3Com CoreBuilder 3500, Implementation Manual

Page: 210 / 592

210 C
HAPTER
10: P
ACKET
F
ILTERING
Packet Filtering
Overview
The packet filtering feature allows a switch to make a permit-or-deny
decision for each packet based on the packet contents. Use packet filters
to control traffic on your network segments to:
■
Improve LAN performance.
■
Implement LAN security controls.
■
Shape traffic flow to emulate virtual LAN (VLAN) behavior. See
Chapter 9.
What Can You Filter? Before you create a packet filter, you must decide which part of the
packet you want to use for your filtering decisions. You can filter on any
data in the first 64 bytes of the
frame . You can filter Ethernet, Fast
Ethernet, Fiber Distributed Data Interface (FDDI), or Gigabit Ethernet
frames by the destination address, source address, type, length, or any
attribute within the first 64 bytes. Keep in mind that the offsets may
differ between FDDI and Ethernet, so the same filter may not work on all
interfaces. Ethernet and FDDI packet fields are shown in Figure 35.
Figure 35 Ethernet and FDDI Packet Fields
Destination
Address
(6 octets)
Source Address
(6 octets)
Type/Length
(Ethernet Type field if > 1500;
802.3 Length field if - 1500)
0 6 12 14 25
octets
Ethernet Packet
Internal Packet Data
64
Filter First 64 Bytes of Frame
0 6 12 25
octets
FDDI Packet
Destination
Address
(6 octets)
Source
Address
(6 octets)
Internal Packet Data
Filter First 64 Bytes of Frame
64