MTUs

MTUs

There are three different types of MTUs for an interface-

1. Interface/Hardware MTU- Every interface is set to an MTU size by default. Interface MTU value is the measure of a packet size that can be forwarded without being fragmented.

The native Gigabit Ethernet ports can be configured to a maximum interface MTU to 9216 bytes; default is 1500 bytes.

The interface MTU can be changed from interface configuration using following command-

Router(config-if)# mtu <value>

2. IP MTU- IP MTU specifies whether a non-labelled packet forwarded through the interface needs to be fragmented. IP MTU tracks interface MTU. IP MTU has to be lower than or equal to interface MTU. If interface MTU is changed, IP MTU is changed automatically. However, if IP MTU is changed (lower than or equal to interface MTU), it will not change the interface MTU and will appear in the running configuration.

IP MTU defaults to interface MTU when the router first boots up.

IP MTU can be changed from interface configuration mode using following command-

Router(config-if)# ip mtu <value>

3. MPLS MTU- MPLS MTU specifies whether a labelled packet (packet with shim-header) forwarded through the interface needs to be fragmented. MPLS MTU can be higher than interface MTU, however the router warns such a change because that could result in data corruption, high CPU utilization and packet drops.

If the interface MTU is less than 1524 bytes, you can set the maximum MPLS MTU to 24 bytes more than the interface MTU. For example, if the interface MTU is 1520 bytes, the maximum MPLS MTU can be set to 1544 bytes.

However, if the interface MTU is equal to or more than 1524 bytes, then you can set the maximum MPLS MTU equal to interface MTU. For example, if the interface MTU is 1600 bytes, the maximum MPLS MTU can be set to 1600 bytes.

MPLS MTU defaults to interface MTU when the router first boots up. If interface MTU is changed, the MPLS MTU is also set to that value automatically. However, the reverse is not true.

MPLS MTU can be changed from interface configuration mode using following command-

Router(config-if)# mpls mtu <value>

Cisco recommends that if MPLS is used, the MPLS MTU command is configured on the interface to a value less than or equal to interface MTU.

MRU- Maximum Receive Unit

• MRU is a Cisco-proprietary parameter which implicitly means MTU per FEC.
• It informs the LSR how big a packet (in bytes) can be for a particular FEC (i.e destination prefix) that can still be forwarded out without being fragmented.
• The MRU changes per prefix depending on the label operation performed on the prefix.
• To view the value of MRU for an FEC, use show mpls forwarding-table <prefix> detail command.