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 435
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