Mtu; Raw And Tagged Modes - Alcatel-Lucent 7705 Service Manual

Ethernet VLL (Epipe) Services

MTU

The largest maximum transmission unit (MTU) supported on an Ethernet port is 2102 bytes
for null encapsulated ports and 2106 bytes for dot1q encapsulated ports. The default MTU
for a Gigabit Ethernet port is 1572 bytes; whereas, the default MTU for a 10/100 Ethernet
port is 1514 or 1518 bytes, depending on the encapsulation type setting (null or dot1q).
Network-facing Ethernet ports must support a larger MTU than access-facing Ethernet ports
in order to account for the pseudowire headers that are added to the access Ethernet frames.
The following list gives the worst-case MTU sizes for Ethernet VLLs over Ethernet port(s)
under various configurations, where the worst case is the largest MTU size required in order
to carry the payload:
Note: Since it is not practical to split a Layer 2 Ethernet frame into smaller frames, the
access port (SAP) MTU must be smaller than the service and network port MTU. If the
access port MTU is larger than the tunnel MTU, the Ethernet VLL does not come into service
and remains in the inoperative state. See
for VLL service.

Raw and Tagged Modes

An Ethernet PW operates in one of two modes: raw or tagged. Raw and tagged modes relate
to the way the router handles VLAN tags embedded in the header of an Ethernet frame. Both
modes are supported by the 7705 SAR.
Raw and tagged modes are configured using the
the
tagged mode, choose
VLAN tags can provide service-affecting information about a frame. Service-affecting
means that information in the tag affects the forwarding decisions that are made to route the
packet. The port connected to the attachment circuit (AC) can be configured for
dot1q
tag received at the SAP (from the AC) as not service affecting; when configured for dot1q,
received tags are service affecting.
Page 162
• Access, null mode: 1514 bytes (1500 bytes payload)
• Access, dot1q mode: 1518 bytes (1500 bytes payload)
• Network, null mode: 1572 bytes (1514 bytes payload)
• Network, dot1q mode: 1572 bytes (1518 bytes payload)
command. To configure raw mode, choose the
spoke-sdp
.
vlan
operation. When the port is configured for
MTU Settings on page 186
vc-type {ether|vlan}
ether
, the 7705 SAR treats any attached
null
7705 SAR OS Services Guide
for information on MTU
parameter under
option; to configure
or
null