3com 2200 User Manual

The following examples of using the packet filter language start with basic 
packet filter concepts.

This filter operates on the destination address field of a frame. It 
allows packets to be forwarded that are destined for stations with an 
Organizationally Unique Identifier (OUI) of 08-00-02. To customize this filter 
to another OUI value, change the literal value loaded in the last 

 

instruction. Note that the OUI must be padded with an additional 00 to fill 
out the literal to 4 bytes.

name

“Forward to 08-00-02”

pushField.l

0

# Get first 4 bytes of 
# destination address

pushLiteral.l

0xffffff00

# Set up mask to isolate first 
# 3 bytes

and

# Top of stack now has OUI

pushLiteral.l

0x08000200

# Load OUI value

eq

# Check for match

This filter operates on the source address field of a frame. It allows packets 
to be forwarded that are from stations with an OUI of 08-00-02. To 
customize this filter to another OUI value, change the literal value loaded in 
the last

 instruction. Note that the OUI must be padded with an 

additional 00 to fill out the literal to 4 bytes.

name

“Forward from 08-00-02”

pushField.l

6

# Get first 4 bytes of source 
# address

pushLiteral.l

0xffffff00

# Set up mask to isolate first 
# 3 bytes

and

# Top of stack now has OUI

pushLiteral.l

0x08000200

# Load OUI value

eq

# Check for match

This filter operates on the length field of a frame. It allows packets to be 
forwarded that are less than 400 bytes in length. To customize this filter to 
another length value, change the literal value loaded in the 

 

instruction.

name

“Forward < 400”

pushField.w

12

# Get length field

pushLiteral.w

400

# Load length limit

lt

# Check for frame length < limit