3Com 4007, Руководство по внедрению, Страница: 307 / 684

Packet Filtering Overview 307
Figure 24 Ethernet and FDDI Packet Fields
When Is a Filter
Applied? — Paths
Packets travel on many different paths through the switch. You can
control to which path a filter is applied.
Input Packet Filtering: Receive Path
Input packet filtering applies to packets immediately upon reaching the
switch port, before they reach the switch’s internal forwarding processing
(
receive path ). Because the packets never enter the switch, the switch
itself is protected against an external attack.
Output Packet Filtering: Transmit Path
Output packet filtering applies to packets after they have been through
the switch’s internal forward processing (
transmit path ).
Internal Packet Filtering: Receive Internal Path
Internal packet filtering applies to packets intended for the switch itself
(such as pings, Telnet packets, and so forth) on the
receive internal path .
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