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 Reference" 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.