Interfaces and Hardware Component Configuration Guide for Cisco 8000 Series Routers, Cisco IOS XR Releases

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Interfaces and Hardware Component Configuration Guide for Cisco 8000 Series Routers, Cisco IOS XR Releases

IP MTU

Updated: July 31, 2026

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This topic defines IP MTU, explains differences from Ethernet MTU, provides calculation guidelines, and introduces Maxinum Receive Unit (MRU) for Cisco routers.

In IP protocol, Maximum Transmission Unit (MTU) refers to the maximum size of an IP packet. The IP MTU is a network parameter that

  • defines the maximum size of an IP packet transmitted without fragmentation

  • includes IP headers while excluding data link layer headers, also known as Ethernet headers, and

  • defaults to 1500 bytes on router interfaces when IP is enabled. However, you can configure the IP MTU to different value as well.

The system supports IP MTU, IPv4 and IPv6, on Q200-based systems on these Cisco 8000 Series router and line card(s):

  • 8201-32FH

  • 88-LC0-36FH-M

How is Ethernet MTU different from IP MTU?

Ethernet MTU defines the maximum packet size that an interface supports, while IP MTU defines the MTU size of an IP packet.

How is IP MTU calculated?

The following scenarios provide information on how the system calculates IP MTU size, when:

  • Ethernet MTU is not configured, the system sets:

    • Default value as 1514 bytes on a physical or bundle main interface

    • 1518 bytes for single tag VLAN interface

    • 1522 bytes for double tag VLAN (QinQ) interface

    Note

    In this case, IP MTU value will be 1500 bytes for IPv4 and IPv6 MTUs.

  • Ethernet MTU is configured as X bytes, the system sets:

    • X bytes on a physical or bundle main interface

    • X+4 bytes for single tag VLAN interface

    • X+8 bytes for double tag VLAN (QinQ) interface

    Note

    In this case, IP MTU is X-14 bytes for IPv4 and IPv6 MTUs.

The following are some of the important guidelines for IP MTU size:

  • When no Ethernet MTU and IP MTU is configured, the default value is 1500B.

  • When Ethernet MTU and no IP MTU is configured, IP MTU value in the hardware is Ethernet MTU-14B.

  • IP MTU value can’t be more than Ethernet MTU value. For example, if the Ethernet MTU value is 3000 bytes and you configure IP MTU value as 5000 bytes. The system sets the IP MTU value as 2986 (3000-14).

What is Maximum Receive Unit (MRU) and how is it different from MTU?

MRU is the largest packet size that an interface can receive. This is an ingress parameter. Usually, MRU equals MTU. However, you can’t configure an MRU value. The Ethernet MTU, also known as a Layer 2 (L2) value that you configure on a physical interface is also applied as the MRU of that physical interface.

IP MTU support across platforms

The following table lists Ethernet MTU, IPv4, and IPv6 MTU support across various platforms and their limitations as applicable:

Table 1. IP MTU support across platforms

ASIC

Ethernet MTU check

IPv4 and IPv6 MTU

Q100-based

Ethernet MTU check on an egress interface is not supported.

Supported and the system derives IP MTU value from Ethernet MTU.

Q200-based

Ethernet MTU check on an egress interface is not supported.

Supported

Note

IPv4 and IPv6 can have their own separate MTU values.

P100-based

Ethernet MTU check on an egress interface is not supported.

Supported

Note

IPv4 and IPv6 can have their own separate MTU values.

IP MTU checks

Cisco IOS XR supports

  • MRU checks

  • Ethernet MTU checks

  • IP MTU checks

These checks decide if an IP payload packets needs fragmentation or not. Ethernet MTU and IP MTU check is applied on all egress traffic for each packet that flows in the system. In the forwarding plane, also known as dataplane, the IP packet length is compared against IP MTU value applied on the L3 forwarding interface. Full packet length is compared against Ethernet MTU value applied on the physical port.

This table describes IP MTU check to each forwarding flow on different systems.

Table 2. IP MTU check

ASIC

IP MTU check

Q100-based

These interfaces implement IP MTU checks and provide fragmentation for all IP payload packets. A single value for both IPv4 and IPv6 is supported.

Q200-based

These interfaces implement IP MTU checks and provide fragmentation for all IP payload packets.

Mixed system with Q100 and Q200-based line cards

Interfaces with Q100-based system provides its behavior and interfaces with Q200-based systems provides its behavior as described in this table.

P100-based

These interfaces implement IP MTU checks and provide fragmentation for all IP payload packets.

IP MTU scale

These are the scale support and limitations for IP MTU configurations:

  • Ethernet MTU configuration is allowed on every physical or bundle interface without any scale limits.

