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). This module describes how to configure ARP processes on the
Cisco ASR 9000 Series Aggregation Services Router.
For a complete
description of the ARP commands listed in this module, refer to the
Cisco ASR 9000 Series Aggregation Services Router IP Addresses
and Services Command ReferenceTo locate documentation of other
commands that appear in this module, use the command reference master index, or
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.
restrictions apply to configuring ARP :
Resolution Protocol (RARP) is not supported.
Due to a hardware
limitation in the Ethernet SPA interfaces installed on all routers, when a
packet contains a wrong destination address, the corresponding SPA drops the
packet even if the ingress packet count is already incremented in the output of
ARP throttling is
is the rate limiting of ARP packets in Forwarding Information Base (FIB).
The following additional restrictions apply when configuring the
Direct Attached Gateway Redundancy (DAGR) feature on Cisco ASR 9000 Series
IPv6 is not
are not supported.
interfaces are not supported.
Process Restart is not supported.
Failover is not supported.
Information About Configuring ARP
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.
Address Resolution on a Single LAN
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.
Address Resolution When Interconnected by a Router
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.
ARP and Proxy
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.
Cisco IOS XR software also supports a form of ARP called local proxy ARP.
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
The target IP
address is not on the same physical network (LAN) on which the request is
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.
When local proxy ARP is enabled, the
networking device responds to ARP requests that meet all the following
The target IP
address in the ARP request, the IP address of the ARP source, and the IP
address of the interface on which the ARP request is received are on the same
Layer 3 network.
The next hop for
the target IP address is through the same interface as the request is received.
Typically, local proxy ARP is used to
resolve MAC addresses to IP addresses in the same Layer 3 network such as,
private VLANs that are Layer 2-separated. Local proxy ARP supports all types of
interfaces supported by ARP and unnumbered interfaces.
ARP Cache Entries
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.
Direct Attached Gateway Redundancy
Direct Attached Gateway Redundancy (DAGR) allows third-party redundancy schemes on connected devices to use gratuitous ARP as a failover signal, enabling the ARP process to advertise an new type of route in the Routing Information Base (RIB). These routes are distributed by Open Shortest Path First (OSPF).
Sometimes part of an IP network requires redundancy without routing protocols. A prime example is in the mobile environment, where devices such as base station controllers and multimedia gateways are deployed in redundant pairs, with aggressive failover requirements (subsecond or less), but typically do not have the capability to use native Layer 3 protocols such as OSPF or Intermediate System-to-Intermediate System (IS-IS) protocol to manage this redundancy. Instead, these devices assume they are connected to adjacent IP devices over an Ethernet switch, and manage their redundancy at Layer 2, using proprietary mechanisms similar to Virtual Router Redundancy Protocol (VRRP). This requires a resilient Ethernet switching capability, and depends on mechanisms such as MAC learning and MAC flooding.
DAGR is a feature that enables many of these devices to connect directly to Cisco ASR 9000 Series Routers without an intervening Ethernet switch. DAGR enables the subsecond failover requirements to be met using a Layer 3 solution. No MAC learning, flooding, or switching is required.
Since mobile devices' 1:1 Layer 2 redundancy mechanisms are proprietary, they do not necessarily conform to any standard. So although most IP mobile equipment is compatible with DAGR, interoperability does require qualification, due to the possibly proprietary nature of the Layer 2 mechanisms with which DAGR interfaces.
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
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.
RP/0/RSP0/CPU0:router(config)# arp 192.168.7.19 0800.0900.1834 arpa alias
Creates a static
ARP cache entry associating the specified 32-bit IP address with the specified
48-bit hardware address.
alias entry is created, then any interface to which the entry
is attached will act as if it is the owner of the specified addresses, that is,
it will respond to ARP request packets for this network layer address with the
data link layer address in the entry.
commit—Saves the configuration changes and remains
within the configuration session.
end—Prompts user to take one of these actions:
Yes— Saves configuration changes and exits the
No—Exits the configuration session without
committing the configuration changes.
Cancel—Remains in the configuration session,
without committing the configuration changes.
Enabling Proxy ARP
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.