| Description |
This article describes the expected behavior of the hardware switch interface on FortiGates when a member port is removed. |
| Scope | FortiGates featuring Shared Media Interfaces. |
| Solution |
A hardware switch on FortiGate serves as a virtual switch interface, aggregating multiple ports to function as a single interface. Supported models include a default hardware switch, typically labeled as either 'internal' or 'lan'. This hardware switch operates at the chipset level.
Ports within the same hardware switch behave as if they are on a single physical switch within the same broadcast domain. Member ports can be reassigned to other switches or utilized as standalone interfaces. For more detailed information, refer to the official document:
In most scenarios, adding or removing member ports from a hardware switch should not cause a flap, provided at least one port remains operational.
However, there is a specific scenario where a 1-second flap occurs when removing a member port (even if that port is already down) while other ports remain up.
Explanation of the Scenario: Certain FortiGates, such as the FortiGate-100F and 90G, feature a Shared Media Interface, allowing two distinct physical connectors to link to a single logical interface. For further information, see: Technical Tip: Understanding Shared Media interfaces on the FortiGate
When a Shared Media Interface is included in a hardware switch, removing any member port (even if down) will cause the hardware switch to flap.
For example, in the below FortiGate 101F, ports 17-20 are designated as Shared Media Interface:
Ports 18-20 are part of the hardware switch along with port 2, which is down:
When port 2 is removed from the 'lan' switch interface, the system event indicates that the 'lan' interface has gone down and back up. This behavior will also occur when port 2 is added back. If any VLAN interfaces or overlay services are configured on top of the hardware switch, they will also experience a brief disruption during the change.
Conversely, if ports 1 and 2 are included in the hardware switch, and port 1 is up, removing port 2 will not cause a flap since port 1 is not a Shared Media Interface.
Conclusion: In summary, the behavior described is expected for the hardware switch interface with Shared Media Interfaces on FortiGate devices. When making this kind of change to the hardware switch interface that carries production traffic, it is advisable to schedule a maintenance window to minimize any potential impact on network traffic. |
