IP Addressing: ARP Configuration Guide, Cisco IOS Release 15M&T
Monitoring and Maintaining ARP Information
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Monitoring and Maintaining ARP Information

Contents

Monitoring and Maintaining ARP Information

Last Updated: December 3, 2012

The Monitoring and Maintaining ARP Information feature document describes the practices involved in monitoring and maintaining arp information.

Address Resolution Protocol (ARP) is an Internet protocol used to map an IP address to a MAC address. ARP finds the MAC address, also known as the hardware address, of an IP-routed host from its known IP address and maintains this mapping information in a table. The router uses this IP address and MAC address mapping information to send IP packets to the next-hop router in the network.

ARP information monitoring and maintenance capabilities improves the management tools for ARP support in a Cisco IOS environment:

  • To better support ARP analysis activities, the ARP administrative facilities provide detailed information about and granular control over ARP information. This information can be used to investigate issues with ARP packet traffic, ARP high availability (HA), or ARP synchronization with Cisco Express Forwarding adjacency.
  • The ARP debug trace facility enables ARP packet debug trace for individual types of ARP events. The ARP debugging provides filtering of ARP entries for a specified interface, for hosts that match an access list, or for both.
  • For increased security against ARP attacks, trap-based enabling of ARP system message logging can be configured per interface to alert network administrators of possible anomalies.
  • To prevent the possibility of system instability due to memory exhaustion, the number of ARP entries that can be learned by the system can be limited. This feature is supported only on the Cisco 7600 platform, starting from Cisco IOS Release 12.2(33)SRD3.

No configuration tasks are associated with these additional ARP information monitoring and maintenance capabilities. The ARP-related enhancements introduced by this functionality are expanded forms of existing ARP management tasks.

Finding Feature Information

Your software release may not support all the features documented in this module. For the latest caveats and feature information, see Bug Search Tool and the release notes for your platform and software release. To find information about the features documented in this module, and to see a list of the releases in which each feature is supported, see the feature information table at the end of this module.

Use Cisco Feature Navigator to find information about platform support and Cisco software image support. To access Cisco Feature Navigator, go to www.cisco.com/go/cfn. An account on Cisco.com is not required.

Restrictions for Monitoring and Maintaining ARP Information

For Cisco IOS Release 12.4(11)T, the restrictions described in the following sections apply to the ARP information monitoring and maintenance capabilities:

ARP High Availability

The ARP subsystem supports ARP HA on Cisco networking devices that support dual Route Processors (RPs) for redundant processing capability. However, ARP HA is limited to the synchronization of dynamically learned ARP entries from the active RP to the standby RP. Statically configured ARP entries are not synchronized to the standby RP.

ARP Security Against ARP Attacks

The ARP subsystem supports a method for detecting a possible ARP attack by monitoring the number of ARP table entries for specific interfaces. However, no router-level security feature can prevent Man-in-the-Middle (MiM) types of ARP-spoofing attacks, which are a form of wiretapping attack where the attacker intercepts and selectively modifies communicated data to masquerade as one or more of the entities involved in a communication association. There are no ARP features to be implemented to resolve this security issue. Protecting the router from ARP attacks is best handled in switches through the ARP Access Control List (ACL) filters rather than at the router level.

Information About Monitoring and Maintaining ARP Information

Overview of Monitoring and Maintaining ARP Information

ARP information monitoring and maintenance capabilities improves the management tools for ARP support in a Cisco IOS environment. For information about the entire ARP feature, see the Additional References. The following sections summarize the ARP subsystem enhancements introduced in Cisco IOS Release 12.4(11)T:

ARP Information Display Enhancements

The ARP information display capabilities have been expanded to support display of selected ARP entries, ARP entry details, and other ARP information.

Display of Selected ARP Entries

ARP table entries can be selected for display based on the following criteria:

  • Virtual Private Network (VPN) routing and forwarding (VRF) instance
  • ARP mode type
  • Host or network
  • Router interface

In Cisco IOS software versions prior to Release 12.4(11)T, the show arp command displays the entire ARP table.

