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Chapter 27 IP Source Guard
MGS3520 Series User’s Guide
237
Trusted ports are connected to DHCP servers or other switches. The Switch discards DHCP packets
from trusted ports only if the rate at which DHCP packets arrive is too high. The Switch learns
dynamic bindings from trusted ports.
Note: The Switch will drop all DHCP requests if you enable DHCP snooping and there are
no trusted ports.
Untrusted ports are connected to subscribers. The Switch discards DHCP packets from untrusted
ports in the following situations:
• The packet is a DHCP server packet (for example, OFFER, ACK, or NACK).
• The source MAC address and source IP address in the packet do not match any of the current
bindings.
• The packet is a RELEASE or DECLINE packet, and the source MAC address and source port do not
match any of the current bindings.
• The rate at which DHCP packets arrive is too high.
27.1.1.2 DHCP Snooping Database
The Switch stores the binding table in volatile memory. If the Switch restarts, it loads static
bindings from permanent memory but loses the dynamic bindings, in which case the devices in the
network have to send DHCP requests again. As a result, it is recommended you configure the DHCP
snooping database.
The DHCP snooping database maintains the dynamic bindings for DHCP snooping and ARP
inspection in a file on an external TFTP server. If you set up the DHCP snooping database, the
Switch can reload the dynamic bindings from the DHCP snooping database after the Switch
restarts.
You can configure the name and location of the file on the external TFTP server. The file has the
following format:
Figure 137
DHCP Snooping Database File Format
The <initial-checksum> helps distinguish between the bindings in the latest update and the
bindings from previous updates. Each binding consists of 72 bytes, a space, and another checksum
that is used to validate the binding when it is read. If the calculated checksum is not equal to the
checksum in the file, that binding and all others after it are ignored.
<initial-checksum>
TYPE DHCP-SNOOPING
VERSION 1
BEGIN
<binding-1> <checksum-1>
<binding-2> <checksum-1-2>
...
...
<binding-n> <checksum-1-2-..-n>
END
Содержание MGS3520 Series
Страница 18: ...18 PART I User s Guide ...
Страница 71: ...71 PART II Technical Reference ...
Страница 118: ...Chapter 9 VLAN MGS3520 Series User s Guide 118 Figure 58 Advanced Application VLAN Port Based VLAN Setup Port Isolation ...
Страница 189: ...Chapter 22 Policy Rule MGS3520 Series User s Guide 189 Figure 107 Policy Example EXAMPLE ...
Страница 408: ...Appendix B Common Services MGS3520 Series User s Guide 408 ...
Страница 420: ...Appendix D Legal Information MGS3520 Series User s Guide 420 Environmental Product Declaration ...