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| General IP Routing
C
18
HAPTER
Address Resolution Protocol
A
R
DDRESS
ESOLUTION
P
ARP
ROXY
C
ONFIGURATION
P
ROTOCOL
The router uses its routing tables to make routing decisions, and uses
Address Resolution Protocol (ARP) to forward traffic from one hop to the
next. ARP is used to map an IP address to a physical layer (i.e., MAC)
address. When an IP frame is received by this router (or any standards-
based router), it first looks up the MAC address corresponding to the
destination IP address in the ARP cache. If the address is found, the router
writes the MAC address into the appropriate field in the frame header, and
forwards the frame on to the next hop. IP traffic passes along the path to
its final destination in this way, with each routing device mapping the
destination IP address to the MAC address of the next hop toward the
recipient, until the packet is delivered to the final destination.
If there is no entry for an IP address in the ARP cache, the router will
broadcast an ARP request packet to all devices on the network. The ARP
request contains the following fields similar to that shown in this example:
Table 45: Address Resolution Protocol
destination IP address
10.1.0.19
destination MAC address ?
source IP address
10.1.0.253
source MAC address
00-00-ab-cd-00-00
When devices receive this request, they discard it if their address does not
match the destination IP address in the message. However, if it does
match, they write their own hardware address into the destination MAC
address field and send the message back to the source hardware address.
When the source device receives a reply, it writes the destination IP
address and corresponding MAC address into its cache, and forwards the IP
traffic on to the next hop. As long as this entry has not timed out, the
router will be able forward traffic directly to the next hop for this
destination without having to broadcast another ARP request.
Also, if the switch receives a request for its own IP address, it will send
back a response, and also cache the MAC of the source device's IP address.
Use the IP > ARP (Configure General) page to enable Proxy ARP for specific
VLAN interfaces.
CLI R
EFERENCES
"ip proxy-arp" on page 1430
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C
U
OMMAND
SAGE
When a node in the attached subnetwork does not have routing or a
default gateway configured, Proxy ARP can be used to forward ARP requests
to a remote subnetwork. When the router receives an ARP request for a
remote network and Proxy ARP is enabled, it determines if it has the best
route to the remote network, and then answers the ARP request by
– 686 –
- Quick Links:
- Connecting to the Switch
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