3Com 4007, Implementation Manual, Page: 328 / 684

328 C
HAPTER
15: P
ACKET
F
ILTERING
reject 1 byte Conditionally rejects the packet being examined. Pops
a byte from the stack. If its value is non-zero, the
packet is rejected and filter evaluation ends
immediately; otherwise, the filter evaluation continues
with the next instruction.
Use
reject
with
and
and
or
operators when you
have sequential tests and you would like the filter to
reject a packet before the entire expression has been
evaluated. Using
reject
can significantly improve the
performance of certain types of filters. See
“Implementing Sequential Tests in a Packet Filter”
earlier in the chapter for more information.
shiftl (shift left) 1 byte Pops two values from the stack and shifts the first
operand left by the number of bits specified by the
second operand. Bits shifted out of the left side of the
operand are discarded, and zeros are shifted in from
the right. The resulting value is pushed back onto the
stack. The contents of the top of the stack determines
the size of the first operand and the size of the result.
The second operand is always 1 byte and only the low
5 bits of the byte are used as the shift count.
shiftr (shift right) 1 byte Pops two values from the stack and shifts the first
operand right by the number of bits specified by the
second operand. Bits shifted out of the right side of the
operand are discarded, and zeros are shifted in from
the left. The resulting value is pushed back onto the
stack. The contents of the top of the stack determines
the size of the first operand and the size of the result.
The second operand is always 1 byte and only the low
5 bits of the byte are used as the shift count.
Table 66 Packet Filtering Opcodes (continued)
Opcode
Memory
Requirements
Description