Display of ARP Entry Details

The following detailed ARP information can be displayed:

  • Adjacency notification--This information can be used to investigate issues with ARP packet traffic, ARP HA, or ARP notification for Cisco Express Forwarding adjacency. If the ARP subsystem needs to synchronize an ARP entry with Cisco Express Forwarding adjacency, that information is included when the affected entry is displayed.
  • Associated interface for floating static ARP entries--If the ARP subsystem succeeds in finding the associated interface for a floating static ARP entry, that information can be included when the affected entry is displayed.
  • Application subblocks--If an application-specific ARP entry is displayed, information about the subblock data can be included in the display.

The show ip arp command, introduced in Cisco IOS Release 9.0, allows you to display only certain ARP table entries based on specified criteria (IP address, interface, or hardware address). However, that command does not display the ARP entry modes, Cisco Express Forwarding adjacency notification information, or the associated interface for floating static ARP entries.

Display of Other ARP Information

The following ARP information--other than the contents of the ARP table entries--can be displayed:

  • ARP table summary statistics--The numbers of entries in the table of each mode type and per interface.
  • ARP HA status and statistics--Different types of switchover statistics are displayed based on the current state and recent activities of the RP.

ARP Information Refresh Enhancements

In Cisco IOS software versions prior to Release 12.4(11)T, the clear arp command refreshes all nonstatic entries in the ARP table. The ARP information refresh facility enables you to manage selected ARP information:

  • Refresh all nonstatic ARP table entries
  • Refresh nonstatic ARP table entries associated with a particular interface
  • Refresh nonstatic ARP table entries for a particular IP address in a particular VRF
  • Reset ARP HA statistics

ARP Debug Trace Enhancements

In Cisco IOS software versions prior to Release 12.4(11)T, the debug arp command supports debugging information for ARP packet traffic only. The ARP debug trace facility now provides more detailed selection and filter options for ARP debug trace.

Debug Trace for Selected ARP Events

The ARP debugging information can be enabled for the following types of ARP events:

  • ARP table entry events
  • ARP table events
  • ARP interface interactions
  • ARP HA events
Support for Filtering Debug Trace by Interface or Access List

The debug arp command supports debug trace filtering as defined by the debug list command. This enhancement enables ARP debugging information to be focused on desired debugging information based on a specific router interface, an access list of IP addresses, or both.

ARP Security Enhancement

When trap-based enabling of ARP system message logging (syslog) output is configured, the router monitors the number of dynamically learned ARP table entries for each interface and triggers ARP logging whenever the number of learned ARP entries for a particular interface exceeds the preconfigured value.

Such syslog traps can in turn alert network administrators (via protocols such as Simple Network Management Protocol (SNMP)) with the identity of the affected interface and the number of learned ARP entries over that interface. The administrator can then investigate why the ARP table has grown to the configured thresholds, and take the necessary action to resolve possible security breaching. Alternatively, the router can take self-defense actions automatically, with the action depending on the severity, from more frequent refreshing to shutting down the interface port.


Note


This router-level security feature can help detect a MiM ARP-spoofing attack, but it cannot prevent such an attack. There are no ARP features to be implemented to resolve this security issue. Protecting the router from ARP attacks is best handled in switches through the ARP-ACL filters rather than at the router level.

Address Resolution Protocol

ARP was developed to enable communications on an internetwork, as defined by RFC 826. Routers and Layer 3 switches need ARP to map IP addresses to MAC hardware addresses so that IP packets can be sent across networks. The following sections provide background information about ARP:

ARP Broadcast and Response Process

Before a device sends a datagram to another device, it looks in its own ARP information to see if there is a MAC address and corresponding IP address for the destination device. If there is no entry, the source device sends a broadcast message to every device on the network. Each device compares the IP address to its own. Only the device with the matching IP address replies to the sending device with a packet containing the MAC address for the device. The source device adds the destination device MAC address to its ARP table for future reference, creates a data-link header and trailer that encapsulates the packet, and proceeds to transfer the data.

When the destination device lies on a remote network, one beyond another router, the process is the same except that the sending device sends an ARP request for the MAC address of the default gateway. After the address is resolved and the default gateway receives the packet, the default gateway broadcasts the destination IP address over the networks connected to it. The router on the destination device network uses ARP to obtain the MAC address of the destination device and delivers the packet.

ARP Caching

Because the mapping of IP addresses to MAC addresses occurs at each hop (router) on the network for every datagram sent over an internetwork, performance of the network could be compromised. To minimize broadcasts and limit wasteful use of network resources, ARP caching was implemented.

