Dell Force10 S25-01-GE-24V FTOS 8.4.2.6 Configuraiton Guide for the E-Series T - Page 341

Drop DHCP packets on snooped VLANs only Binding table entries are deleted when a lease expires, or the relay agent encounters a DHCPRELEASE. Starting with FTOS Release 8.2.1.1, line cards maintain a list of snooped VLANs. When the binding table fills, DHCP packets are dropped only on snooped-VLANs, while such packets will be forwarded across non-snooped VLANs. Since 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. Once the table usage falls below the max limit of 4000 entries, new IP address assignments are allowed. View the number of entries in the table with the show ip dhcp snooping binding command. This output displays the snooping binding table created using the ACK packets from the trusted port. Figure 14-7. Command example: show ip dhcp snooping binding Force10#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 Gi 0/2 10.1.1.252 00:00:4d:57:e6:f6 172800 D Vl 10 Gi 0/1 10.1.1.253 00:00:4d:57:f8:e8 172740 D Vl 10 Gi 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 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 accepts ARP request and replies from any device, and 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. A spoofed ARP message is one in which MAC address in the sender hardware address field and the IP address in the sender protocol field are strategically chosen by the attacker. For example, in an MITM attack, the attacker sends a client an ARP message containing the attacker's MAC address and the gateway's IP address. The client then thinks that the attacker is the gateway, and sends all internet-bound Dynamic Host Configuration Protocol | 341