DHCP lease state synchronization optimization

In a stateful BNG dual homing setup, Multi-Chassis Synchronization (MCS) is used to synchronize the subscriber state between the active and standby BNG, including the DHCP lease states, using configure redundancy multi-chassis options sub-mgmt configuration. For IPoE subscribers the synchronization includes DHCPv4 and DHCPv6 lease states and for PPPoE subscribers the synchronization includes DHCPv6 lease states.

The DHCP lease states are synchronized as follows:

when the lease is created in the system
at every DHCP renewal
when the lease is removed from the system

With short lease times in a scaled deployment, MCS synchronization creates additional load on the control plane. Short least times typically occur when the lease-split feature is enabled because a short lease time is used between the DHCP client and DHCP relay agent and a long lease time is used between the DHCP relay agent and DHCP server. With lease split enabled, the MCS application synchronizes the DHCP renewals following the short lease speed.

To reduce the control plane load in scaled multi-chassis redundant BNG deployments with short DHCP leases, a DHCP lease time threshold can be configured to control the eligibility of a DHCP lease for MCS synchronization at renewal:

# configure redundancy multi-chassis options sub-mgmt dhcp-leasetime-threshold
  - dhcp-leasetime-threshold [days <days>] [hrs <hours>] [min <minutes>] [sec <seconds>]
  - no dhcp-leasetime-threshold
 <days>               : [0..1]
 <hours>              : [0..23]
 <minutes>            : [0..59]
 <seconds>            : [0..59]

An active DHCP lease time threshold per multi-chassis peer is determined as the smallest value configured on either of the redundant BNGs:

# show redundancy multi-chassis sync peer 192.0.2.1
===============================================================================
Multi-chassis Peer Table
===============================================================================
Peer
-------------------------------------------------------------------------------
Peer IP Address         : 192.0.2.1
Peer Name               : mcs-pe2-pe1
Description             : (Not Specified)
Authentication          : Disabled
Source IP Address       : 192.0.2.2
Admin State             : Enabled
Sub-mgmt options        :
  DHCP lease threshold  : Active (10 minutes)
    Local / Remote      : 10 minutes / 15 minutes

The DHCP lease time threshold is inactive when unconfigured or unsupported on at least one end of the multi-chassis peer:

# show redundancy multi-chassis sync peer 192.0.2.1
===============================================================================
Multi-chassis Peer Table
===============================================================================
Peer
-------------------------------------------------------------------------------
Peer IP Address         : 192.0.2.1
Peer Name               : mcs-pe2-pe1
Description             : (Not Specified)
Authentication          : Disabled
Source IP Address       : 192.0.2.2
Admin State             : Enabled
Sub-mgmt options        :
  DHCP lease threshold  : Inactive
    Local / Remote      : 10 minutes / --

DHCP leases with lease time committed by the DHCP server less than or equal to the active DHCP lease time threshold are not synchronized at renewal, if only the remaining lease time is changed.

When lease split is active, the following rules apply if the short lease time is less than or equal to the active DHCP lease time threshold:

The DHCP lease is not synchronized when the DHCP client renewal or rebind is proxied, only the remaining short lease time is changed, and at least one DHCP server is reachable.
The DHCP lease is always synchronized when the DHCP client renewal or rebind is relayed to the DHCP server.

After an MCS redundancy switchover, DHCP leases that are flagged to skip MCS synchronization are granted the full lease time in the new active BNG. This could lead to a temporary address conflict when a client disconnects ungracefully immediately after such a switchover as illustrated in the following scenario:

A DHCP client lease with 15 minutes lease time is active on a redundant BNG pair with active DHCP lease time threshold equalling 20 minutes.
Five minutes after the last DHCP client renewal, an SRRP switchover occurs. At the new active BNG, the DHCP lease is eligible to be extended to the DHCP server committed lease time of 15 minutes while the client and server have a remaining lease time of 10 minutes.
If the client disconnects ungracefully before the next renewal (for example, by not sending a DHCP release), the state in the BNG is not cleared and the session lives longer than expected.
The lease in the DHCP server expires 10 minutes after the switchover, while the lease in the BNG is still active. The DHCP server can allocate the same address or prefix to another user, which could create a temporary address conflict in the BNG.

Note:

The MCS DHCP lease time threshold is not applicable for DHCP server failover (using the configure redundancy multi-chassis peer sync local-dhcp-server context) and not applicable for DHCP snooping.