  • IPv4/IPv6 MTU configuration is allowed on every L3 forwarding interface. However, the system applies it as an IP MTU profile with a combination of <IPv4 MTU and IPv6 MTU> set. Each profile takes a unique set of these two MTUs, and supports eight such unique MTUs. The IP MTU profiles are stored identically across all NPUs of a given line card.

  • When system runs out of resources (OOR), the system generates syslog messages while continuing to use the previously configured IP MTU values.

IP MTU configuration guidelines for Q200-based systems

An Ethernet interface has a default of 1514 bytes. IP MTU is always derived from the default Ethernet MTU. If Ethernet MTU is configured, IP MTU is derived from the previously configured Ethernet MTU.

  • If you don’t define any Ethernet MTU configuration, the system assigns the default value of Ethernet packet size of 1514 bytes.

  • For IPv4 and IPv6 MTUs, 1500 bytes is used, which is derived by subtracting 14 bytes of Ethernet header size from its default value.

  • On any interface, if an Ethernet MTU is configured, the new configuration takes higher precedence than the default interface configuration. However, in such a scenario, the new configuration doesn’t get applied on the subinterfaces. The rest of the interfaces continue to work with the default Ethernet MTU configurations.

  • For the double tag (QinQ) packets, MRU and Ethernet MTU value are derived by adding 8 bytes to the configured value.

  • IPv4 and IPv6 MTU values are derived by subtracting 14 bytes of the Ethernet header size out of the configured value.

  • If an IPv4 or IPv6 MTU is configured, it is applied to IP MTU value. MRU or MTU values remain unaffected by this configuration.

  • Configuration restrictions apply to validate that the IP MTU value is smaller than Ethernet MTU value that is configured on an interface.

Note

P100-based systems follow the same guidelines as Q200-based systems.

The following sample table explains how the system calculates the Ethernet MTU, IPv4, and IPv6 MTU values when configured on various interfaces that are Q200-based systems.

Table 3. Interfaces and configurations

Interface type

Default configurations

when, no Ethernet or IP MTU configuration is applied

Ethernet MTU configurations

for example, MTU = 1614

IPv4/IPv6 configurations

for example, IPv4 | IPv6 MTU = 1550, Ethernet MTU = 1614

Any main interface

NA

Configuration that is applied on any main interface and its subinterfaces is effective.

Configuration that is applied on any subinterface is not effective.

NA

Physical port

MRU = 1514 + 8

MRU = 1614 + 8

MRU = 1614 + 8

L3 main interface (physical or bundle)

Ethernet MTU = 1514 + 8

IPv4/IPv6 MTU = 1500

Ethernet MTU = 1614 + 8

IPv4/IPv6 MTU = 1614 - 14

Ethernet MTU = 1614 + 8

IPv4/IPv6 MTU = 1550

L3 sub-interface (physical or bundle)

Ethernet MTU = 1514 + 8

IPv4/IPv6 MTU = 1500
Note

Configuration not applicable on subinterfaces. Takes main interface configuration.

Ethernet MTU = Not applicable

IPv4/IPv6 MTU = 1614 - 14

Ethernet MTU = 1614 + 8

IPv4/IPv6 MTU = 1550

SVI/BVI

IPv4/IPv6 MTU = 1500

Ethernet MTU = Not applicable

IPv4/IPv6 MTU = 1600

IPv4/IPv6 MTU = 1550

Ethernet main interface

Ethernet MTU = 1514 + 8

Ethernet MTU = 1614 + 8

IPv4/IPv6 MTU = Not applicable

Not applicable

Ethernet MTU = 1614 + 8

Ethernet sub-interface

Ethernet MTU = 1514 + 8

Ethernet MTU = 1614 + 8

IPv4/IPv6 MTU = Not applicable

Not applicable

Ethernet MTU = 1614 + 8

IP MTU configuration guidelines for Q100-based systems

These are the IP MTU configuration guidelines for Q100-based systems:

  • If you don’t define any MTU configuration in the system, the system assigns default value of 1514 bytes to the Ethernet packets.

  • An MRU of 1522 bytes is set to allow for any double tag packets on an interface.

  • Ethernet MTU check on egress interface is not supported.

  • For IPv4 and IPv6 MTUs, 1500 bytes is used, which is derived by subtracting 14 bytes of Ethernet header size from its default value.

  • If you configure an IP MTU (IPv4/IPv6 MTU) on any interface, the configuration does not take effect.

The following table lists the MTU configurations and MTU calculations on various interfaces for Q100-based systems:

Table 4. Interfaces and configurations

Interface type

Default configurations

Interface MTU configurations

where, MTU = 1614

Any main interface

NA

Configuration applied on any main interface and its subinterfaces is effective.

