White Papers

EXEC Privilege mode
show ip dhcp snooping
View the DHCP snooping statistics with the show ip dhcp snooping command.
Dell#show ip dhcp snooping
IP DHCP Snooping : Enabled.
IP DHCP Snooping Mac Verification : Disabled.
IP DHCP Relay Information-option : Disabled.
IP DHCP Relay Trust Downstream : Disabled.
Database write-delay (In minutes) : 0
DHCP packets information
Relay Information-option packets : 0
Relay Trust downstream packets : 0
Snooping packets : 0
Packets received on snooping disabled L3 Ports : 0
Snooping packets processed on L2 vlans : 142
DHCP Binding File Details
Invalid File : 0
Invalid Binding Entry : 0
Binding Entry lease expired : 0
List of Trust Ports :Te 0/49
List of DHCP Snooping Enabled Vlans :Vl 10
List of DAI Trust ports :Te 0/49
Drop DHCP Packets on Snooped VLANs Only
Binding table entries are deleted when a lease expires or the relay agent encounters a DHCPRELEASE.
Line cards maintain a list of snooped VLANs. When the binding table fills, DHCP packets are dropped only on snooped
VLANs, while such packets are forwarded across non-snooped VLANs. Because DHCP packets are dropped, no new IP address
assignments are made. However, DHCP release and decline packets are allowed so that the DHCP snooping table can decrease
in size. After the table usage falls below the maximum limit of 4000 entries, new IP address assignments are allowed.
To view the number of entries in the table, use the show ip dhcp snooping binding command. This output displays the
snooping binding table created using the ACK packets from the trusted port.
Dell#show ip dhcp snooping binding
Codes : S - Static D - Dynamic
IP Address MAC Address Expires(Sec) Type VLAN Interface
================================================================
10.1.1.251 00:00:4d:57:f2:50 172800 D Vl 10 Te 0/2
10.1.1.252 00:00:4d:57:e6:f6 172800 D Vl 10 Te 0/1
10.1.1.253 00:00:4d:57:f8:e8 172740 D Vl 10 Te 0/3
10.1.1.254 00:00:4d:69:e8:f2 172740 D Vl 10 Te 0/50
Total number of Entries in the table : 4
Dynamic ARP Inspection
Dynamic address resolution protocol (ARP) inspection prevents ARP spoofing by forwarding only ARP frames that have been
validated against the DHCP binding table.
ARP is a stateless protocol that provides no authentication mechanism. Network devices accept ARP requests and replies from
any device. ARP replies are accepted even when no request was sent. If a client receives an ARP message for which a relevant
entry already exists in its ARP cache, it overwrites the existing entry with the new information.
The lack of authentication in ARP makes it vulnerable to spoofing. ARP spoofing is a technique attackers use to inject false
IP-to-MAC mappings into the ARP cache of a network device. It is used to launch man-in-the-middle (MITM), and denial-of-
service (DoS) attacks, among others.
Dynamic Host Configuration Protocol (DHCP)
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