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Appendix b. packet sizes general issues, Network-specific size limits, Default network-specific maximum packet size – IBM SC30-3865-04 User Manual

Page 465: Appendix b. packet sizes, General issues

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Appendix B. Packet Sizes

This appendix discusses the sizes of packets for the various networks and protocols
supported. Included are the following sections:

v

General Issues

v

Network-Specific Size Limits

v

Protocol-Specific Size Limits

v

Changing Maximum Packet Sizes

General Issues

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):

v

The network-types over which the packet must travel.

v

The protocols in use on these networks.

The following factors affect the size of the packet contents:

v

Length of the Data-Link header information that the current network type and
interface require the packet to have.

v

Length of the trailer information (if any) that the current network type and
interface require the packet to have.

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.

Network-Specific Size Limits

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

*: 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 entitledConfiguring and Monitoring Frame Relay Interfaces in
Software User’s Guide .

© Copyright IBM Corp. 1994, 1998

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