ARP caching is the method of storing network addresses and the associated data-link addresses in memory for a period of time as the addresses are learned. This minimizes the use of valuable network resources to broadcast for the same address each time a datagram is sent. The cache entries must be maintained because the information could become outdated, so it is critical that the cache entries are set to expire periodically. Every device on a network updates its tables as addresses are broadcast.

ARP Table

The ARP table provides a database in which a Cisco router caches learned and configured route-mapping information. Each entry in the ARP table is associated with either a local IP address (which represents a device owned by the router) or a remote host IP address (which represents an external device). The contents of the entry define the following ARP-intrinsic information:

  • The association of the 32-bit IP address and 48-bit MAC address of that port
  • Other information needed to support ARP in a Cisco IOS environment (such as link type, VRF table ID, and encapsulation type)

When the router forwards a packet using an IP switching technology such as Cisco Express Forwarding, the ARP table entries supply MAC rewrite information.

ARP Table Entry Modes

Each entry in the ARP table is designated with a mode type. The ARP subsystem supports the basic ARP table entry modes and also introduces new, application-specific modes.

Basic ARP Table Entry Modes

The ARP subsystem uses the following basic ARP table entry modes to organize the ARP entries for ARP-internal processing:

  • Alias--This mode is assigned to an entry that has been explicitly configured by an administrator with a local IP address, subnet mask, gateway, and corresponding MAC address. Static ARP entries are kept in the cache table on a permanent basis. They are best for local addresses that need to communicate with other devices in the same network on a regular basis.
  • Dynamic--This mode is assigned to a dynamically learned entry that was initiated by an ARP request and is associated with an external host. Dynamic ARP entries are automatically added by the Cisco IOS software and maintained for a period of time, then removed. No administrative tasks are needed unless a time limit is added. The default time limit is four hours. If the network has a large number routes that are added and deleted from the cache, the time limit should be adjusted. A dynamic ARP entry is considered "complete" in that the entry contains the MAC address of the external host, as supplied by an ARP reply.
  • Incomplete--This mode is a transient mode for a dynamic ARP entry. This mode indicates an entry that was initiated by an ARP request and is associated with an external host but does not contain a MAC address.
  • Interface--This mode is assigned to an entry for a local IP address that has been derived from an interface.
  • Static--This mode is assigned to an entry that has been explicitly configured by an administrator with an external IP address, subnet mask, gateway, and corresponding MAC address. Static ARP entries are kept in the cache table on a permanent basis. They are best for external devices that need to communicate with other devices in the same network on a regular basis. A static ARP entry is said to be "floating" if it is not associated with any interface when it is configured.

To maintain the validity of dynamically learned routes, the ARP subsystem refreshes dynamic ARP entries periodically (as configured or every four hours by default) so that the ARP table reflects any changed, aged-out, or removed dynamic routes.

To maintain the validity of statically configured routes, the ARP subsystem updates static ARP entries and alias ARP entries once per minute so that the ARP table reflects any changed or removed statically configured routes.

Application-Specific ARP Table Entry Modes

The ARP subsystem uses the application-specific ARP table entry modes to support applications that need to add ARP table entries for their solutions. ARP applications can register with the ARP subsystem to obtain an application type handle. With this handle, the applications can insert ARP entries with the appropriate application-specific entry mode:

  • Simple Application--This mode is assigned to an application-created entry that represents an external device.
  • Application Alias--This mode is assigned to an application-created entry that is associated with a local address.
  • Application Timer--This mode is assigned to an application-created entry that is associated with an external device. The ARP subsystem provides timer-based services to applications that create entries of this mode.

Application-specific entries do not expire, but instead are maintained by the application.

ARP Table Entry Subblocks

The ARP entry subblock structure provides the means to attach non-ARP intrinsic data to selected ARP entries. When an ARP entry inserted into the ARP table requires special, ARP-internal handling, the information needed by the process that performs the special handling is defined in a subblock that is attached to the ARP entry.

The ARP subsystem attaches subblocks to the following types of ARP entries, as needed:

  • Alias, dynamic, and static ARP entries--A subblock is attached to all entries of these types in order to specify information needed by the ARP timer process that coordinates the periodic refresh operation that ensures the validity of the associations between IP addresses and MAC address defined by these entries.
  • Interface ARP entries--A subblock is attached to all interface ARP entries in order to store information about the interface.
  • Simple Application, Application Alias, and Application Timer entries--An application that creates an ARP entry can include any application-specific data necessary for its work, such as timer structures for timer services or data structure pointers for grouping related subblocks.

