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Address resolution is the process of mapping network addresses to Media Access Control (MAC) addresses. This process is accomplished using the Address Resolution Protocol (ARP).
 Note |
For a complete description of the ARP commands listed in this module, refer to the IP Addresses and Services Command Reference for Cisco NCS 6000 Series Routers. |
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Release |
Modification |
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Release 5.0.0 |
This feature was introduced. |
You must be in a user group associated with a task group that includes the proper task IDs. The command reference guides include the task IDs required for each command. If you suspect user group assignment is preventing you from using a command, contact your AAA administrator for assistance.
The following restrictions apply to configuring ARP :
Reverse Address Resolution Protocol (RARP) is not supported.
To configure ARP, you must understand the following concepts:
A device in the IP can have both a local address (which uniquely identifies the device on its local segment or LAN) and a network address (which identifies the network to which the device belongs). The local address is more properly known as a data link address, because it is contained in the data link layer (Layer 2 of the OSI model) part of the packet header and is read by data-link devices (bridges and all device interfaces, for example). The more technically inclined person will refer to local addresses as MAC addresses, because the MAC sublayer within the data link layer processes addresses for the layer.
To communicate with a device on Ethernet, for example, Cisco IOS XR software first must determine the 48-bit MAC or local data-link address of that device. The process of determining the local data-link address from an IP address is called address resolution.
The following process describes address resolution when the source and destination devices are attached to the same LAN:
End System A broadcasts an ARP request onto the LAN, attempting to learn the MAC address of End System B.
The broadcast is received and processed by all devices on the LAN, including End System B.
Only End System B replies to the ARP request. It sends an ARP reply containing its MAC address to End System A.
End System A receives the reply and saves the MAC address of End System B in its ARP cache. (The ARP cache is where network addresses are associated with MAC addresses.)
Whenever End System A needs to communicate with End System B, it checks the ARP cache, finds the MAC address of System B, and sends the frame directly, without needing to first use an ARP request.
The following process describes address resolution when the source and destination devices are attached to different LANs that are interconnected by a router (only if proxy-arp is turned on):
End System Y broadcasts an ARP request onto the LAN, attempting to learn the MAC address of End System Z.
The broadcast is received and processed by all devices on the LAN, including Router X.
Router X checks its routing table and finds that End System Z is located on a different LAN.
Router X therefore acts as a proxy for End System Z. It replies to the ARP request from End System Y, sending an ARP reply containing its own MAC address as if it belonged to End System Z.
End System Y receives the ARP reply and saves the MAC address of Router X in its ARP cache, in the entry for End System Z.
When End System Y needs to communicate with End System Z, it checks the ARP cache, finds the MAC address of Router X, and sends the frame directly, without using ARP requests.
Router X receives the traffic from End System Y and forwards it to End System Z on the other LAN.
Two forms of address resolution are supported by Cisco IOS XR software: Address Resolution Protocol (ARP) and proxy ARP, as defined in RFC 826 and RFC 1027, respectively.
ARP is used to associate IP addresses with media or MAC addresses. Taking an IP address as input, ARP determines the associated media address. After a media or MAC address is determined, the IP address or media address association is stored in an ARP cache for rapid retrieval. Then the IP datagram is encapsulated in a link-layer frame and sent over the network.
When proxy ARP is disabled, the networking device responds to ARP requests received on an interface only if one of the following conditions is met:
The target IP address in the ARP request is the same as the interface IP address on which the request is received.
The target IP address in the ARP request has a statically configured ARP alias.
When proxy ARP is enabled, the networking device also responds to ARP requests that meet all the following conditions:
The target IP address is not on the same physical network (LAN) on which the request is received.
The networking device has one or more routes to the target IP address.
All of the routes to the target IP address go through interfaces other than the one on which the request is received.
ARP establishes correspondences between network addresses (an IP address, for example) and Ethernet hardware addresses. A record of each correspondence is kept in a cache for a predetermined amount of time and then discarded.
You can also add a static (permanent) entry to the ARP cache that persists until expressly removed.
This section contains instructions for the following tasks:
ARP and other address resolution protocols provide a dynamic mapping between IP addresses and media addresses. Because most hosts support dynamic address resolution, generally you need not to specify static ARP cache entries. If you must define them, you can do so globally. Performing this task installs a permanent entry in the ARP cache. Cisco IOS XR software uses this entry to translate 32-bit IP addresses into 48-bit hardware addresses.
Optionally, you can specify that the software responds to ARP requests as if it were the owner of the specified IP address by making an alias entry in the ARP cache.
| Command or Action | Purpose | |||
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Step 1 |
configure |
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Step 2 |
Do one of the following:
Example:or |
Creates a static ARP cache entry associating the specified 32-bit IP address with the specified 48-bit hardware address.
