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Note: You can change the maximum packet size for interfaces other than Ethernet.
Use the network command from the Config> prompt to access the
interface’s configuration commands.
The maximum packet size is the maximum amount of data the protocol forwarder
can pass to the device.
Note: These numbers correspond to the MTUs in 4.2 BSD UNIX.
For an IP packet, this includes the IP header, the UDP or TCP header, and all data.
The packet size in use is displayed when the router’s GWCON memory command
is used. The “Pkt” size is the Network layer packet size. The Hdr (header) and Tlr
(trailer) sizes depend on the networks and their network interfaces.
Protocol-Specific Size Limits
This section explains the protocol-specific size limits.
IP Packet Lengths
The IP protocol specifications do not require a host IP implementation to accept IP
packets of more than 576 octets; however, router IP implementations must
accommodate IP packets of any length up to the limits imposed by the
network-specific packets in use.
Furthermore, router IP performs transparent fragmentation and reassembly of
packets that would otherwise exceed network-specific length restrictions, as
required by the IP specification.
Packet size mismatches do not cause connectivity problems. However, fragment
reassembly does pose a performance penalty, so fragmentation should be avoided
whenever possible.
Changing Maximum Packet Sizes
Normally, the router automatically sets the maximum network layer packet size to
the size of the largest possible packet on all the connected networks. It then adds
any headers and trailers required by the networks to determine the internal buffer
size, which is larger than the network layer size.
Some networks (Token-Ring 4 Mbps and Token-Ring 16 Mbps) allow you to
configure maximum packet sizes. Configuring maximum packet sizes affects the
size of buffers used on the router and this in turn affects the number of buffers
available for a given memory size. Routers automatically determine what size buffer
it is going to need. You can change the maximum Network layer packet size that
the router handles by using the set packet-size command; however, do not use this
command unless specifically directed to by Customer Service.
Packet Sizes
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MRS V3.2 Protocol Config Ref Vol 2