ARP Table Entry Synchronization with Cisco Express Forwarding Adjacency

If Cisco Express Forwarding is enabled on the router, the router maintains forwarding information (outbound interface and MAC header rewrite) for adjacent nodes. A node is said to be adjacent to another node if the node can be reached with a single hop across a link layer (Layer 2). Cisco Express Forwarding stores the forwarding information in an adjacency database so that Layer 2 addressing information can be inserted into link-layer headers attached to the ARP packets.

The ARP table information is one of the sources for Cisco Express Forwarding adjacency. Whenever the ARP subsystem attaches an ARP table entry to an outbound interface with a valid hardware address, the subsystem issues an internal "ARP adjacency" notification. The notification causes an ARP background process to synchronize that ARP entry with Cisco Express Forwarding adjacency via the adjacency database.

Attachment to an outbound interface occurs only for entries in the following modes:

  • Alias
  • Dynamic
  • Floating Static
  • Application Simple
  • Application Timer

The ARP subsystem processes each floating static ARP entry to find the attached interface by using the IP address in the entry to locate the connected or proxy-ARP interface. The addition of this interface information completes the ARP entry so that it can be synchronized with Cisco Express Forwarding adjacency.

ARP Table Size Monitoring per Interface

The ARP protocol can be used as a vehicle to attack router systems. One ARP attack method, spoofing, is applied on the medium to forge the identity of the host. The Cisco IOS routers have implemented a self-defense scheme to protect the router's own interface address. Other features, such as secure ARP and authorized ARP learning, are implemented in some Cisco IOS releases to limit the scope of ARP learning.

Another ARP attack method, denial-of-service (DoS), includes sending ARP packets to the router in an attempt to overwhelm the CPU processing the ARP packets and to deplete system memory by the ARP table entries created as a result of the ARP packets, resulting in a service outage on the network. A high rate of incoming ARP packets can also cause the ARP input queue to fill up quickly and exceed the maximum default or router-configured capacity, causing an out-of-service condition.

One way to detect a possible attempt to breach security through an ARP attack on the router is to monitor the size of the ARP table and trigger an alert when the number of entries reaches a configured threshold. With a simple limit on the overall ARP table size, though, it is difficult to distinguish between a valid ARP packet and a rogue packet. For a more accurate view of the incoming packets, the ARP subsystem monitors the ARP table size at the interface level. Based on the number of nodes the router serves and the number of hosts on an interface, the expected maximum number of interface-specific entries can be determined. If the number of ARP table entries for an interface exceeds the predetermined threshold, that condition might indicate an attempt to breach security through an ARP attack on the router.

ARP High Availability

ARP HA is a function of the Cisco nonstop forwarding (NSF) feature in the Cisco IOS software. On a Cisco networking device that contains dual RPs and has been configured for stateful switchover (SSO), ARP HA provides a method for increasing network availability for processing ARP entries.

This section summarizes the internal processes and data structures that the ARP subsystem uses to implement ARP HA:

Coexistence with Stateful Switchover

In Cisco networking devices that support dual RPs, ARP uses the stateful switchover (SSO) feature in the Cisco IOS software. SSO provides redundancy and synchronization for many Cisco IOS applications and features. SSO takes advantage of RP redundancy by establishing one of the RPs as the active processor while the other RP is designated as the standby processor, and then synchronizing critical state information between them.

Following an initial synchronization between the two processors, SSO dynamically maintains RP state information between the processors. A switchover from the active to the standby processor occurs when the active RP fails, is removed from the networking device, or is manually taken down for maintenance.

Synchronization Queue

The active RP maintains a synchronization queue, which contains two lists of ARP table entries:

  • ARP entries from the main ARP table that are to be synchronized to the standby RP
  • ARP entries from the main ARP table that have already been synchronized to the standby RP

Note


The synchronization queue consists of two lists of links to entries in the main ARP table.

When switchover occurs, the ARP HA process uses the list of not-yet-synchronized entries to determine which of the entries in the redundant ARP table in the new standby RP (originally the active RP) to synchronize with the main ARP table.

If the standby RP reloads, the ARP HA process bulk synchronizes the entire synchronization queue (entries from both of the lists) to the standby RP when the standby RP reboots.

