DHCP and PPPoE behavior in FortiGate HA clusters
This topic outlines how FortiGate HA clusters handle scenarios where units obtain their own IP addresses using DHCP or PPPoE, and how they operate when functioning as a DHCP server or DHCP relay.
It includes the following operational modes:
Each subsection explains the expected behaviors, HA-specific requirements, and operational impacts to ensure consistency, reliability, and seamless failover across the cluster.
FortiGate as a DHCP or PPPoE client
A FortiGate interface can use DHCP or PPPoE to obtain its own IP address. These dynamic addressing methods are supported in HA deployments, but require specific configuration sequencing to ensure stable cluster operation.
HA support for DHCP and PPPoE
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FortiGate fully supports using DHCP or PPPoE on HA cluster interfaces.
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However, these features must be enabled after the HA cluster is formed.
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This ensures that the primary unit negotiates and maintains the correct dynamic IP address on behalf of the cluster.
Risks when enabled before HA formation
If an interface is already using DHCP or PPPoE before HA is enabled, the following issues may occur:
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Incorrect or inconsistent IP assignments across units.
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Failure to reach the DHCP or PPPoE server during HA negotiation.
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Possible disruption in WAN connectivity while the cluster becomes operational.
DHCP relay operation in a FortiGate HA cluster
A FortiGate HA cluster can function as a DHCP relay agent, forwarding DHCP client messages to an external DHCP server. In an HA environment, the relay process is tightly controlled to ensure consistent behavior across the cluster.
Primary unit responsibilities
In both active-passive and active-active HA modes, all DHCP relay sessions are handled exclusively by the primary unit. This ensures deterministic forwarding and prevents duplication of relay traffic.
Failover behavior
If an HA failover occurs:
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Active DHCP relay sessions are not resumed by the new primary. As a result, DHCP clients may need to retransmit their DHCP requests.
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The external DHCP server continues to manage all lease allocations, ensuring no lease conflicts occur.
DHCP server operation in a FortiGate HA cluster
A FortiGate HA cluster can function as a fully integrated DHCP server. In an HA setup, the primary unit is responsible for all DHCP related operations, ensuring consistent lease assignment and preventing conflicts.
Primary unit responsibilities
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Respond to all DHCP requests.
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Maintain the complete DHCP lease database.
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Automatically synchronize the lease database to all subordinate units.
Failover behavior
If an HA failover occurs:
The newly elected primary unit already holds the most up to date DHCP lease database. This prevents issues such as:
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Incorrect IP address assignments
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Duplicate lease allocations
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Lost or stale lease information
Cluster formation and lease database merging
When an HA cluster is first formed:
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Any unit joining as a subordinate might already contain an existing DHCP lease database from prior standalone operation.
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To ensure consistency across the cluster, all DHCP lease databases from all units are merged into a single unified database. This consolidated database is then synchronized to every cluster unit.