The Maximum Transmission Unit (MTU) is the maximum packet size (excluding the data link header) that an interface can transmit. The Layer 2 VPN MTU defines the MTU for the customer facing interface. By default, the MTU size is set to 1500 bytes, which is the MTU size for standard Ethernet frames.
The frame size of the customer packet also includes the data link header and FCS field. The Ethernet data link header includes a minimum of a destination MAC address (six bytes), a source MAC address (six bytes), and an ethertype field (two bytes). The FCS field is four bytes. For a 1500-byte customer payload, the minimum customer frame size is 1518 bytes.
The Layer 2 frame, minus the FCS field, is encapsulated to form an MPLS (Multiprotocol Label Switching) packet to be transmitted to a Layer 2 VPN peer. This encapsulation adds another Ethernet header, an MPLS shim header, and a FCS field. The MPLS shim header usually includes two four-byte MPLS labels. Therefore, this encapsulation adds a minimum of 26 bytes.
The total frame size includes the customer payload, the Ethernet header on the customer facing interface (minimum 14 bytes), and the MPLS encapsulation (minimum 26 bytes). For a 1500-byte customer payload, the minimum Layer 2 VPN frame size is 1540 bytes.
The total frame size is restricted by the jumbo frame size. The maximum jumbo frame size that can be configured is 9216 bytes. If the customer payload in the above example is increased to 9176 bytes, the resulting Layer 2 VPN encapsulated frame size is 9216 bytes. Therefore, the maximum practical value of the Layer 2 VPN MTU is 9176 bytes.
There are additional considerations that can lower the maximum practical value of the Layer 2 VPN MTU setting. If the Layer 2 VPN is configured to include the VLAN (Virtual LAN) tag of the customer interface, this increases the total frame size by four bytes. If the MPLS interface used to transmit the encapsulated Layer 2 VPN packet is a tagged VLAN, this also increases the total frame size by four bytes. If either of these configuration options is present, the maximum practical Layer 2 VPN MTU size is reduced accordingly.
The Layer 2 VPN MTU value is signaled to peers as part of the VC FEC information during PW setup. The local value must match the value received from the PW peer. If the two values do not match, the PW is not established.
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