MTU on Cisco Routers: The Layer 2 vs Layer 3 Difference Most Engineers Miss

 

If you’ve ever run show ip interface and show interface on a Cisco router, you may have noticed that both commands report an MTU. In most lab outputs the two numbers match, which makes it easy to assume they are the same thing. They are not. This post explains what each value means, how they relate, and why the distinction matters in real networks.

The Output That Raises the Question

Take a typical Ethernet interface on a Cisco router:

R1#show ip interface eth1/0
Ethernet1/0 is up, line protocol is up
  Internet address is 12.1.1.1/30
  MTU is 1500 bytes
  ...

R1#show interface eth1/0
Ethernet1/0 is up, line protocol is up
  Hardware is AmdP2, address is ca01.518f.001c
  Internet address is 12.1.1.1/30
  MTU 1500 bytes, BW 10000 Kbit/sec, DLY 1000 usec
  ...

Both say 1500 bytes. So why does the router show it twice?

What Is MTU?

MTU (Maximum Transmission Unit) is the largest unit of data an interface will transmit in a single piece. Anything bigger has to be fragmented or dropped. The catch is that “unit” depends on which layer you are talking about, and that is exactly where these two commands differ.

IP MTU (Layer 3)

The MTU shown by show ip interface is the IP MTU. It is the largest IP packet, header plus payload, that the interface will send without fragmenting it.

You configure it under the interface:

interface Ethernet1/0
 ip mtu 1400

After this, any IP packet larger than 1400 bytes leaving that interface is fragmented. If the packet has the DF (Don’t Fragment) bit set, the router drops it and sends back an ICMP “fragmentation needed” message instead.

Interface MTU (Layer 2)

The MTU shown by show interface is the interface MTU, also called the hardware or Layer 2 MTU. It is the largest payload a frame on that link can carry. On Ethernet the default is 1500 bytes, and this figure does not include the 14-byte Ethernet header or the 4-byte FCS.

You configure it like this:

interface Ethernet1/0
 mtu 1600

This is the ceiling for everything riding on the link: IPv4, IPv6, MPLS and so on.

How the Two Values Relate

The relationship is simple but has a few rules worth remembering:

• By default they track each other. If you never configure ip mtu, the IP MTU follows the interface MTU. Change mtu and the IP MTU changes with it.
• IP MTU cannot exceed the interface MTU. The router will not let you set ip mtu larger than what the link can carry.
• Once you set ip mtu explicitly, it becomes independent. From then on, the two numbers can differ.
• Other protocols have their own knobs. ipv6 mtu and mpls mtu are also capped by the interface MTU.

A Quick Example

Suppose you configure:

interface Ethernet1/0
 ip mtu 1400

Then:

• show ip interface reports MTU is 1400 bytes
• show interface reports MTU 1500 bytes

The link can physically carry 1500-byte payloads, but the router will only build IP packets up to 1400 bytes on it.

Why This Matters in Practice

Tunnels and VPNs

GRE and IPsec add extra headers to every packet. If the encapsulated packet grows beyond what the underlying link can carry, you get fragmentation or silent drops. Lowering the IP MTU on the tunnel interface (and often adjusting TCP MSS as well) leaves room for that overhead.

Jumbo Frames

Raising the interface MTU, for example to 9000, enables jumbo frames in data center or storage networks. Every device on the path, including both ends of the link, must support the larger size, or large frames will be dropped.

OSPF Adjacencies

This one catches many engineers during troubleshooting. OSPF routers include their MTU in Database Description (DBD) packets, and a mismatch between neighbors can leave the adjacency stuck in the ExStart or Exchange state. On Cisco IOS this check uses the interface’s IP MTU. You can bypass it with:

interface Ethernet1/0
 ip ospf mtu-ignore

That command works, but fixing the underlying mismatch is the better answer. Ignoring the check can hide a real problem and lead to dropped large packets later.

Quick Reference

	IP MTU	Interface MTU
Layer	3 (IP packet)	2 (frame payload)
Show command	show ip interface	show interface
Configure with	ip mtu	mtu
Default on Ethernet	Follows interface MTU	1500 bytes
Upper limit	Interface MTU	Hardware dependent

Key Takeaways

• Both commands show MTU, but at different layers.
• Interface MTU describes what the link can carry; IP MTU describes what the router will send as an IP packet.
• They match by default, and diverge as soon as ip mtu is configured explicitly.
• When troubleshooting fragmentation, tunnel overhead, or OSPF neighbors stuck in ExStart, check both values on both ends of the link.

The next time you see two identical MTU numbers on a router, you’ll know they are two different settings that simply happen to agree.