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2-4
Cisco Nexus 7000 Series NX-OS Unicast Routing Configuration Guide, Release 4.x
OL-20002-02
Chapter 2 Configuring IPv4
Information About IPv4
Devices that do not use ARP
When a network is divided into two segments, a bridge joins the segments and filters traffic to each
segment based on MAC addresses. The bridge builds its own address table, which uses MAC addresses
only, as opposed to a device, which has an ARP cache that contains both IP addresses and the
corresponding MAC addresses.
Passive hubs are central-connection devices that physically connect other devices in a network. They
send messages out on all their ports to the devices and operate at Layer 1, but do not maintain an address
table.
Layer 2 switches determine which port is connected to a device to which the message is addressed and
send only to that port, unlike a hub, which sends the message out all its ports. However, Layer 3 switches
are devices that build an ARP cache (table).
Reverse ARP
Reverse ARP (RARP) as defined by RFC 903 works the same way as ARP, except that the RARP request
packet requests an IP address instead of a MAC address. RARP often is used by diskless workstations
because this type of device has no way to store IP addresses to use when they boot. The only address that
is known is the MAC address because it is burned into the hardware.
Use of RARP requires an RARP server on the same network segment as the router interface. Figure 2-2
illustrates how RARP works.
Figure 2-2 Reverse ARP
There are several limitations of RARP. Because of these limitations, most businesses use DHCP to assign
IP addresses dynamically. DHCP is cost effective and requires less maintenance than RARP. The
following are the most important limitations:
• Since RARP uses hardware addresses, if the internetwork is large with many physical networks, a
RARP server must be on every segment with an additional server for redundancy. Maintaining two
servers for every segment is costly.
• Each server must be configured with a table of static mappings between the hardware addresses and
IP addresses. Maintenance of the IP addresses is difficult.
• RARP only provides IP addresses of the hosts and not subnet masks or default gateways.
I am device A and sending
a broadcast that uses my
hardware address.
Can somone on the network
tell me what my IP address is?
Okay, your hardware address
is 2222.8048.1644.1234 and
your IP address is 10.0.0.2
RARP server
135218
Device A
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