DHCP Short Cycle Protection

Conditions That Can Cause Failed or Short-Lived DHCP Client Sessions

Conditions that can cause a failed or short-lived client session include:

• Authentication denials from external AAA servers, such as RADIUS or Diameter, due to the absence of a corresponding entry in the RADIUS database or due to improper login attempts.

• Router or external authentication server unreachability due to network failure or misconfiguration.

• Insufficient memory resources to create a dynamic subscriber interface.

• Protocol negotiation failures with the CPE.

• Client logout shortly after a successful login; this action creates a fully negotiated and configured client session before the session is torn down.

How DHCP Short-Cycle Protection Works

DHCP short-cycle protection is disabled on the router by default. When you enable it by including the short-cycle-protection statement at a global, group, or interface level, the router does the following for DHCP sessions on static and dynamic logical interfaces:

1. Detects short-lived client sessions, also referred to as short-cycle events, and locks out the client based on the following events:

   • E0: Time when jdhcpd declares the client session to be active.

   • E1: Time when jdhcpd declares the client session should be torn down.

   • E2: Time when jdhcpd deletes the client session entry from the database.

   A short-cycle event occurs when the interval between E0 and E1 is less than or equal to 60 seconds. When the interval is greater than 60 seconds, the logout is considered normal. If the router declares the session to be short-lived, it adds the client to the lockout database at time E2.

2. Temporarily locks out the specified DHCP client by preventing connection to the router.

   During lockout, the router drops negotiation packets (DHCP discover and solicit messages) from the client until the lockout period expires. When the lockout period expires, the client can resume normal negotiation of the connection.

   You can set a range for the lockout period by specifying a minimum and maximum length with the short-cycle-protection statement. You must specify both a minimum and a maximum value.

3. Tracks the time between a client’s repeated short-cycle events to determine whether to increase the lockout time for a subsequent short-cycle event. The interval between events is compared to the grace time threshold. By default, the grace time threshold is 900 seconds, but it is automatically set to the maximum lockout time if that value is greater than 900 seconds.

   If no subsequent negotiation is attempted within the grace time, the client entry is removed from the lockout database.

   If a subsequent negotiation is attempted before the grace threshold is reached, it is treated as another short-cycle event and the lockout penalty is increased. The penalty is increased exponentially each time the negotiation is attempted within the grace time.

The initial lockout period is based on the configured minimum value. Additional penalties are calculated as follows, where n is the number of consecutive short-cycle events that occur within the grace time:

Lockout time = (Lockout minimum time) x [2^(n-1)]

For example, with a minimum duration of 1 second and a maximum duration of 300 seconds, the initial lockout period is 1 second; subsequent penalties increase to 2 seconds, then 4 seconds, 8 seconds, 16 seconds, 32 seconds, 64 seconds, 128 seconds, 256 seconds and finally 300 seconds. The final lockout period is 300 seconds instead of 512 seconds because no penalty can exceed the maximum value of the lockout range.

If the lockout time reaches the maximum, then it stays at that value for each subsequent lockout period until the time between short-cycle events is greater than the grace threshold.

Termination of the Lockout Condition

When a DHCP client is locked out, the lockout condition persists until all lockout timers have expired, except when any of the following occurs:

• You administratively clear the lockout condition by issuing one of the following operational commands:

  • clear dhcp relay lockout-entries

  • clear dhcp server lockout-entries

  • clear dhcpv6 relay lockout-entries

  • clear dhcpv6 server lockout-entries

• You reset the FPC on which the client session undergoing lockout is configured.

• You reset the Routing Engine.

When any of these events occurs, jdhcpd terminates lockout and clears the lockout history for all affected client sessions. The released clients are allowed to negotiate again. Because there is no retained history, the lockout period starts with the minimum value if a subsequent short-cycle event occurs for one of these clients.

When a dynamic VLAN or demux VLAN logical interface is removed from an underlying physical interface that is configured with remove-when-no-subscribers, the lockout of affected clients persists until all the timers have expired. If the logical interface is recreated before all timers expire, then the lockout state is applied to the re-created logical interfaces.

Benefits of Using DHCP Short Cycle Protection

• Reduces excessive control plane loading on the router and authentication, authorization, and provisioning loading on the external authority server.

• Reduces the resources required to process DHCP control packets and to negotiate and terminate short-lived connections.

• Temporarily defers subsequent attempts for clients with failed or short-lived client sessions in favor of sessions can complete successfully and last for more than a short duration.

• Reduces the resources required to authenticate and terminate these connections on external authentication servers, such as RADIUS and Diameter.

• Enables lockout of a single failed or short-lived DHCP session without disrupting other DHCP sessions on the same interface.

  Because DHCP short-cycle protection identifies each client session by its unique client ID, the router can lock out only the offending DHCP client while enabling other DHCP clients on the same interface to successfully negotiate the connection.

Purpose

View or clear information about DHCP short-cycle protection operations.

Use the supported show and clear commands to manage and display information about the short-cycle protection operations for the DHCP relay agent and the DHCP local server. You can display information about all locked-out entries or about only individual entries identified by their database index number.

Action

• To display short-cycle protection information for DHCPv4 or DHCPv6 relay agent:

• To clear short-cycle protection information for DHCPv4 or DHCPv6 relay agent:

• To display short-cycle protection information for DHCPv4 or DHCPv6 local server:

• To clear short-cycle protection information for DHCPv4 or DHCPv6 local server:

Meaning

When you include the all option with these show commands, information is provided for each client entry in the lockout database, such as the index number that corresponds to the entry in the database, the client identification key, the state of the lockout, how many seconds until the current state is over, how long the current state has been in effect, and how many consecutive times the client has been locked out.

When you want to remove information from the lockout database for a particular client, you must first issue the corresponding show command with the all option to determine the index for the client entry. Then you can specify that index with the clear command.

In the following example, you display all locked-out client entries for DHCPv4 relay agent to find the index number for a particular client, then you clear only that entry and verify that it is deleted:

In the following example, you display all locked-out client entries for DHCPv6 local server, then you clear all entries and verify that they are deleted: