Connectivity Guide

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.
NOTE: Dynamic ARP inspection (DAI) uses entries in the L2SysFlow CAM region, a sub-region of SystemFlow. One CAM
entry is required for every DAI-enabled VLAN. You can enable DAI on up to 16 VLANs on a system.
Configuring Dynamic ARP Inspection
To enable dynamic ARP inspection, use the following commands.
1. Enable DHCP snooping.
2. Validate ARP frames against the DHCP snooping binding table.
INTERFACE VLAN mode
arp inspection
To view entries in the ARP database, use the show arp inspection database command.
DellEMC#show arp inspection database
Protocol Address Age(min) Hardware Address Interface VLAN CPU
---------------------------------------------------------------------
Internet 10.1.1.251 - 00:00:4d:57:f2:50 Te 1/2 Vl 10 CP
Internet 10.1.1.252 - 00:00:4d:57:e6:f6 Te 1/1 Vl 10 CP
Internet 10.1.1.253 - 00:00:4d:57:f8:e8 Te 1/3 Vl 10 CP
Internet 10.1.1.254 - 00:00:4d:69:e8:f2 Te 1/5 Vl 10 CP
DellEMC#
To see how many valid and invalid ARP packets have been processed, use the show arp inspection statistics
command.
DellEMC#show arp inspection statistics
Dynamic ARP Inspection (DAI) Statistics
---------------------------------------
Valid ARP Requests : 0
304
Dynamic Host Configuration Protocol (DHCP)