MTU Explained [L2 MTU vs L3 MTU vs MSS]
Contents
TCP/IP PDU Overview
Let’s first review the PDUs (Protocol Data Units) in the TCP/IP model:

- Link Layer: Frame
- Network Layer: Packet
- Transport Layer TCP: Segment
- Transport Layer UDP: UDP Datagram
- Application Layer: Data
So, what is the MTU:
The maximum transmission unit (MTU) is the size of the largest protocol data unit (PDU) that can be communicated in a single network-layer transaction.
Maximum transmission unit (MTU) defines the largest size of packets that an interface can transmit without the need to fragment. IP packets larger than the MTU must go through IP fragmentation procedures.
In simple words, The MTU, or ‘Maximum Transmission Unit’, is the largest block of data that can be transmitted in IP Packet without fragmentation.
MTU (L2 MTU)
The Ethernet II L2 standard sets the maximum frame size at 1518 bytes. Ethernet headers are 18 bytes (Destination MAC, Source MAC, Ether Type, and CRC), which gives a maximum 1500 bytes for the packet.
- MTU is the max packet size supported by an interface.
- In simple words, MTU is the maximum packet size inside a layer 2 frame.

- L2MTU indicates the maximum size of the frame without Ethernet header that can be sent by an interface.
- Usually Layer 2 MTU affects all the interfaces of a switch. Configured in global config mode.
Each interface has a default maximum packet size or MTU size. This number generally defaults to the largest size possible for that interface type.
The default MTU (L2MTU) depends on the Layer 2 protocol. If we specify: L2 frame is Ethernet then maximum frame size is 1518, then the default MTU is 1500 Bytes.
For example, default MTU:
- Ethernet: 1500
- Token Ring: 4464
- ATM: 4470
IP MTU (L3 MTU)
The L3 network protocol has its own MTU concept. The maximum size of a packet with an IP header that can transfer a network interface without fragmentation is called IP MTU.
Layer 3 MTU that affects SVIs and routed interfaces of a switch with IP addresses on them and originating IP traffic that uses these interfaces as GW for routing between networks.
In case of cisco router, when setting the MTU under the physical interface: it’s the maximum size of any packet (not only IP MTU). if we set the IP MTU, we are specifying the maximum size of IP packet inside the frame (generally Ethernet frame).

The IP Packet includes IP Header (Destination IP and Source IP), TCP/UDP Header, and the Data Payload.
So, As you can see in the previous figure, the MTU is limited by the Maximum Ethernet II Frame size (which can be other L2 frame types, not only Ethernet II).
MTU vs IP MTU
The difference MTU and IP MTU is that:
- MTU is setting the maximum size of the packet for any Layer 3 protocol (inside the layer 2 frame) on the interface
- IP MTU is setting the maximum size of the IP packet inside the frame on the interface which can be forwarded without fragmentation.
Let’s take example on Cisco Routers, On an interface you can configure the MTU and IP MTU:
The following is the interface MTU, it’s 1500 bytes by default for Ethernet.
R1#show interfaces fastEthernet 0/1 | include MTU
MTU 1500 bytes, BW 100000 Kbit/sec, DLY 100 usec,
The IP MTU is also 1500 bytes by default. Once you get above 1500 bytes your router will start fragmenting the IP packets.
R1#show ip interface fastEthernet 0/1 | include MTU
MTU is 1500 bytes
Note: The IP MTU can be equal or less than the MTU.

TCP MSS (Maximum Segment Size)
TCP MSS defines the largest amount of data (payload) that a TCP segment can carry in a single transmission without fragmentation. It does not include TCP headers, IP headers, or other protocol overhead. The MSS value is negotiated during the TCP three-way handshake.
IP MTU = IP Header(20) + TCP/UDP Header (20) + TCP MSS
- Typically for IPv4 1500 MTU:
1500 = IP Header(20) + TCP/UDP Header (20) + TCP MSS (1460)
- If IPv6 is used (where the IP header is 40 bytes instead of 20):
MSS = 1500 – (40 + 20) = 1440 bytes.

MSS Negotiation in TCP Handshake
When two devices establish a TCP connection, they exchange MSS values in the SYN packets during the three-way handshake.

- Client SYN: Contains MSS option (e.g., MSS = 1460).
- Server SYN-ACK: Server also sends its MSS value (e.g., MSS = 1400).
- Final ACK: MSS is now determined. Each side uses the smaller MSS value for data transmission.
You can refer to following basic illustration for TCP 3 way handshake process with wireshark:
==> The lowest MSS value from both ends determines the largest TCP segment sent in either direction.
Common considerations
- MTU Mismatch: If a part of the network has an MTU smaller than expected, packets exceeding that size will be fragmented or dropped depending on DF bit setting.
- Path MTU Discovery (PMTUD) Issues: If PMTUD is blocked by firewalls (ICMP filtering), large packets can’t be adjusted dynamically.
- VPN & Tunnels: Encapsulation like GRE, IPsec reduces the effective MTU, which require to lower MSS to avoid fragmentation