Backup ARP Table

The standby RP maintains a backup ARP table, which stores backup ARP entries that the standby RP receives from the active RP. During a switchover, the ARP HA process monitors the interface up events. For interfaces that come up, the process searches the backup table on the new active RP (originally the standby RP) for the related ARP entries. The process then adds any related backup ARP entries to the main ARP table.

ARP HA State Machine

The ARP HA process is controlled by an event-driven state machine that consists of two halves: one half for the active RP and the other half for the standby RP. When a switchover occurs, the standby RP transitions to the active half of the state machine. The state machine tracks the status of active/standby synchronization and switchover.

The active half of the state machine can be in any one of the following states:

  • ARP_HA_ST_A_BULK--Transient state in which the active RP waits for the standby RP to signal that it has finished processing of the entries sent by the bulk-synchronization operation.
  • ARP_HA_ST_A_SSO--Transient state in which the new active RP waits for the signal to be fully operational.
  • ARP_HA_ST_A_UP--Active state in which the active RP does not send entries to the standby RP. The active RP transitions into this state either because the standby RP has not come up yet or because a previous synchronization has failed.
  • ARP_HA_ST_A_UP_SYNC--Active state in which the active RP sends entries from the synchronization queue to the standby RP. The active RP transitions into this state when the number of entries to be synchronized reaches a threshold or when the synchronization timer expires, whichever occurs first.

The standby half of the state machine contains the following states:

  • ARP_HA_ST_S_BULK--Transient state in which the standby RP processes the entries sent by the bulk-synchronization operation. After the active RP signals that it has finished sending entries, the standby RP transitions into the ARP_HA_ST_S_UP state and then signals back to the active RP that it has finished processing the entries sent by the bulk-synchronization operation.
  • ARP_HA_ST_S_UP--Active state in which the standby RP processes the incremental ARP synchronization entries from the active RP. When the switchover occurs, the standby RP transitions to the ARP_HA_ST_A_SSO state.

These states and recent activities of the RP can be displayed for monitoring the ARP HA activities.

How to Monitor and Maintain ARP Information

Displaying ARP Table Entry Information

To display ARP table entry information, use the show arp summary, show arp, and show arp application commands:

  • Step 2 is useful for obtaining a high-level view of the contents of the ARP table.
  • Step 3 and Step 4 are useful for displaying the contents of all ARP table entries and any entry subblocks.
  • Step 5 is useful for displaying ARP table information about external applications that are supported by ARP and are running on registered clients.
SUMMARY STEPS

1.    enable

2.    show arp summary

3.    show interfaces [summary]

4.    show arp [[vrf vrf-name] [[arp-mode] [[ip-address [mask]] [interface-type interface-number]]]] [detail]

5.    show arp application [application-id] [detail]


DETAILED STEPS
Step 1   enable

This command enables privileged EXEC mode:



Example:
Router> enable
Step 2   show arp summary

This command displays the total number of ARP table entries, the number of ARP table entries for each ARP entry mode, and the number of ARP table entries for each interface on the router:



Example:
Router# show arp summary
 
Total number of entries in the ARP table: 10.
Total number of Dynamic ARP entries: 4.
Total number of Incomplete ARP entries: 0.
Total number of Interface ARP entries: 4.
Total number of Static ARP entries: 2.
Total number of Alias ARP entries: 0.
Total number of Simple Application ARP entries: 0.
Total number of Application Alias ARP entries: 0.
Total number of Application Timer ARP entries: 0.
Interface              Entry Count
Ethernet3/2                      1
Ethernet3/1                      4
Ethernet3/0                      3
Step 3   show interfaces [summary]

This command lists all the interfaces configured on the router or access server. The valid interface types and numbers can vary according to the router and the interfaces on the router. To list all the interfaces configured on a particular router, use the show interfaces command with the summary keyword. This information is useful if you will be displaying the ARP table entries for a particular router interface.