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Step 3 |
commit |
Cisco IOS XR software uses proxy ARP (as defined in RFC 1027) to help hosts with no knowledge of routing determine the media addresses of hosts on other networks or subnets. For example, if the router receives an ARP request for a host that is not on the same interface as the ARP request sender, and if the router has all of its routes to that host through other interfaces, then it generates a proxy ARP reply packet giving its own local data-link address. The host that sent the ARP request then sends its packets to the router, which forwards them to the intended host. Proxy ARP is disabled by default; this task describes how to enable proxy ARP if it has been disabled.
| Command or Action | Purpose | |
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Step 1 |
configure |
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Step 2 |
interface type number Example: |
Enters interface configuration mode. |
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Step 3 |
proxy-arp Example: |
Enables proxy ARP on the interface. |
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Step 4 |
commit |
You can configure an interface or a sub-interface to learn only the ARP entries from its local subnet.
Use the following procedure to configure local ARP learning on an interface.
Enter the interface configuration mode.
Router(config)# interface GigabitEthernet 0/0/0/1Configure the IPv4/IPv6 address for the interface.
Router(config-if)# ipv4 address 12.1.3.4 255.255.255.0Configure local ARP learning on the interface.
Router(config-if)# arp learning localEnable the interface and commit your configuration.
Router(config-if)# no shut
Router(config-if)# commit
RP/0/0/CPU0:Dec 12 13:41:16.580 : ifmgr[397]: %PKT_INFRA-LINK-3-UPDOWN : interface GigabitEthernet 0/0/0/1, changed state to Down
RP/0/0/CPU0:Dec 12 13:41:16.683 : ifmgr[397]: %PKT_INFRA-LINK-3-UPDOWN : interface GigabitEthernet 0/0/0/1 changed state to Up Confirm your configuration.
Router(config-if)# show running-configuration
..
Building configuration...
!! IOS XR Configuration 0.0.0
!! Last configuration change at Mon Dec 12 13:41:16 2016
!interface GigabitEthernet 0/0/0/1
ipv4 address 12.1.3.4 255.255.255.0
arp learning local
!Verify if local ARP learning is working as configured on the interface.
Router(config-if)# do show arp idb gigabitEthernet 0/0/0/1 location 0/0/CPU0
Thu Dec 15 10:00:11.733 IST
GigabitEthernet 0/0/0/1 (0x00000040):
IPv4 address 12.1.3.4, Vrf ID 0x60000000
VRF Name default
Dynamic learning: Local
Dynamic entry timeout: 14400 secs
Purge delay: off
IPv4 caps added (state up)
MPLS caps not added
Interface not virtual, not client fwd ref,
Proxy arp not configured, not enabled
Local Proxy arp not configured
Packet IO layer is NetIO
Srg Role : DEFAULT
Idb Flag : 2146444
IDB is Complete(Optional) You can monitor the ARP traffic on the interface.
Router(config-if)# do show arp idb gigabitEthernet 0/0/0/1 location 0/0/CPU0
Thu Dec 15 10:13:28.964 IST
ARP statistics:
Recv: 0 requests, 0 replies
Sent: 0 requests, 1 replies (0 proxy, 0 local proxy, 1 gratuitous)
Subscriber Interface:
0 requests recv, 0 replies sent, 0 gratuitous replies sent
Resolve requests rcvd: 0
Resolve requests dropped: 0
Errors: 0 out of memory, 0 no buffers, 0 out of sunbet
ARP cache:
Total ARP entries in cache: 1
Dynamic: 0, Interface: 1, Standby: 0
Alias: 0, Static: 0, DHCP: 0
IP Packet drop count for GigabitEthernet0_0_0_1: 0
The following sections provide references related to ARP.
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Related Topic |
Document Title |
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ARP commands |
ARP Commands module in IP Addresses and Services Command Reference for Cisco NCS 6000 Series Routers |
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Getting started material |
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Related Topic |
Document Title |
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QoS commands: complete command syntax, command modes, command history, defaults, usage guidelines, and examples |
Quality of Service Commands module in Modular QoS Command Reference for Cisco NCS 6000 Series Routers |
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Class-based traffic shaping, traffic policing, low latency queuing, and MDDR |
Configuring Modular Quality of Service Congestion Management module in Modular QoSConfiguration Guide for Cisco NCS 6000 Series Routers |
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Standards |
Title |
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No new or modified standards are supported by this feature, and support for existing standards has not been modified by this feature. |
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MIBs |
MIBs Link |
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To locate and download MIBs, use the Cisco MIB Locator found at the following URL and choose a platform under the Cisco Access Products menu: https://mibs.cloudapps.cisco.com/ITDIT/MIBS/servlet/index |
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RFCs |
Title |
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RFC 826 |
Ethernet Address Resolution Protocol: Or converting network protocol addresses to 48.bit Ethernet address for transmission on Ethernet hardware |
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RFC 1027 |
Using ARP to implement transparent subnet gateways |
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Description |
Link |
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