Configuration applied on any sub-interface is not effective.

Physical port

MRU = 1514 + 8

MRU = 1614 + 8

L3 main interface (physical or bundle)

Ethernet MTU check on egress interface is not supported.

IPv4/IPv6 MTU = 1522 applied on full packet size

Ethernet MTU check on egress interface is not supported.

IPv4/IPv6 MTU = 1622 applied on full packet size

L3 subinterface (physical or bundle)

Ethernet MTU check on egress interface is not supported.

IPv4/IPv6 MTU = 1522 applied on full packet size
Note

Configuration not applicable on sub-interfaces. Takes main interface configuration.

Ethernet MTU = Not applicable

IPv4/IPv6 MTU = 1622 applied on full packet size

SVI and BVI

Ethernet MTU is not applicable

IPv4/IPv6 MTU = 1522 applied on full packet size

Ethernet main interface

Ethernet MTU check on egress interface is not supported.

IPv4/IPv6 MTU is not applicable

Ethernet subinterface

Ethernet MTU check on egress interface is not supported.

IPv4/IPv6 MTU is not applicable

GRE tunnel

Ethernet MTU check on egress interface is not supported.

IPv4/IPv6 MTU = 1522 applied on full packet size

Limitations for IP MTU

These are the limitations and feature support for IP MTU:

  • MPLS MTU is not supported.

  • GRE/IPinIP MTU is not supported.

    Note

    GRE interface uses the configured L3 MTU. If an egress packet is above the configured L3 MTU value, the packet is discarded.

  • Multicast MDT MTU is not supported.

  • Ethernet MTU configuration on sub-interfaces is not supported.

  • Ethernet MTU configuration on BVI interfaces is not supported.

Although CLI command for the unsupported features might be available on your router, they are not functional.

Configure IP MTU

To configure IPv4, or IPv6 MTU, you must be in the configuration mode on an L3 interface.

Procedure

1.

Enter global configuration mode.

Example:

Router#configure terminal
2.

Use interface interface-path-id to enter interface configuration mode for the Layer 3 interface. commit , end

Example:

Router#interface HundredGigE0/0/0/33
3.

Use the ipv4 mtu mtu-value command to configure the IPv4 MTU value.

Example:

Router(config-if)# ipv4 mtu 9642

To configure MTU value for IPv6, use ipv6 mtu mtu-value command.
4.

Save the configuration.

Example:

Router(config-if)# commit
Router(config-if)# end
5.

Use show ipv4 interface interface-path-id , show interfaces interface-path-id commands to verify the configuration

Example:

Router:abc#
Router:abc#sh ipv4 int HundredGigE0/0/0/33
Thu Apr 28 16:54:10.820 UTC
HundredGigE0/0/0/33 is Shutdown, ipv4 protocol is Down
  Vrf is default (vrfid 0x60000000)
  Internet address is 10.10.10.91/29
  MTU is 9642 (2000 is available to IP)
Router:abc#sh int HundredGigE0/0/0/33    
Thu Apr 28 16:57:23.390 UTC
HundredGigE0/0/0/33 is administratively down, line protocol is administratively down
  Interface state transitions: 0
  Hardware is HundredGigE, address is e41f.7bde.123c (bia e41f.7bde.123c)
  Internet address is 194.19.242.94/29
  MTU 9642 bytes, BW 100000000 Kbit (Max: 100000000 Kbit)
     reliability 255/255, txload 0/255, rxload 0/255
  Encapsulation ARPA,
  Full-duplex, 100000Mb/s, 100GBASE-AOC, link type is force-up
  output flow control is off, input flow control is off
  Carrier delay (up) is 10 msec
  loopback not set,
  ARP type ARPA, ARP timeout 04:00:00
  Last input never, output never
  Last clearing of "show interface" counters never
  30 second input rate 0 bits/sec, 0 packets/sec
  30 second output rate 0 bits/sec, 0 packets/sec
     0 packets input, 0 bytes, 0 total input drops
     0 drops for unrecognized upper-level protocol
     Received 0 broadcast packets, 0 multicast packets
              0 runts, 0 giants, 0 throttles, 0 parity
     0 input errors, 0 CRC, 0 frame, 0 overrun, 0 ignored, 0 abort
     0 packets output, 0 bytes, 0 total output drops
     Output 0 broadcast packets, 0 multicast packets
     0 output errors, 0 underruns, 0 applique, 0 resets
     0 output buffer failures, 0 output buffers swapped out
     0 carrier transitions


Router:abc#
Verifies that the configured IP MTU value is available to IP on the interface.

Verify these output values

  • Internet address is 194.19.242.94/29.

  • MTU 9642 bytes appears in the interface output.