Product and Software: This article applies to all Aruba controllers and ArubaOS versions.
Aruba APs use an MTU discovery mechanism to set an appropriate MTU for the GRE tunnel used for each BSSID.
The AP sends the GRE packet of size 1578 with the do not fragment (DF) bit set. If the controller receives the packet, it returns the packet to the AP. Upon receipt, the AP sends a message to the controller that the GRE tunnel MTU is now 1578. The controller applies the MTU of 1578 to the tunnel.
The AP MTU discovery mechanism only tries a packet size of 1578 with the DF bit set. If the MTU discovery fails, that is, the packet is either not received by the controller or the return packet not received by the AP, the MTU of the tunnel is 1500. If the MTU discovery is successful, then the MTU is 1578.
The MTU can be explicitly set in the AP system profile to a value in the range of 1024-1578. If the MTU is explicitly set, the GRE tunnel MTU is the value specified in the system profile and the MTU discovery mechanism is not used. If the MTU is explicitly set, the user should make sure the maximum packet size is able to traverse the entire network between the controller and the AP.
The value of the GRE tunnel is independent of the MTU on the VLAN that the controller uses to communicate to the AP. The MTU on the VLAN can be 1500 and the MTU of the GRE tunnel can be 1578, as long as the MTU discovery successfully sends the 1578 byte frame from the AP to the controller and back again. The controller can still receive packets larger than 1500 bytes even though the MTU of the VLAN is 1500.