3Com corebuilder 3500 Implementation Manual page 229

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Table 33 Packet Filtering Opcodes (continued)

Requirements

Pushes the current top of the stack onto the stack (that

is, it reads the top of the stack and pushes the value

onto the stack, which effectively duplicates the item

currently on top of the stack). The size of the contents

of the stack determines the size of the push.

Use pushTop for each additional comparison you

intend to make with the current top of the stack. The

pushTop instruction makes a copy of the field more

efficiently than if you use a second pushField

instruction.

If you are writing a filter that is going to check the

same offset more than once, such as checking the

Ethernet type field to filter multiple protocols, use the

following guidelines. Assume that you want to filter

DEC LAT, IP, and ARP traffic on a port. Rather than use

multiple pushField .w 12 commands to look at the

12th offset where the Ethernet type field resides, use

multiple pushTop commands, as shown here:

Original Filter:

pushField.w 12

pushLiteral.w 0x6004

pushField.w 12

pushLiteral.w 0x0800

pushField.w 12

pushLiteral.w 0x0806

Shortened Filter:

PushField.w 12

pushLiteral.w 0x6004

pushLiteral.w 0x0800

pushLiteral.w 0x0806

The Packet Filtering Language