by including the appropriate statements at the [edit routing-instances <routing-instance-name> system services dhcp-local-server reconfigure] hierarchy level for all DHCPv4 clients. To override this global configuration for only the DHCP clients serviced by a specified group of interfaces, you can include the statements with different values at the [edit routing-instances <routing-instance-name> system services dhcp-local-server group group-name reconfigure] hierarchy level for DHCPv4 clients.

Include the attempts statement to specify how many times the local server sends the forcerenew or reconfigure message to initiate client reconfiguration. Include the timeout statement to set the interval between the first and second attempts. The interval between each subsequent attempt doubles the previous value. For example, if the first value is 2, the first retry is attempted 2 seconds after the first attempt fails. The second retry is attempted 4 seconds after the first retry fails. The third retry is attempted 8 seconds after the second retry fails, and so on.

By default, the DHCP client’s original configuration is restored if all of the reconfiguration attempts fail. Include the clear-on-abort statement to delete the client instead.

You can force the local server to initiate the reconfiguration process for clients by issuing the request dhcp server reconfigure command for DHCPv4 clients. Command options determine whether reconfiguration is then attempted for all clients or specified clients.

Events that take place while a reconfiguration is in process take precedence over the reconfiguration. Table 1 lists the actions taken in response to several different events.