3Com LANPLEX 6000 User Manual page 197

The Ethernet and FDDI packet fields in Figure 14-1 are used as operands in
the packet filter. The two simplest operands are described in Table 14-3.
Table 14-3 Packet Filter Operands
Operand Description
packet field A field in the packet that can reside at any offset.
The size of the field can be 1, 2, 4, or 6 bytes.
Typically, you only specify a 6-byte field when you
want the filter to examine a 48-bit address.
constant A literal value to which you are comparing a packet
field. As with a field, a constant can be 1, 2, 4, or 6
bytes.
The operators that you specify in the packet filter allow the filter to make
a logical decision about whether the packet should be forwarded or
discarded. These operators are described in Table 14-4.
Table 14-4 Packet Filter Operators
Operator
equal
not equal
less than
less than or equal
greater than
greater than or equal
and
or
exclusive or
not
shift left
shift right
The operators and, or, and exclusive or are bit-wise operators. This means
that each bit of the operands are logically compared to produce the
resulting bit.
Result
true if operand 1 = operand 2
true if operand 1 ≠ operand 2
true if operand 1 < operand 2
true if operand 1 ≤ operand 2
true if operand 1 > operand 2
true if operand 1 ≥ operand 2
operand 1 bit-wise AND operand 2
operand 1 bit-wise OR operand 2
operand 1 bit-wise XOR operand 2
true if operand 1 = false
operand 1 SHIFT LEFT operand 2
operand 1 SHIFT RIGHT operand 2
Creating Packet Filters
Opcode to Use
pushField
pushLiteral
Opcode to Use
eq
ne
lt
le
gt
ge
and
or
xor
not
shiftl
shiftr
14-7