Alcatel-Lucent 7705 - Table 28: MTU Points and Descriptions

VLL Services
7705 SAR OS Services Guide Page 187
Table 29 aids in calculating MTU values for various configurations and operating scenarios.
Table 28: MTU Points and Descriptions
Key MTU Point Description
1 Access port
MTU
The access port MTU value is a configurable value that accounts for the L2 header and
the payload. The default access port MTU value for the following Fast Ethernet port
SAP encapsulations is:
• Null: 1514 bytes (payload = 1500 bytes, L2 header = 14 bytes)
• dot1q: 1518 bytes (payload = 1500 bytes, L2 header = 18 bytes)
2 SAP MTU The SAP MTU value is not a configurable value. It is set at the SAP by the 7705 SAR
operating system. It defines the service payload capability of the service and is
automatically set to be the same value as the access port MTU.
3 Service MTU The service MTU value is a configurable value and is the same size as the VLL payload.
The service MTU is sometimes called the VC-type MTU in the 7705 SAR
documentation set. In Figure 34, NP stands for network processor.
For CESoPSN with CAS service, ensure that the service MTU is set to a value large
enough to account for the extra bytes appended to the packet payload for CAS bits. See
Structured T1/E1 CES with CAS on page 152 for more information.
4 Path MTU The path MTU is configured at the SDP. It is the maximum that the SDP can transmit
without rejecting and discarding the packet. The path MTU value is derived from the
network port MTU value by subtracting the Layer 2 and Layer 2.5 overhead values (for
MPLS) and the Layer 2 and Layer 3 overhead values (for GRE).
If the network port SDP binding is Ethernet, then the following equations hold:
• For MPLS:
Path MTU = Port MTU - (Ethernet header [14 bytes or 18 bytes] + Tunnel
header + PW header)
•For GRE:
Path MTU = Port MTU - (Ethernet header [14 bytes or 18 bytes] + IP header [20
bytes] + Tunnel header [4 bytes] + PW header [4 bytes])
5 Network port
MTU
The network port MTU is a configurable value equal to the payload plus all headers (L2,
IP (for GRE), tunnel and PW), up to the maximum supported value (hardware limit) of
1572 bytes.
Note: Ethernet QinQ is not supported in this release and is shown in this table for reference
purposes only.