About the MTU
The MTU specifies the maximum frame payload size that the Firepower Threat Defense device can transmit on a given Ethernet interface. The MTU value is the frame size without Ethernet headers, VLAN tagging, or other overhead. For example, when you set the MTU to 1500, the expected frame size is 1518 bytes including the headers, or 1522 when using VLAN. Do not set the MTU value higher to accommodate these headers.
Path MTU Discovery
The Firepower Threat Defense device supports Path MTU Discovery (as defined in RFC 1191), which lets all devices in a network path between two hosts coordinate the MTU so they can standardize on the lowest MTU in the path.
MTU and Fragmentation
For IPv4, if an outgoing IP packet is larger than the specified MTU, it is fragmented into 2 or more frames. Fragments are reassembled at the destination (and sometimes at intermediate hops), and fragmentation can cause performance degradation. For IPv6, packets are typically not allowed to be fragmented at all. Therefore, your IP packets should fit within the MTU size to avoid fragmentation.
For UDP or ICMP, the application should take the MTU into account to avoid fragmentation.
Note: The Firepower Threat Defense device can receive frames larger than the configured MTU as long as there is room in memory.
MTU and Jumbo Frames
A larger MTU lets you send larger packets. Larger packets might be more efficient for your network. See the following guidelines:
- Matching MTUs on the traffic path—We recommend that you set the MTU on all Firepower Threat Defense device interfaces and other device interfaces along the traffic path to be the same. Matching MTUs prevents intermediate devices from fragmenting the packets.
- Accommodating jumbo frames—A jumbo frame is an Ethernet packet larger than the standard maximum of 1522 bytes (including Layer 2 header and VLAN header), up to 9216 bytes. You can set the MTU up to 9198 bytes to accommodate jumbo frames. The maximum is 9000 for Firepower Threat Defense Virtual.
Note: Increasing the MTU assigns more memory for jumbo frames, which might limit the maximum usage of other features, such as access rules. If you increase the MTU above the default 1500 on ASA 5500-X series devices or Firepower Threat Defense Virtual, you must reboot the system. You do not need to reboot Firepower 2100 series devices, where jumbo frame support is always enabled.