MTU configuration guidelines

Observe the following general rules when planning your service and physical MTU configurations:

The router must contend with MTU limitations at many service points. The physical (access and network) port, service, and SDP MTU values must be individually defined.
Identify the ports that are designated as network ports intended to carry service traffic.
MTU values should not be modified frequently.
MTU values must conform to both of the following conditions:
- The service MTU must be less than or equal to the SDP path MTU.
- The service MTU must be less than or equal to the access port (SAP) MTU.
When the network group encryption (NGE) feature is enabled, additional bytes because of NGE packet overhead must be considered. See the ‟NGE Packet Overhead and MTU Considerations” section in the 7450 ESS, 7750 SR, 7950 XRS, and VSR Services Overview Guide for more information.

Default MTU values

MTU default values shows the default MTU values which are dependent upon the (sub-) port type, mode, and encapsulation.

Table 1. MTU default values
Port type	Mode	Encap type	Default (bytes)
Ethernet	access	null	1514
Ethernet	access	dot1q	1518
Fast Ethernet	network	—	1514
Other Ethernet	network	—	9212^¹

¹ The default MTU for Ethernet ports other than Fast Ethernet is actually the lesser of 9212 and any MTU limitations imposed by hardware which is typically 16K.

Modifying MTU defaults

MTU command options must be modified on the service level as well as the port level.

The service-level MTU command options configure the service payload (Maximum Transmission Unit – MTU) in bytes for the service ID overriding the service-type default MTU.
The port-level MTU command options configure the maximum payload MTU size for an Ethernet port, LAG, or SONET/SDH SONET path (sub-port) or TDM port/channel.

The default MTU values must be modified to ensure that packets are not dropped because of frame size limitations. The service MTU must be less than or equal to both the SAP port MTU and the SDP path MTU values. When an SDP is configured on a network port using default port MTU values, the operational path MTU can be less than the service MTU. In this case, enter the show service sdp command to check the operational state. If the operational state is down, then modify the MTU value accordingly.

Configuration example

In order for the maximum length service frame to successfully travel from a local ingress SAP to a remote egress SAP, the MTU values configured on the local ingress SAP, the SDP (GRE or MPLS), and the egress SAP must be coordinated to accept the maximum frame size the service can forward. For example, the targeted MTU values to configure for a distributed Epipe service (ALA-A and ALA-B) are shown in MTU configuration example.

Because ALA-A uses Dot1q encapsulation, the SAP MTU must be set to 1518 to be able to accept a 1514 byte service frame (see MTU default values for MTU default values). Each SDP MTU must be set to at least 1514 as well. If ALA-A’s network port (2/1/1) is configured as an Ethernet port with a GRE SDP encapsulation type, then the MTU value of network ports 2/1/1 and 3/1/1 must each be at least 1556 bytes (1514 MTU + 28 GRE/Martini + 14 Ethernet). Finally, the MTU of ALA-B’s SAP (access port 4/1/1) must be at least 1514, as it uses null encapsulation.

MTU configuration example values shows example MTU configuration values.

Table 2. MTU configuration example values
	ALA-A		ALA-B
	Access (SAP)	Network	Network	Access (SAP)
Port (slot/MDA/port)	1/1/1	2/1/12	3/1/1	4/1/1
Mode or ECAP-type	dot1q	network	network	null
MTU	1518	1556	1556	1514