Dell S4048T, Configuration Manual

Page: 363 / 1277

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. However, the ExaScale default CAM profile allocates only nine entries to the
L2SysFlow region for DAI. You can configure 10 to 16 DAI-enabled VLANs by allocating more CAM space
to the L2SysFlow region before enabling DAI.
SystemFlow has 102 entries by default. This region is comprised of two sub-regions: L2Protocol and
L2SystemFlow. L2Protocol has 87 entries; L2SystemFlow has 15 entries. Six L2SystemFlow entries are used
by Layer 2 protocols, leaving nine for DAI. L2Protocol can have a maximum of 100 entries; you must
expand this region to capacity before you can increase the size of L2SystemFlow. This is relevant when
you are enabling DAI on VLANs. If, for example, you want to enable DAI on 16 VLANs, you need seven
more entries; in this case, reconfigure the SystemFlow region for 122 entries using the
layer-2 eg-acl
value fib value frrp value ing-acl value learn value l2pt value qos value
system-flow 122
command.
The logic is as follows:
L2Protocol has 87 entries by default and must be expanded to its maximum capacity, 100 entries, before
L2SystemFlow can be increased; therefore, 13 more L2Protocol entries are required. L2SystemFlow has 15
entries by default, but only nine are for DAI; to enable DAI on 16 VLANs, seven more entries are required.
87 L2Pr 13 additional L2Pr 15 L2Syst 7 additional L2SystemFlow equals 122.
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
Dynamic Host Configuration Protocol (DHCP) 363