proxy ARP
2013-08-09 13:27
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Contents
IntroductionPrerequisites
Requirements
Components
Used
Conventions
How Does Proxy ARP Work?
Network
Diagram
Advantages of Proxy ARP
Disadvantages of Proxy ARP
Related Information
Related Cisco Support Community Discussions
Introduction
This document explains the concept of proxy Address Resolution Protocol (ARP). Proxy ARP is the technique in which one host, usually a router, answers ARP requests intended for another machine. By "faking" its identity, the router accepts responsibilityfor routing packets to the "real" destination. Proxy ARP can help machines on a subnet reach remote subnets without the need to configure routing or a default gateway. Proxy ARP is defined in RFC
1027
.
Prerequisites
Requirements
This document requires an understanding of the ARP and Ethernet environment.Components Used
The information in this document is based on these software and hardware versions:Cisco IOS® Software Release 12.2(10b)
Cisco 2500 Series Routers
The information presented in this document was created from devices in a specific lab environment. All of the devices used in this document started with a cleared (default) configuration. If you are working in a live network, ensure that you understand
the potential impact of any command before using it.
Conventions
Refer to Cisco Technical Tips Conventions formore information on document conventions.
How Does Proxy ARP Work?
This is an example of how proxy ARP works:Network Diagram
The Host A (172.16.10.100) on Subnet A needs to send packets to Host D (172.16.20.200) on Subnet B. As shown in the diagram, Host A has a /16 subnet mask. What this means is that Host A believes that it is directly connected to all of network 172.16.0.0.
When Host A needs to communicate with any devices it believes are directly connected, it sends an ARP request to the destination. Therefore, when Host A needs to send a packet to Host D, Host A believes that Host D is directly connected, so it sends an ARP
request to Host D.
In order to reach Host D (172.16.20.200), Host A needs the MAC address of Host D.
Therefore, Host A broadcasts an ARP request on Subnet A, as shown:
Sender's MAC Address | Sender's IP Address | Target MAC Address | Target IP Address |
---|---|---|---|
00-00-0c-94-36-aa | 172.16.10.100 | 00-00-00-00-00-00 | 172.16.20.200 |
as the destination address. Since the ARP request is a broadcast, it reaches all the nodes in the Subnet A, which includes the e0 interface of the router, but does not reach Host D. The broadcast does not reach Host D because routers, by default, do not forward
broadcasts.
Since the router knows that the target address (172.16.20.200) is on another subnet and can reach Host D, it replies with its own MAC address to Host A.
Sender's MAC Address | Sender's IP Address | Target MAC Address | Target IP Address |
---|---|---|---|
00-00-0c-94-36-ab | 172.16.20.200 | 00-00-0c-94-36-aa | 172.16.10.100 |
are always unicast to the original requester.
Upon receipt of this ARP reply, Host A updates its ARP table, as shown:
IP Address | MAC Address |
---|---|
172.16.20.200 | 00-00-0c-94-36-ab |
hosts in Subnet A is populated with the MAC address of the router for all the hosts on Subnet B. Hence, all packets destined to Subnet B are sent to the router. The router forwards those packets to the hosts in Subnet B.
The ARP cache of Host A is shown in this table:
IP Address | MAC Address |
---|---|
172.16.20.200 | 00-00-0c-94-36-ab |
172.16.20.100 | 00-00-0c-94-36-ab |
172.16.10.99 | 00-00-0c-94-36-ab |
172.16.10.200 | 00-00-0c-94-36-bb |
The interface of the Cisco must be configured to accept and respond to proxy ARP. This is enabled by default. The no ip proxy-arp command must be configured on the interface of the router connected to the
ISP router. Proxy ARP can be disabled on each interface individually with the interface configuration command no ip proxy-arp, as shown:
Router# configure terminal Enter configuration commands, one per line. End with CNTL/Z. Router(config)# interface ethernet 0 Router(config-if)# no ip proxy-arp Router(config-if)# ^Z Router#
In order to enable proxy ARP on an interface, issue the ip proxy-arp interface configuration command.
Note: When Host B (172.16.10.200/24) on Subnet A tries to send packets to destination Host D (172.16.20.200) on Subnet B, it looks into its IP routing table and routes the packet accordingly. Host B (172.16.10.200/24) does not ARP
for Host D IP address 172.16.20.200 because it belongs to a different subnet than what is configured on Host B ethernet interface 172.16.20.200/24.
Advantages of Proxy ARP
The main advantage of proxy ARP is that it can be added to a single router on a network and does not disturb the routing tables of the other routers on the network.Proxy ARP must be used on the network where IP hosts are not configured with a default gateway or do not have any routing intelligence.
Disadvantages of Proxy ARP
Hosts have no idea of the physical details of their network and assume it to be a flat network in which they can reach any destination simply by sending an ARP request. But using ARP for everything has disadvantages. These are some of the disadvantages:It increases the amount of ARP traffic on your segment.
Hosts need larger ARP tables in order to handle IP-to-MAC address mappings.
Security can be undermined. A machine can claim to be another in order to intercept packets, an act called "spoofing."
It does not work for networks that do not use ARP for address resolution.
It does not generalize to all network topologies. For example, more than one router that connects two physical networks.
Refer to the Enabling Proxy ARP section
of Configuring IP Addressing for more information about configuring
proxy ARP.
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