Example:
Router# show interfaces summary
 
*: interface is up
 IHQ: pkts in input hold queue     IQD: pkts dropped from input queue
 OHQ: pkts in output hold queue    OQD: pkts dropped from output queue
 RXBS: rx rate (bits/sec)          RXPS: rx rate (pkts/sec)
 TXBS: tx rate (bits/sec)          TXPS: tx rate (pkts/sec)
 TRTL: throttle count
  Interface               IHQ   IQD  OHQ   OQD  RXBS RXPS  TXBS TXPS TRTL
-------------------------------------------------------------------------
  FastEthernet1/0          0     0    0     0     0    0     0    0    0
  ATM2/0                   0     0    0     0     0    0     0    0    0
* Ethernet3/0              0     0    0     0     0    0     0    0    0
* Ethernet3/1              0     0    0     0     0    0     0    0    0
* Ethernet3/2              0     0    0     0     0    0     0    0    0
  Ethernet3/3              0     0    0     0     0    0     0    0    0
  Serial4/0                0     0    0     0     0    0     0    0    0
  Serial4/1                0     0    0     0     0    0     0    0    0
  Serial4/2                0     0    0     0     0    0     0    0    0
  Serial4/3                0     0    0     0     0    0     0    0    0
  Fddi5/0                  0     0    0     0     0    0     0    0    0
* Loopback0                0     0    0     0     0    0     0    0    0
Step 4   show arp [[vrf vrf-name] [[arp-mode] [[ip-address [mask]] [interface-type interface-number]]]] [detail]

This command displays all ARP table entries or only the ARP table entries that meet the optional selection criteria.

Tip   

The valid interface types and numbers can vary according to the router and the interfaces on the router. Use the appropriate interface specification, typed exactly as it is displayed under the Interface column of the show interfaces command output, to replace the interface-type and interface-number arguments in the show arp command.



Example:
Router# show arp vrf vrf1 dynamic 209.165.200.225 e3/1 detail
 
ARP entry for 209.165.200.225, link type IP.
  Dynamic, via Ethernet3/1, last updated 147 minutes ago.
  Encap type is ARPA, hardware address is 0050.d173.e881, 6 bytes long.
  ARP subblocks:
  * Dynamic ARP Subblock
    Entry will be refreshed in 109 minutes and 52 seconds.
    It has 2 chances to be refreshed before it is purged.
    Entry is complete.
  * IP ARP Adjacency
    Adjacency (for 209.165.200.225 on Ethernet3/1) was installed.
    Connection ID: 0
Step 5   show arp application [application-id] [detail]

This command displays ARP table information for a specific ARP application or for all applications supported by ARP and running on registered clients:



Example:
Router# show arp application detail
 
Number of clients registered: 8
Application         ID        Num of Subblocks
ARP Backup          200       0
Application         ID        Num of Subblocks
IP ARP Adj Conn ID  201       0
Application         ID        Num of Subblocks
IP Subscriber       202       0
Application         ID        Num of Subblocks
LEC                 203       0
Application         ID        Num of Subblocks
DHCPD               204       0
Application         ID        Num of Subblocks
DSS                 205       0
Application         ID        Num of Subblocks
IP Mobility         206       0
Application         ID        Num of Subblocks
IP ARP Adjacency    207       5
          
ARP entry for 209.165.200.226, link type IP.
  Static.
    Subblock data:
    Adjacency (for 209.165.200.226 on Ethernet3/1) was withdrawn.
    Connection ID: 0
ARP entry for 209.165.200.227, link type IP.
  Dynamic, via Ethernet3/0.
    Subblock data:
    Adjacency (for 209.165.200.227 on Ethernet3/0) was installed.
    Connection ID: 0
ARP entry for 209.165.200.228, link type IP.
  Dynamic, via Ethernet3/0.
    Subblock data:
    Adjacency (for 209.165.200.228 on Ethernet3/0) was installed.
    Connection ID: 0
ARP entry for 209.165.200.225, link type IP.
  Dynamic, via Ethernet3/1, in VRF vrf1.
    Subblock data:
    Adjacency (for 209.165.200.225 on Ethernet3/1) was installed.
    Connection ID: 0
ARP entry for 209.165.200.229, link type IP.
  Dynamic, via Ethernet3/1, in VRF vrf1.
    Subblock data:
    Adjacency (for 209.165.200.229 on Ethernet3/1) was installed.
    Connection ID: 0

Displaying ARP HA Status and Statistics

To display the ARP HA status and statistics for a Cisco networking device that contains dual RPs and has been configured for SSO, use the show arp ha command. Different HA details are displayed, depending on the current RP state:

  • The active RP that was the active RP from last time the router was rebooted
  • The active RP that was a standby RP and became the active RP after an SSO occurred
  • The standby RP
SUMMARY STEPS

1.    enable

2.    show arp ha


DETAILED STEPS
Step 1   enable

This command enables privileged EXEC mode:



Example:
Router> enable
Step 2   show arp ha

This command displays the ARP HA status and statistics collected for an HA-capable platform, such as a Cisco 7600 series router, that has been configured for SSO. The output from this command depends on the current and most recent states of the RP.

