MTU configuration guidelines

MTU default and maximum values lists the default and maximum MTU values for Fast Ethernet ports, cellular ports, and the WLAN interface.

For information about how to modify the MTU defaults, see "Modifying MTU defaults" in the 7450 ESS, 7750 SR, 7950 XRS, and VSR Interface Configuration Guide.

The cellular port IP layer MTU is derived from the PDN router interface that is configured for the cellular port. By default, the PDN router interface MTU is not set. To operate the cellular interface without failures, an MTU value that is less than or equal to 1486 bytes must be configured (using the ip-mtu command) for the PDN router interface. For information about configuring the PDN router interface, see "PDN router interface command descriptions" in the 7705 SAR-Hm and SAR-Hmc Main Configuration Guide.

Mobile networks often require a strict IP layer MTU for the LTE interface that is less than or equal to1486 bytes. Consult with the cellular service provider about the correct IP layer MTU value to set for the associated PDN router interface.

The SAP MTU settings must also correctly account for the PDN router interface IP layer MTU, as services that are transported over the cellular interface are impacted by this configuration.

For example, if a cellular provider allows an IP layer MTU of 1486 bytes, the following calculations and values must be considered when setting up services over a cellular port.

For BGP and T-LDP protocols, the MTU of protocol packets must be set to 1486 bytes or less. For information about BGP path MTU discovery and LDP path MTU discovery, see the path-mtu-discovery command in the 7450 ESS, 7750 SR, 7950 XRS, and VSR Classic CLI Command Reference Guide.

For Layer 3 services over a VPRN service using GRE transport, the SAP MTU must be set as follows:

• SAP MTU = {1486 bytes - (GRE packet overhead) - (VPRN service label)}

• SAP MTU = {1486 bytes- (24 bytes) - (4 bytes)}

• SAP MTU = 1458 bytes

For Layer 3 services over a VPRN service using GRE transport with NGE enabled, the SAP MTU must be set as follows:

• SAP MTU = {1486 bytes - (GRE packet overhead) - (VPRN service label) - (NGE overhead)}

• SAP MTU = {1486 bytes - (24 bytes) - (4 bytes) - (77 bytes)}

• SAP MTU = 1381 bytes

For Layer 2 services over a VPLS service using GRE transport, the SAP (port) MTU must be set as follows:

• SAP MTU = {1486 bytes - (GRE packet overhead) - (VPLS service label)}

• SAP MTU = {1486 bytes - (24 bytes) - (4 bytes)}

• SAP MTU = 1458 bytes

For Layer 2 services over a VPLS service using GRE transport with NGE enabled, the SAP MTU must be set as follows:

• SAP MTU = {1486 bytes - (GRE packet overhead) - (VPLS service label) - (NGE overhead)}

• SAP MTU = {1486 bytes - (24 bytes) - (4 bytes) - (77 bytes)}

• SAP MTU = 1381 bytes

For Layer 2 services using an Epipe VLL/VPWS service with a control word, an additional 4 bytes of overhead is required for the control word. Therefore, the following SAP (port) MTU must be set as follows:

• SAP MTU = {1486 bytes - (GRE packet overhead) - (VLL service label) - (CTL word)}

• SAP MTU = {1486 bytes - (24 bytes) - (4 bytes) - (4 bytes)}

• SAP MTU = 1454 bytes

For Layer 2 services using an Epipe VLL/VPWS service with a control word and NGE enabled, an additional 4 bytes of overhead is required for the control word and 4 bytes of NGE overhead is added. Therefore, the following SAP (port) MTU must be set as follows:

• SAP MTU = {1486 bytes - (GRE packet overhead) - (VLL service label) - (CTL word) – (NGE overhead)}

• SAP MTU = {1486 bytes - (24 bytes) - (4 bytes) - (4 bytes) - (81 bytes)}

• SAP MTU = 1373 bytes

The SAP MTU of Layer 2 services can be increased to accommodate larger packets that are closer to the Ethernet port maximum MTU value by using GRE SDP fragmentation and reassembly. See "GRE SDP tunnel fragmentation and reassembly" in the 7705 SAR-Hm and SAR-Hmc Main Configuration Guide.

The WLAN port MTU value is set to 1500 bytes and cannot be changed. Since cellular ports have a lower MTU with a maximum of 1486 bytes, and WLAN traffic from stations connected to the WLAN AP is carried over an IP/MPLS service that adds additional overhead to traffic traveling over cellular ports, the operator must understand the requirements of the MTU for their applications in order to successfully use the WLAN interface.

For example, when the WLAN interface AP is connected to the Nokia WLAN GW using an Epipe service, there is at most 1454 bytes available to carry a Layer 2 packet for the WLAN AP packet that includes a Layer 2 header of 14 bytes (see MTU considerations over a cellular port for information about the SAP MTU of an Epipe service over a cellular port). In order to successfully send these packets over a cellular port without further modification, the MTU of the IP payload in the WLAN AP Layer 2 packet must be restricted to 1440 bytes.

The MTU of the WLAN interface can be handled in one of two ways:

• by modifying the MTU value on clients that are connecting to the WLAN AP such that they send traffic that conforms to the service MTU of the IP/MPLS transport service, minus the 14 byte Layer 2 overhead

• by configuring GRE SDP fragmentation and reassembly on the node to allow packets that require an MTU greater than that available on the cellular interface to be fragmented and reassembled when carried over the cellular interface