3Com 2200 manual Packet Filter Operands Description Opcode Packet field, PushField, PushLiteral

Creating Packet Filters 12-7

The Ethernet and FDDI packet fields in Figure 12-1 are used as operands in the packet filter. The two simplest operands are described in Table 12-3.

Table 12-3 Packet Filter Operands

Operand	Description	Opcode
packet field	A field in the packet that can reside at any offset.	pushField
	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	pushLiteral
	field. As with a field, a constant can be 1, 2, 4, or 6
	bytes long.

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 12-4.

Table 12-4 Packet Filter Operators

Operator	Result	Opcode
equal	true if operand 1 = operand 2	eq
not equal	true if operand 1 ≠ operand 2	ne
less than	true if operand 1 < operand 2	lt
less than or equal	true if operand 1 ≤ operand 2	le
greater than	true if operand 1 > operand 2	gt
greater than or equal	true if operand 1 ≥ operand 2	ge
and	operand 1 bit-wise AND operand 2	and
or	operand 1 bit-wise OR operand 2	or
exclusive or	operand 1 bit-wise XOR operand 2	xor
not	true if operand 1 = false	not
shift left	operand 1 SHIFT LEFT operand 2	shiftl
shift right	operand 1 SHIFT RIGHT operand 2	shiftr

The operators and, or, and exclusive or are bit-wise operators. Each bit of

the operands is logically compared to produce the resulting bit.