What is MTU?

Maximum Transmission Unit (MTU) is the largest packet size, in bytes, that a network interface or a network path can transmit without breaking it into smaller pieces. Standard Ethernet MTU is 1500 bytes; VOS interfaces accept values from 72 through 9000 bytes, including jumbo frames for high-throughput links.

Key takeaways

• MTU is the largest packet an interface can send before fragmentation.
• Standard Ethernet MTU is 1500 bytes; jumbo frames raise it to 9000.
• A mismatch between peers can stall protocols like OSPF adjacency.
• Path MTU Discovery finds the smallest MTU along an end-to-end route.
• VOS interfaces accept configured MTU values from 72 through 9000 bytes.

On this page: Why it matters · How it works · Types · MTU & Versa · FAQ

Why is MTU important?

MTU sets the ceiling on how much data a packet can carry, shaping both efficiency and reliability. Too low, and transfers waste bandwidth on overhead – more packets, more processing, lower throughput. Too high for a link that cannot carry it, and packets get dropped or fragmented, adding latency and CPU cost at every hop that reassembles them.

MTU mismatches are also a classic cause of failures that look unrelated to packet size. Two OSPF routers exchange their interface MTU during Link State Database synchronization; if the values do not match, the neighbors never complete the handshake and the session sticks in an intermediate state. Because the symptom looks nothing like the cause, MTU is among the first things engineers check when a working link stops passing traffic.

What is MTU?

MTU applies at the interface: it is the largest protocol data unit – payload plus headers – the interface can send or receive in one frame. Ethernet’s historical default is 1500 bytes, set by early hardware and still the safe assumption across the public internet. Jumbo frames raise that ceiling, commonly to 9000 bytes, on links and hardware that support them end to end.

When a packet exceeds the MTU of a link it must cross, the device either fragments it, or – if the Don’t Fragment bit is set – drops it and returns an ICMP “fragmentation needed” message. Path MTU Discovery uses that mechanism: a sender starts at its own MTU and backs off on each fragmentation-needed response, converging on the smallest MTU along the path.

Types of MTU

Standard Ethernet MTU (1500 bytes) is the default for IPv4 networks and the safest choice when a path’s capability is unknown, since almost every device and carrier supports it. Jumbo frames (up to 9000 bytes) reduce per-packet overhead on high-throughput links such as data-center interconnects, but every device along the path must support the larger size or packets fragment. Tunnel-reduced MTU accounts for encapsulation overhead: IPsec, GRE, and VXLAN tunnels add headers, so the usable MTU inside the tunnel is smaller than the interface’s – a common source of “works for small traffic, fails on large packets.” IPv6 minimum MTU (1280 bytes) is a protocol floor: every IPv6 link must support at least that size, and since IPv6 routers do not fragment in transit, Path MTU Discovery becomes mandatory.

MTU & Versa

VOS interfaces expose MTU as a direct configuration field on every interface type. When you configure interfaces on a VOS device – WAN and LAN Ethernet ports, subinterfaces, tunnel and GRE interfaces – the MTU field accepts 72 through 9000 bytes, with some subinterfaces extending to 9600 and no default until set. The same field appears in Concerto: configuring LAN interfaces for wired and aggregate Ethernet interfaces exposes MTU in that same range under Link Configuration.

MTU mismatch is also a named failure mode in Versa’s OSPF troubleshooting guidance: when two peers have different interface MTUs, the adjacency sticks in EXSTART during Database Description exchange instead of completing. The fix is to check the local MTU against the remote peer’s advertised value and align the two – after which the adjacency comes up normally.

Frequently asked questions

What is the standard Ethernet MTU size?

The standard Ethernet MTU is 1500 bytes, set by early Ethernet hardware limits and still the default assumption across the public internet. It is small enough that virtually every switch, router, and carrier link supports it without any special configuration on either end.

What are jumbo frames?

Jumbo frames are Ethernet frames larger than the standard 1500-byte MTU, typically up to 9000 bytes. They reduce per-packet header overhead on high-throughput links, but every device along the path must support the same size, or packets fragment or drop.

What happens if MTU is mismatched between two connected devices?

Traffic up to the smaller MTU passes normally, but larger packets are dropped or fragmented depending on the DF bit. In routing protocols like OSPF, an MTU mismatch can prevent neighbors from completing adjacency, since both sides must agree on the interface MTU.

What is Path MTU Discovery?

Path MTU Discovery (PMTUD) is the process a sender uses to find the smallest MTU along a route to a destination. It sends packets with the Don’t Fragment bit set and reduces packet size whenever a router along the path reports the packet is too large.

What MTU range do Versa VOS interfaces support?

VOS interfaces accept MTU values from 72 through 9000 bytes on most interface types, with some subinterfaces supporting up to 9600 bytes. There is no default value – you set it explicitly whenever you configure an interface, whether through Director or Concerto.

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