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    Preservation of DHCP Proxy Client Bindings During Stateful SRP Switchover

    The Dynamic Host Configuration Protocol (DHCP) proxy client model is used for the centralized management of IP addresses for Point-to-Point Protocol (PPP) and DHCP subscriber sessions. Typically, service providers configure a common pool on a DHCP server to assign IP addresses to both PPP and DHCP subscribers for optimal and effective utilization of IP addresses. The DHCP proxy client in JunosE Software supports stateful switch route processor (SRP) switchover. When the DHCP proxy client is configured on a virtual router, the proxy client requests an IP address from the DHCP server on behalf of the PPP subscriber. Then, the obtained IP address is transmitted to the PPP subscriber.

    The DHCP proxy client also maintains the lease time for the allocated IP address on behalf of the PPP subscriber to determine the period for which the client binding needs to be maintained. When the PPP subscriber session is terminated, the proxy client returns the allocated IP address to the DHCP server.

    The authentication, authorization, and accounting (AAA) server handles the communication between the PPP subscriber and the DHCP proxy client. The AAA server obtains the IP address from the DHCP proxy client and forwards it to the PPP subscriber. The DHCP proxy client maintains a state machine for each PPP subscriber for which it has obtained an IP address from the DHCP server.

    The state machine handles the protocol message exchanges between the PPP subscriber and the DHCP proxy client and maintains the DHCP lease for the allocated IP addresses. To enable DHCP proxy client bindings to be preserved across a warm restart of the router, the state information for clients is saved in nonvolatile storage (NVS) so that the contents of the NVS in the primary and standby SRP modules are synchronized. Mirroring of state information occurs from memory on the active SRP module to memory on the standby SRP module by way of transactions. When a transaction is committed on the active SRP module, the data associated with the transaction is sent to the standby SRP module. Such a mirroring of the state information enables the standby SRP module that takes over as the primary module after a stateful SRP switchover operation to handle requests from subscribers seamlessly and without disruption.

    The following configuration details are mirrored from the primary SRP module to the standby module:

    • A list of client bindings along with their states
    • IP addresses of the DHCP proxy client and PPP subscriber
    • Last transaction ID
    • Lease time of IP addresses allocated to subscribers
    • Lease time that is remaining for each client binding so that the standby SRP module, when it takes over as the primary module, can continue with the DHCP lease without renewing the lease

    When the standby SRP module takes over as the primary module after a stateful SRP switchover operation, it continues to handle DHCP lease renewal requests from existing clients based on their states and processes state transitions without any disruption. Lease times on existing client bindings are preserved across a stateful SRP switchover operation.

    Note: The maximum number of DHCP proxy client bindings that are stored on the router chassis is 48,000 for ERX routers, 64,000 for E120 routers, and 96,000 for E320 routers.

    Restoration of Client Bindings from Mirrored Storage

    The mirroring application is used to synchronize the configuration information available on the SRP modules. The mirroring state machine resides on both the primary and secondary SRP modules. Mirroring of client binding information enables the DHCP proxy client to service client requests after a warm restart without any interruption.

    When you perform a stateful SRP switchover operation, a warm restart of the DHCP proxy client application takes place. During this process, all client bindings that were mirrored are restored on the standby SRP module that starts functioning as the primary module. For each proxy client binding that is restored from the mirrored storage containers on the newly active primary module, the DHCP proxy client queries the AAA application by using the user profile to determine whether the subscriber is still logged in. If the AAA server responds that the subscriber session is not active, in cases when the subscriber logs out during a stateful SRP switchover operation and the DHCP proxy client did not receive a notification about the logout, the client binding is removed from the DHCP proxy client. Because the AAA application supports stateful SRP switchover, AAA is synchronized with the PPP module and stale bindings are not retained.

    Published: 2014-08-12