7705 SAR Interfaces
98
Interface Configuration Guide
3HE 11011 AAAC TQZZA Edition: 01
For an Ethernet Adapter card, all frames received at an ingress network or access
port are policed against the MRU for the ingress adapter card, regardless of the
configured MTU. Any frames larger than the MRU are discarded and the “Too Long
Frame” and “Error Stats” counters in the port statistics display are incremented.
At network egress, frames are checked against the configured port MTU. If the frame
exceeds the configured port MTU and the DF bit is set, then the “MTU Exceeded”
discard counter will be incremented on the ingress IP interface statistics display, or
on the MPLS interface statistics display if the packet is an MPLS packet.
For example, on adapter cards that do not support an MTU greater than 2106 bytes,
fragmentation is not supported for frames greater than the maximum supported MTU
for that card (that is, 2106 bytes). If the maximum supported MTU is exceeded, the
following occurs.
• An appropriate ICMP reply message (Destination Unreachable) is generated by
the 7705 SAR. The router ensures that the ICMP generated message cannot be
used as a DOS attack (that is, the router paces the ICMP message).
• The appropriate statistics are incremented.
Jumbo frames offer better utilization of an Ethernet link because as more payload is
packed into an Ethernet frame of constant size, the ratio of overhead to payload is
minimized.
From the traffic management perspective, large payloads may cause long delays, so
a balance between link utilization and delay must be found. For example, for ATM
VLLs, concatenating a large number of ATM cells when the MTU is set to a very high
value could generate a 9-kbyte ATM VLL frame. Transmitting a frame that large
would take more than 23 ms on a 3-Mb/s policed Ethernet uplink.
Dot1q 9728
QinQ 9732
Table 11 Maximum MTU (or MRU) per Ethernet Encapsulation Type
Encapsulation Maximum MTU (bytes)
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