This appendix discusses the sizes of packets for the various networks and protocols supported. Included are the following sections:
For the purposes of this discussion, the packets that the routers handle consist of user data and header information.
The amount of user data within a packet is limited by the amount of header information on the packet. The amount of header information depends on (at least):
The following factors affect the size of the packet contents:
On any given network, the sum of the maximum data size together with header and trailer sizes will equal the network's maximum packet size. When routing between networks of different maximum packet size, fragmentation of the packet may result.
Given the information in the previous section, the maximum amount of
network layer data supported by each data link layer (network interface) can
be determined. Table 77 lists the default maximum packet sizes for common interface
types.
Table 77. Default Network-Specific Maximum Packet Size
| Network Type (Data Link) | Network Layer max packet size (bytes) | Length of Network Header | Information Trailer |
|---|---|---|---|
| Token-Ring 4 Mbps | 2052 | 22 | 0 |
| Token-Ring 16 Mbps | 2052 | 22 | 0 |
| Ethernet | 1500 | 18 | 4 |
| PPP | 2046 | 2 | 0 |
| Frame Relay | 2048* | variable | 2 |
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| *: | For Frame Relay interfaces, you configure the maximum frame size not the network layer maximum packet size. To determine the maximum network layer packet size for a protocol, see the description of the set frame-size command in the chapter entitled Configuring and Monitoring Frame Relay Interfaces in Nways Multiprotocol Access Services Software User's Guide . |
| 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.
This section explains the protocol-specific size limits.
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.
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.