Multiple Ipv4 Addresses; Address Resolution Protocol - Cisco Nexus 3000 Series Configuration Manual

Information About 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 3 k - d o c f e e d b a c k @ c i s c o . c o m
The IP feature in the Cisco NX-OS system is responsible for handling IPv4 packets, as well as
forwarding of IPv4 packets, which includes IPv4 unicast/multicast route lookup, reverse path forwarding
(RPF) checks, and software access control list (ACL) forwarding. The IP feature also manages the
network interface IP address configuration, duplicate address checks, static routes, and packet
send/receive interface for IP clients.
This section includes the following topics:
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Multiple IPv4 Addresses

The Cisco NX-OS system supports multiple IP addresses per interface. You can specify an unlimited
number of secondary addresses for a variety of situations. The most common situations are as follows:
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If any switch on a network segment uses a secondary IPv4 address, all other switches on that same
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network interface must also use a secondary address from the same network or subnet. The inconsistent
use of secondary addresses on a network segment can quickly cause routing loops.

Address Resolution Protocol

Networking switches and Layer 3 switches use Address Resolution Protocol (ARP) to map IP (network
layer) addresses to (Media Access Control [MAC]-layer) addresses to enable IP packets to be sent across
networks. Before a switch sends a packet to another switch, it looks in its own ARP cache to see if there
is a MAC address and corresponding IP address for the destination switch. If there is no entry, the source
switch sends a broadcast message to every switch on the network.
Cisco Nexus 3000 Series NX-OS Unicast Routing Configuration Guide, NX-OS Release 5.0(3)U1(1)
2-2
Multiple IPv4 Addresses, page 2-2
Address Resolution Protocol, page 2-2
ARP Caching, page 2-3
Static and Dynamic Entries in the ARP Cache, page 2-3
Devices that Do Not Use ARP, page 2-4
Reverse ARP, page 2-4
Proxy ARP, page 2-5
Local Proxy ARP, page 2-5
ICMP, page 2-5
Virtualization Support, page 2-6
When there are not enough host IP addresses for a particular network interface. For example, if your
subnet allows up to 254 hosts per logical subnet, but on one physical subnet you must have 300 host
addresses, then you can use secondary IP addresses on the routers or access servers to allow you to
have two logical subnets using one physical subnet.
Two subnets of a single network might otherwise be separated by another network. You can create
a single network from subnets that are physically separated by another network by using a secondary
address. In these instances, the first network is extended, or layered on top of the second network.
A subnet cannot appear on more than one active interface of the router at a time.
Chapter 2 Configuring IPv4