This article explains how Jumbo Frames work and how MTU behavior is implemented across Alta Labs routers, switches, and access points.

What Are Jumbo Frames?

A Jumbo Frame is an Ethernet frame larger than the standard Maximum Transmission Unit (MTU) of 1500 bytes.

• Standard Ethernet MTU: 1500 bytes.
• Common Jumbo MTU: 9000 bytes (9K).

Jumbo frames allow more data to be transmitted per packet, reducing per-packet overhead in high-throughput LAN environments.

Jumbo frames are not supported across the public internet. Internet traffic is expected to use a 1500-byte MTU, and larger frames will be fragmented or dropped.

Default Behavior by Alta Labs Product Type

Routers

• Default MTU: 1500.
• Jumbo toggle: Available.
• Scope: LAN traffic only.
• MTU when enabled: 9216.
• TCP MSS clamping to path MTU is automatically enabled on WAN interfaces.

When Jumbo Frames are enabled, the router automatically applies an MTU of 9216 bytes to all LAN interfaces, including VLANs.

Where to Enable Jumbo Frames

To enable Jumbo Frames, go to Network > Route10 config > Ports > Advanced > Jumbo Frames, then toggle the setting on or off. This setting applies only to LAN interfaces. WAN behavior is not affected.

Jumbo Frames toggle under Ports > Advanced. This setting applies to LAN interfaces only.

PPPoE WAN Behavior

When using PPPoE:

• Effective MTU: 1492 bytes.
• Based on 8 bytes of PPPoE overhead.
• Larger WAN MTU configurations are not supported.

Even if Jumbo Frames are enabled on LAN interfaces, PPPoE WAN traffic remains limited to 1492 bytes.

TCP MSS Behavior

For TCP traffic, Maximum Segment Size (MSS) is automatically derived from the interface MTU.

In standard IPv4 deployments:

• 1500 MTU → 1460 MSS.
• 1492 MTU (PPPoE) → 1452 MSS.

Path MTU Discovery (PMTUD) ensures packets are sized appropriately across the network path. Manual MSS configuration is not required in typical deployments. Route10 automatically clamps TCP MSS to the detected path MTU on each WAN interface to prevent Path MTU Discovery black-hole issues and to ensure compatibility with jumbo frame-enabled LAN hosts.

Access Points

• MTU: 1500 (fixed).
• No Jumbo configuration option.

Why Jumbo Frames Are Not Used on Wi-Fi

Wireless networks:

• Use aggregation and retransmission mechanisms.
• Operate with diverse client capabilities.
• Experience variable link quality.

Jumbo frames provide no practical benefit over Wi-Fi and may introduce compatibility issues. Access points therefore operate at the standard 1500 MTU.

Switches

• Maximum supported frame size: 12288 bytes.
• Always enabled.
• Not configurable.

Switches operate at Layer 2 and forward frames transparently. The 12288-byte ceiling ensures compatibility with:

• 9K jumbo LAN environments.
• 9216 routed frames.
• VLAN-tagged traffic.

This does not force devices to use jumbo frames. It simply permits them.

When Should Jumbo Frames Be Used?

Jumbo frames may provide benefit in controlled LAN environments where:

• All devices support 9000 MTU.
• Traffic consists of large sustained transfers.
• The network operates at 10GbE or higher.
• CPU efficiency per packet matters.

Common examples include:

• NAS or SAN traffic.
• Editing video directly from shared storage.
• Large RAW media transfers.
• Virtualization and hypervisor clusters.
• Backup servers.

On high-speed links, Jumbo frames significantly reduce packet rate. For example:

• 1500 MTU at 10Gbps ≈ ~812,000 packets per second. 
• 9000 MTU at 10Gbps ≈ ~135,000 packets per second.

Lower packet rate reduces interrupt load and processing overhead.

When Jumbo Frames Should Not Be Used

Jumbo frames are not typically recommended for:

• Mixed device environments.
• General home or office networks.
• WAN traffic.
• Wi-Fi networks.

All devices in the traffic path must support jumbo frames. If even one device does not, packets may be fragmented or dropped.

Why 1500 Bytes Is the Safe Default

1500 bytes remains the industry interoperability baseline.

It:

• Is universally supported.
• Matches ISP WAN expectations.
• Prevents potential problems induced by fragmentation.
• Avoids Path MTU Discovery (PMTUD) black-hole issues.
• Works across VLANs, tunnels, and VPNs.
• Ensures compatibility with IoT and embedded devices.

For most environments, leaving MTU at 1500 is the safest and recommended configuration.

Example System Output (Reference Only)

The Jumbo Frames setting does not require any command-line configuration. The examples below are provided for visual reference only.

When Jumbo Frames are enabled on an Alta Labs Route10, LAN interfaces reflect an MTU of 9216:

eth0: ... mtu 9216
eth1: ... mtu 9216

When disabled:

eth0: ... mtu 1500
eth1: ... mtu 1500

On switch products, standard Linux utilities such as ip link show display the Linux interface MTU (1500). This reflects the Linux interface configuration and does not represent the switch hardware’s maximum supported frame size.

Switches support up to 12288-byte frames by default. This value is fixed and does not require configuration. These examples are shown for reference only.

Example system output showing LAN MTU set to 9216 when Jumbo Frames are enabled on Route10 as well as the configured frame size on our switch products. 

Summary

• 1500 bytes is the standard default.
• 9000 bytes is the common jumbo standard.
• Routers enable 9216 when Jumbo Frames are toggled on (LAN only).
• PPPoE WAN remains limited to 1492.
• TCP MSS is automatically derived from MTU.
• Switches allow up to 12288.
• Access Points operate at 1500 only.
• Jumbo Frames should only be enabled on Route10 in validated, end-to-end compatible LAN environments.