The service MTU (Maximum Transmit Unit) in Nokia SR is determined by different criteria in the routers. This article explains why there is different MTU between a network port, spoke-SDP (Service Distribution Point) and an Access port in a service and their relations.
In this example an Epipe service is created with an spoke-SDP and a SAP (Service Access Point). In the example we will aim to use the maximum MTU-size is that is configurable in the router.
The first step is the network port. The default value for and network port is 9212, witch is the maximum MTU for a network port. The network MTU can be changed under the physical interface if needed.
SDP
When MPLS is used for transport the maximum MTU size for an SDP is 9190. SDP MTU indicates the maximum length of a packet that a customer can send over the SDP.
22 bytes is removed from the network MTU due to transportation overhead. The overhead contains information about how the frame will be forwarded and is explained in the table below.
| Bytes | Function |
|---|---|
| 14 | Ethernet header |
| 4 | MPLS Transport label |
| 4 | MPLS Service label |
SDP MTU indicates the maximum MTU of the payload that can be transmitted through the SDP.
Service MTU
Service MTU is the maximum length allowed for a packets payload inside of the service. The service MTU must be smaller, or the same value as the spoke-sdp. Standard MTU of an Epipe is 1514 and can be changed to a maximum value of 9212. If an higher value is selected than the SDP MTU, an error flag “PathMTUTooSmall” will appear and the spoke-sdp and Epipe service will change to operational status down.
Access port
An access port standard MTU is 1514, the size of an standard Ethernet frame. This can be increased to a maximum value of 9212. The SAP inside the service will be operationally up as long as the access port MTU is the same or greater than the service MTU.
The service and SAP will be operationally up if the access port is configured with MTU 9212 and the service-mtu at 1514, but if packets at SAP-ingress exceeds 1514 bytes there will be dropped by the service.
If an encapsulation is used, for example a VLAN, to determine witch traffic that will belong to an service, an additional 4 bytes must be added to the access port MTU. This 4 bytes are later removed at SAP ingress and are not carried in the service MTU. If Q-in-Q is used, 8 bytes must be added to the access port MTU.