7705 SAR Interfaces
100
Interface Configuration Guide
3HE 11011 AAAC TQZZA Edition: 01
For example, if a packet arrives on a 7705 SAR-A and is to be forwarded to a TDM
port that has a maximum port MTU of 2090 bytes and a channel group configured for
PPP with the port MTU of 1000 bytes (PPP port MTU), the following may occur.
• If the arriving packet is 800 bytes, then forward the packet.
• If the arriving packet is 1400 bytes and the DF bit is 0, then forward the packet,
which will be fragmented to the PPP port MTU size.
• If the arriving packet is 2500 bytes and the DF bit is 0, then forward the packet,
which will be fragmented to the PPP port MTU size.
3.2.12.3.3 Multicast Support for Jumbo Frames
Jumbo frames are supported in a multicast configuration as long as all adapter cards
in the multicast group support jumbo frames. If an adapter card that does not support
jumbo frames is present in the multicast group, the replicated multicast jumbo frame
packet will be discarded by the fabric because of an MRU error of the fabric port (RX).
The multicast group replicates the jumbo frame for all adapter cards, regardless of
whether they support jumbo frames, only when forwarding the packet through the
fabric. The replicated jumbo frame packet is discarded on adapter cards that do not
support jumbo frames.
3.2.12.3.4 PMC Jumbo Frame Support
For the Packet Microwave Adapter card (PMC), ensure that the microwave hardware
installed with the card supports the corresponding jumbo frame MTU. If the
microwave hardware does not support the jumbo frame MTU, it is recommended that
the MTU of the PMC port be set to the maximum frame size that is supported by the
microwave hardware.
3.2.12.4 Default Port MTU Values
Table 12 displays the default and maximum port MTU values that are dependent
upon the port type, mode, and encapsulation type.
Note: The 7705 SAR now supports a lower IP MTU value of 128 bytes (from the original
512-byte minimum). The IP MTU is derived from the port MTU configuration for network
ports. This lower IP MTU is supported only on Ethernet encapsulated ports. Refer to the
7705 SAR Services Guide, “Bandwidth Optimization for Low-speed Links” for information.
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