Dell Force10 E300, Configuration Manual, Page: 326 / 1262

326 | Dynamic Host Configuration Protocol
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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: DAI uses entries in the L2SysFlow CAM region, a sub-region of SystemFlow. One CAM entry is
required for every DAI-enabled VLAN, and you can enable DAI on up to 16 VLANs on a system. However,
the ExaScale default CAM profile allocates only 9 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, and L2SystemFlow has 15 entries. Six L2SystemFlow entries are used by
Layer 2 protocols, leaving 9 for DAI. L2Protocol can have a maximum of 100 entries, and this region must be
expanded 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 7 more entries; in this case,
reconfigure the SystemFlow region for 122 entries:
layer-2 eg-acl
value
fib
value
frrp
value
ing-acl
value
learn
value
l2pt
value
qos
value
system-flow 122
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 9 are for DAI; to enable DAI on 16 VLANs, 7 more entries are required. 87 L2Pr 13
additional L2Pr 15 L2Syst 7 additional L2SystemFlow equals 122.
Step Task Command Syntax Command Mode
1 Enable DHCP Snooping.
2 Validate ARP frames against the
DHCP Snooping binding table.
arp inspection
INTERFACE VLAN