Active RP

The following is sample output from the show arp ha command on the active RP that has been the active RP since the last time the router was rebooted. ARP HA statistics are displayed for the active state only.



Example:
Router# show arp ha
 
ARP HA in active state (ARP_HA_ST_A_UP_SYNC).
  4 ARP entries in the synchronization queue.
  No ARP entry waiting to be synchronized.
  4022 synchronization packets sent.
  No error in allocating synchronization packets.
  No error in sending synchronization packets.
  No error in encoding interface names.

Active RP That Was Previously a Standby RP

The following is sample output from the show arp ha command on the active RP that had been the standby RP and became the active RP after the most recent SSO occurred. ARP HA statistics are displayed for the active state and also for the previous standby state.



Example:
Router# show arp ha
 
ARP HA in active state (ARP_HA_ST_A_UP_SYNC).
  4 ARP entries in the synchronization queue.
  No ARP entry waiting to be synchronized.
  4022 synchronization packets sent.
  No error in allocating synchronization packets.
  No error in sending synchronization packets.
  No error in encoding interface names.
Statistics collected when ARP HA in standby state:
  No ARP entry in the backup table.
  5 synchronization packets processed.
  No synchronization packet dropped in invalid state.
  No error in decoding interface names.
  4 ARP entries restored before timer.
  No ARP entry restored on timer.
  No ARP entry purged since interface is down.
  No ARP entry purged on timer.

Standby RP

The following is sample output from the show arp ha command on the standby RP. ARP HA statistics are displayed for the standby state only.



Example:
Router# show arp ha
 
ARP HA in standby state (ARP_HA_ST_S_UP).
  4 ARP entries in the backup table.
  4005 synchronization packets processed.
  No synchronization packet dropped in invalid state.
  No error in decoding interface names.

Refreshing Dynamically Learned ARP Table Entries

Refresh dynamically learned ARP table entries to ensure the validity of the IP address and MAC address mapping information and to immediately age out any stale entries (dynamic ARP entries that have expired but have not yet been aged out by the default, timer-based process).

The scope of the refresh operation can be limited to the entries that match any one of the following selection criteria:

  • ARP cache entries for a specific interface
  • ARP cache entries for the global VRF and for a specific host
  • ARP cache entries for a named VRF and for a specific host
SUMMARY STEPS

1.    enable

2.    show interfaces [summary]

3.    clear arp-cache [interface type number | [vrf vrf-name] ip-address]


DETAILED STEPS
  Command or Action Purpose
Step 1
enable


Example:

Router> enable

 

Enables privileged EXEC mode.

  • Enter your password if prompted.
 
Step 2
show interfaces [summary]


Example:

Router# show interfaces summary

 

(Optional) Lists all the interfaces configured on the router or access server.

  • To list the interfaces in a summary table, use the summary keyword. This form of the command output is useful if you will be refreshing the ARP table entries for a particular router interface.
 
Step 3
clear arp-cache [interface type number | [vrf vrf-name] ip-address]


Example:

Router# clear arp-cache 192.0.2.240

 

Refreshes all dynamically created ARP table entries or only the dynamically created ARP table entries that meet the selection criteria.

 

Setting the Maximum Limit for Learned ARP Table Entries

Limiting the number of ARP entries that can be learned by the system helps to prevent the possibility of system instability due to memory exhaustion.

The default behavior of the system is not to enforce any such maximum limit on the number of learned ARP entries in the system. Under these normal circumstances, the number of ARP entries learned from each interface is related to the number of directly connected hosts on the LAN. Scaling ARP entries to a very large number can have the following major impacts on the device:

  • Increase in CPU time to process the ARP packets and to age ARP entries
  • Significantly increased memory consumption in system memory and hardware table memories (for hardware forwarding platforms), which could lead to memory fragmentation and exhaustion

When the number of ARP entries that can be created by the system is not limited, memory exhaustion can cause system instability. Setting a maximum limit for the number of learned ARP table entries can help prevent this scenario from arising.

