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Chapter 4
IP Networking
Figure 4-3
IP Address 192.168.1.20
Subnet Mask 255.255.255.0
ARP matches higher-level IP addresses to the physical addresses of the destination host. It uses a lookup
table (called ARP cache) to perform the translation. When the address is not found in the ARP cache, a
broadcast is sent out on the network with a special format called the ARP request. If one of the machines
on the network recognizes its own IP address in the request, it sends an ARP reply back to the requesting
host. The reply contains the physical hardware address of the receiving host. The requesting host stores
this address in its ARP cache so that all subsequent datagrams (packets) to this destination IP address
can be translated to a physical address.
Proxy ARP enables one LAN-connected ONS 15454 to respond to the ARP request for ONS 15454s not
connected to the LAN. (ONS 15454 Proxy ARP requires no user configuration.) For this to occur, the
DCC-connected ONS 15454s must reside on the same subnet. When a LAN device sends an ARP request
to an ONS 15454 that is not connected to the LAN, the gateway ONS 15454 returns its MAC address to
the LAN device. The LAN device then sends the datagram for the remote ONS 15454 to the MAC
address of the proxy ONS 15454. The proxy ONS 15454 uses its routing table to forward the datagram
to the non-LAN ONS 15454. The routing table is built using the OSPF IP routing protocol. (An OSPF
example is presented in Scenario 6.)
November 2001
Scenario 3: Using Proxy ARP
CTC Workstation
IP Address 192.168.1.100
Subnet Mark at CTC Workstation 255.255.255.0
Default Gateway = N/A
ONS 15454 #2
Default Router = N/A
Static Routes = N/A
ONS 15454 #1
IP Address 192.168.1.10
Subnet Mask 255.255.255.0
Default Router = N/A
Static Routes = N/A
SONET RING
Cisco ONS 15454 Installation and Operations Guide
ONS 15454 IP Addressing Scenarios
LAN A
ONS 15454 #3
IP Address 192.168.1.30
Subnet Mask 255.255.255.0
Default Router = N/A
Static Routes = N/A
4-5
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