Cabletron Systems CyberSWITCH CSX500 Specifications download pdf (Page 49)

Bridging and Routing Configuration

QuickSET Configuration Guide 33

Proxy ARP — This checkbox lets you disable the Proxy ARP function.

Figure 32, below shows an example of Proxy ARP (Address Resolution Protocol). In this

example, hosts A, and C need to communicate, and the only route is through host B as shown. The

IP Addresses that deﬁne the link between hosts A and B use the same subnet number as the

Ethernet LAN,(i.e., 192.168.54.0) through which hosts B and C communicate. Host C assumes

that the IP Address 192.168.53.4 has an Ethernet connection on the LAN.

Figure 32 Proxy ARP Function

When the IP Protocol layer of host C receives a packet addressed to 192.168.53.4, host C puts an

ARP broadcast on the LAN asking the host of that address to respond with its MAC (Machine

Address and Control Layer) address. Because host A is not on the LAN, host C receives no

response, and believes it cannot communicate with that IP Address.

Proxy ARP solves this problem by putting entries into the ARP mapping table on host B that maps

the IP Address 192.168.53.4 to the MAC address of host B on the LAN.

Now when host C broadcasts a request for the MAC address of IP Address 192.168.53.4, host B

will respond with its MAC address. Host C will send packets for 192.168.53.4 to host B’s MAC

address, and host B will forward these packets to Host A.

Ethernet

192.168.54.0

192.168.53.4

Host A

192.168.54.2

Host B

192.168.54.3

Host C

192.168.53.1

Proxy ARP

Map Entries

PPP link

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Cabletron Systems CyberSWITCH CSX500 Specifications Page 49