Mtu Configuration Guidelines; Ip Fragmentation - Alcatel-Lucent 7705 Interface Configuration Manual

Port Features

MTU Configuration Guidelines

Because of all the services overhead (that is, pseudowire/VLL, MPLS tunnel, dot1q and
dot1p overhead), it is crucial that configurable variable frame size be supported for end-to-
end service delivery.
Observe the following general rules when planning your service and physical Maximum
Transmission Unit (MTU) configurations.
For information on configuring the MTU for access and network ports, SDP path, and
service, refer to the 7705 SAR OS Services Guide.
For the Ethernet Adapter card, all received frames on an ingress network or access port are
policed against 1576 bytes (1572 + 4 bytes of FCS), regardless of the port MTU. Any frames
longer than 1576 bytes are discarded and the "Too Long Frame" and "Error Stats" counters
in the port statistics menu are incremented.
At network egress, Ethernet frames are policed against the configured port MTU. If the
frame exceeds the configured port MTU, the "interface out discards" counter in the port
statistics menu is incremented.

IP Fragmentation

IP fragmentation is used to fragment a packet that is larger than the MTU of the egress
interface, so that the packet can be transported over that interface.
The router fragments or discards the IP packets based on whether the DF (Do not fragment)
bit is set in the IP header. If the packet that exceeds the MTU cannot be fragmented, the
packet is discarded and an ICMP message "Fragmentation Needed and Don't Fragment was
Set" is sent back to the source IP address.
Page 40
• The 7705 SAR must contend with MTU limitations at many service points. The
physical (access and network) port, service, and SDP MTU values must be
individually defined.
• The ports that will be designated as network ports intended to carry service traffic
must be identified.
• MTU values should not be modified frequently.
• MTU values must conform to both of the following conditions:
→ the service MTU must be less than or equal to the SDP path MTU
→ the service MTU must be less than or equal to the access port (SAP) MTU
7705 SAR OS Interface Configuration Guide