3Com 2200 manual Packet Filter, Examples, Packet filter concepts

Packet Filter Examples A-9

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

Destination This filter operates on the destination address field of a frame. It Address Filter 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 pushLiteral.l 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

Source Address This filter operates on the source address field of a frame. It allows packets Filter 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 pushLiteral.l 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

Length Filter 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 pushLiteral.w instruction.

name	“Forward	< 400”
pushField.w	12	#	Get length		field
pushLiteral.w	400	#	Load	length	limit
lt		#	Check	for frame length < limit