Specifications

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Cisco Nexus 3000 Series NX-OS Unicast Routing Configuration Guide, Release 5.0(3)U2(2)
OL-25782-02
Chapter 2 Configuring IPv4
Information About IPv4
Proxy ARP
Proxy ARP enables a switch that is physically located on one network appear to be logically part of a
different physical network connected to the same switch or firewall. Proxy ARP allows you to hide a
switch with a public IP address on a private network behind a router and still have the switch 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 switches on a subnet reach remote
subnets without configuring routing or a default gateway.
When switches 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
switches 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 switch and it receives an ARP request, it identifies the request as a
request for a system that is not on the local LAN. The switch responds as if it is the remote destination
for which the broadcast is addressed, with an ARP response that associates the MAC address of the
switch with the IP address of the remote destination. The local switch 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 switch. By default, Proxy ARP is disabled.
Local Proxy ARP
You can use local Proxy ARP to enable a switch 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 switch 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 5.0(3) support enabling or disabling gratuitous ARP
requests or ARP cache updates.
Glean Throttling
When forwarding an incoming IP packet in a line card, if the Address Resolution Protocol (ARP) request
for the next hop is not resolved, the line card forwards the packets to the supervisor (glean throttling).
The supervisor resolves the MAC address for the next hop and programs the hardware.
The Cisco Nexus 7000 Series device hardware has glean rate limiters to protect the supervisor from the
glean traffic. If the maximum number of entries is exceeded, the packets for which the ARP request is
not resolved continues to be processed in the software instead of getting dropped in the hardware.
When an ARP request is sent, the software adds a /32 drop adjacency in the hardware to prevent the
packets to the same next-hop IP address to be forwarded to the supervisor. When the ARP is resolved,
the hardware entry is updated with the correct MAC address. If the ARP entry is not resolved before a
timeout period, the entry is removed from the hardware.