Proxy Arp; Local Proxy Arp; Gratuitous Arp; Configuring Ipv4 - Cisco Nexus 7000 Series Configuration Manual

Chapter 2

Configuring IPv4

S e n d d o c u m e n t c o m m e n t s t o n e x u s 7 k - d o c f e e d b a c k @ c i s c o . c o m .

Proxy ARP

Proxy ARP enables a device that is physically located on one network appear to be logically part of a
different physical network connected to the same device or firewall. Proxy ARP allows you to hide a
device with a public IP address on a private network behind a router, and still have the device appear to
be on the public network in front of the router. By hiding its identity, the router accepts responsibility
for routing packets to the real destination. Proxy ARP can help devices on a subnet reach remote subnets
without configuring routing or a default gateway.
When devices are not in the same data link layer network but in the same IP network, they try to transmit
data to each other as if they are on the local network. However, the router that separates the devices does
not send a broadcast message because routers do not pass hardware-layer broadcasts and the addresses
cannot be resolved.
When you enable Proxy ARP on the device and it receives an ARP request, it identifies the request as a
request for a system that is not on the local LAN. The device responds as if it is the remote destination
for which the broadcast is addressed, with an ARP response that associates the device's MAC address
with the remote destination's IP address. The local device believes that it is directly connected to the
destination, while in reality its packets are being forwarded from the local subnetwork toward the
destination subnetwork by their local device. By default, Proxy ARP is disabled.

Local Proxy ARP

You can use local Proxy ARP to enable a device to respond to ARP requests for IP addresses within a
subnet where normally no routing is required. When you enable local Proxy ARP, ARP responds to all
ARP requests for IP addresses within the subnet and forwards all traffic between hosts in the subnet. Use
this feature only on subnets where hosts are intentionally prevented from communicating directly by the
configuration on the device to which they are connected.

Gratuitous ARP

Gratuitous ARP sends a request with identical source IP address and destination IP address to detect
duplicate IP addresses. Cisco NX-OS Release 4.0(3) and later releases support enabling or disabling
gratuitous ARP requests or ARP cache updates.
ICMP
You can use ICMP to provide message packets that report errors and other information that is relevant
to IP processing. ICMP generates error messages, such as ICMP destination unreachable messages,
ICMP Echo Requests (which send a packet on a round trip between two hosts) and Echo Reply messages.
ICMP also provides many diagnostic functions and can send and redirect error packets to the host. By
default, ICMP is enabled.
Some of the ICMP message types are as follows:
•
•
•
•
OL-20002-02
Network error messages
Network congestion messages
Troubleshooting information
Timeout announcements
Cisco Nexus 7000 Series NX-OS Unicast Routing Configuration Guide, Release 4.x
Information About IPv4
2-5