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22-6
Catalyst 3560 Switch Software Configuration Guide
OL-8553-06
Chapter 22 Configuring Dynamic ARP Inspection
Configuring Dynamic ARP Inspection
Dynamic ARP Inspection Configuration Guidelines
These are the dynamic ARP inspection configuration guidelines:
• Dynamic ARP inspection is an ingress security feature; it does not perform any egress checking.
• Dynamic ARP inspection is not effective for hosts connected to switches that do not support
dynamic ARP inspection or that do not have this feature enabled. Because man-in-the-middle
attacks are limited to a single Layer 2 broadcast domain, separate the domain with dynamic ARP
inspection checks from the one with no checking. This action secures the ARP caches of hosts in the
domain enabled for dynamic ARP inspection.
• Dynamic ARP inspection depends on the entries in the DHCP snooping binding database to verify
IP-to-MAC address bindings in incoming ARP requests and ARP responses. Make sure to enable
DHCP snooping to permit ARP packets that have dynamically assigned IP addresses. For
configuration information, see
Chapter 21, “Configuring DHCP Features and IP Source Guard.”
When DHCP snooping is disabled or in non-DHCP environments, use ARP ACLs to permit or to
deny packets.
• Dynamic ARP inspection is supported on access ports, trunk ports, EtherChannel ports, and private
VLAN ports.
Note Do not enable Dynamic ARP inspection on RSPAN VLANs. If Dynamic ARP inspection is
enabled on RSPAN VLANs, Dynamic ARP inspection packets might not reach the RSPAN
destination port.
• A physical port can join an EtherChannel port channel only when the trust state of the physical port
and the channel port match. Otherwise, the physical port remains suspended in the port channel. A
port channel inherits its trust state from the first physical port that joins the channel. Consequently,
the trust state of the first physical port need not match the trust state of the channel.
Conversely, when you change the trust state on the port channel, the switch configures a new trust
state on all the physical ports that comprise the channel.
• The operating rate for the port channel is cumulative across all the physical ports within the channel.
For example, if you configure the port channel with an ARP rate-limit of 400 pps, all the interfaces
combined on the channel receive an aggregate 400 pps. The rate of incoming ARP packets on
EtherChannel ports is equal to the sum of the incoming rate of packets from all the channel
members. Configure the rate limit for EtherChannel ports only after examining the rate of incoming
ARP packets on the channel-port members.
The rate of incoming packets on a physical port is checked against the port-channel configuration
rather than the physical-ports configuration. The rate-limit configuration on a port channel is
independent of the configuration on its physical ports.
If the EtherChannel receives more ARP packets than the configured rate, the channel (including all
physical ports) is placed in the error-disabled state.
• Make sure to limit the rate of ARP packets on incoming trunk ports. Configure trunk ports with
higher rates to reflect their aggregation and to handle packets across multiple dynamic ARP
inspection-enabled VLANs. You also can use the ip arp inspection limit none interface
configuration command to make the rate unlimited. A high rate-limit on one VLAN can cause a
denial-of-service attack to other VLANs when the software places the port in the error-disabled
state.
• When you enable dynamic ARP inspection on the switch, policers that were configured to police
ARP traffic are no longer effective. The result is that all ARP traffic is sent to the CPU.