Courseiva
InterfacesInterface Config

mtu [bytes]

Sets the maximum transmission unit (MTU) size for an interface, controlling the largest packet that can be transmitted without fragmentation.

Syntax·Interface Config
mtu [bytes]

When to Use This Command

  • Adjusting MTU to match a VPN tunnel endpoint to avoid fragmentation overhead.
  • Reducing MTU on a serial link to improve performance with small packets.
  • Setting a larger MTU on a Gigabit Ethernet interface to support jumbo frames for data transfers.
  • Aligning MTU with a service provider's requirement for a WAN circuit.

Command Examples

Set MTU to 1400 on GigabitEthernet0/1

interface GigabitEthernet0/1 mtu 1400
Router(config-if)# mtu 1400
Router(config-if)# end
Router# show interfaces gigabitEthernet 0/1 | include MTU
  MTU 1400 bytes

The command sets the MTU to 1400 bytes. The output from 'show interfaces' confirms the new MTU value.

Reset MTU to default on Serial0/0/0

interface Serial0/0/0 no mtu
Router(config-if)# no mtu
Router(config-if)# end
Router# show interfaces serial 0/0/0 | include MTU
  MTU 1500 bytes

The 'no mtu' command resets the MTU to the default value (1500 bytes for most interfaces). The output shows the default MTU.

Understanding the Output

The 'show interfaces' command displays the current MTU value for the interface. Look for the line starting with 'MTU' followed by the byte count. A typical default is 1500 bytes. Values below 1500 may indicate intentional reduction for tunnels or QoS. Values above 1500 (e.g., 9000) indicate jumbo frame support, which requires all devices in the path to support the larger MTU. If the MTU is set incorrectly, packets may be fragmented or dropped, causing performance issues or connectivity loss.

CCNA Exam Tips

1.

Remember that the default MTU on Cisco IOS is 1500 bytes for most interfaces.

2.

The 'mtu' command is configured under interface configuration mode, not globally.

3.

On serial interfaces, the default MTU is also 1500, but can be changed for efficiency.

4.

Be aware that changing MTU can affect routing protocols like OSPF, which use MTU to determine adjacency formation.

Common Mistakes

Setting MTU too low causes excessive fragmentation and performance degradation.

Setting MTU too high on a link that doesn't support jumbo frames causes packet drops.

Forgetting to apply the same MTU on both ends of a tunnel or link, leading to mismatched MTU issues.

Related Commands

show interfaces

Displays detailed status and statistics for all interfaces or a specific interface, used to verify interface operational state, errors, and performance.

Practice for the CCNA 200-301

Test your knowledge with hundreds of CCNA practice questions covering all exam domains.

Practice CCNA Questions