H3C Technologies H3C S5560 Series Switches - Manual (Page 85)

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Figure 26 Trusted and untrusted ports
In a cascaded network as shown in Figure 27 , to save system resources, you can enable only the
untrusted ports directly connected to the DHCP clients to record DHCP snooping entries.
Figure 27 Trusted and untrusted ports in a cascaded network
DHCP snooping support for Option 82
Option 82 records the location information about the DHCP client so the administrator can locate the
DHCP client for security and accounting purposes. For more information about Option 82, see " Relay
agent option (Option 82) ."
DHCP snooping uses the same strategies as the DHCP relay agent to handle Option 82 for DHCP
request messages, as shown in Table 4 . If a response returned by the DHCP server contains Option 82,
DHCP snooping removes Option 82 before forwarding the response to the client. If the response contains
no Option 82, DHCP snooping forwards it directly.
DHCP snooping
Switch A
DHCP snooping
Switch C
DHCP client
Host D
DHCP client
Host C
DHCP client
Host B
DHCP server
Device
DHCP snooping
Switch B
GE1/0/4 GE1/0/2
GE1/0/3
GE1/0/1 GE1/0/2
GE1/0/3
GE1/0/4 GE1/0/2 GE1/0/1
GE1/0/3
GE1/0/1
DHCP client
Host A
GE1/0/1
Untrusted ports enabled to record snooping entries
Untrusted ports disabled from recording snooping entries
Trusted ports