Reference Guide

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 version 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 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.
FTOS#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 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.
A spoofed ARP message is one in which the 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 packets to it. Likewise, the attacker sends the gateway an ARP
message containing the attacker’s MAC address and the client’s IP address. The gateway then thinks that the attacker
is the client and forwards all packets addressed to the client to it. As a result, the attacker is able to sniff all packets to
and from the client.
Other attacks using ARP spoofing include:
Broadcast An attacker can broadcast an ARP reply that specifies FF:FF:FF:FF:FF:FF as the gateway’s MAC
address, resulting in all clients broadcasting all internet-bound packets.
MAC flooding An attacker can send fraudulent ARP messages to the gateway until the ARP cache is
exhausted, after which, traffic from the gateway is broadcast.
Denial of service An attacker can send a fraudulent ARP messages to a client to associate a false MAC address
with the gateway address, which would blackhole all internet-bound packets from the client.
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