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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 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 L2Protocol + 13 additional L2Protocol + 15 L2SystemFlow + 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
Examples of Viewing the ARP Information
To view entries in the ARP database, use the show arp inspection database command.
Dell#show arp inspection database
Protocol Address Age(min) Hardware Address Interface VLAN CPU
Dynamic Host Configuration Protocol (DHCP)
335