Once the limit is set, upon reaching the learn ARP entry threshold limit or 80 percent of the configured maximum limit, the system will generate a syslog message with a priority set to Level 3 (LOG_NOTICE). Upon reaching the configured maximum limit, the system will:

  • Start discarding newly learned ARP entries
  • Generate a syslog message with a priority set to Level 3 (LOG_NOTICE). The administrator will have to take appropriate action.

When learned ARP entries in the ARP table drop down from the maximum limit to the permit threshold limit or 95 percent of the maximum, a syslog message is generated to notify the system administrator that the ARP table is back in the normal operational state.

Before You Begin
  • Consult the support documentation for the router to determine the maximum number of ARP entries that can be learned and entered in the ARP table before setting it at the command-line interface (CLI).

Note


  • The maximum limit for the number of learned ARP entries is platform dependent.

Note


The setting of a maximum limit for learned ARP table entries limit functionality is supported only on the Cisco 7600 platform. This support started in Cisco IOS Release 12.2(33)SRD3.
>

SUMMARY STEPS

1.    enable

2.    configure terminal

3.    ip arp entry learn max-limit


DETAILED STEPS
  Command or Action Purpose
Step 1
enable


Example:

Router> enable

 

Enables privileged EXEC mode.

  • Enter your password if prompted.
 
Step 2
configure terminal


Example:

Router# configure terminal

 

Enters global configuration mode.

 
Step 3
ip arp entry learn max-limit


Example:

Router# ip arp entry learn 256

 

Setting the maximum number of learned ARP entries for the platform.

 

Resetting ARP HA Statistics

This task allows the user to reset the ARP HA statistics. It may be useful when debugging the ARP HA subsystem.

SUMMARY STEPS

1.    enable

2.    clear arp-cache counters ha


DETAILED STEPS
  Command or Action Purpose
Step 1
enable


Example:

Router> enable

 

Enables privileged EXEC mode.

  • Enter your password if prompted.
 
Step 2
clear arp-cache counters ha


Example:

Router# clear arp-cache counters ha

 

Resets the ARP HA statistics.

 

Enabling Debug Trace for ARP Transactions

Perform this task to enable debug trace for ARP transactions to monitor the ARP subsystem.

Debug trace can be enabled for all IP ARP packet traffic, or it can be enabled for an individual type of ARP event, such as:

  • ARP entry events
    • Any dynamic ARP entry event
    • Any interface ARP entry event
    • Any static ARP entry event
    • Any ARP entry subblock event
  • ARP table events
    • ARP table operations (entry insertion, modification, or deletion)
    • ARP table timer events
    • ARP table database events (database read/write events)
  • ARP HA events
  • ARP interface events
    • ARP/Cisco Express Forwarding Adjacency interface transactions
    • ARP Application interface transactions

Debug Filtering Support

The amount of ARP debug information displayed is filtered according to the interface and access list specified by the debug list command.

SUMMARY STEPS

1.    enable

2.    debug list [list] [interface]

3.    debug arp [vrf vrf-name | global] [arp-entry-event | arp-table-event | ha | interface-interaction]

4.    show debugging

5.    no debug arp [vrf vrf-name | global] [arp-entry-event | arp-table-event | ha | interface-interaction]


DETAILED STEPS
  Command or Action Purpose
Step 1
enable


Example:

Router> enable

 

Enables privileged EXEC mode.

  • Enter your password if prompted.
 
Step 2
debug list [list] [interface]


Example:

Router# debug list 1102 serial

 

(Optional) Enables the filtering of ARP debugging information (or debugging information for any of the other protocols supported by this command) by using either or both of the following criteria:

  • To display debugging information for a specific interface rather than for all interfaces on a router, identify the interface by using the interface argument. If the interface needs to be configured, use the interface command.
  • To display information for a specific type of packet rather than for all packets, identify the packet details by using the list argument to identify an extended ACL. The ACL specifies a source MAC Ethernet address, the destination MAC Ethernet address, and arbitrary bytes in the packet. If the extended access list needs to be configured, use the access-list (extended-ibm) command.
 
Step 3
debug arp [vrf vrf-name | global] [arp-entry-event | arp-table-event | ha | interface-interaction]


Example:

Router# debug arp static

 

Enables debug trace for ARP packets.

  • Enables debug trace for one of the following specific types of ARP events:
    • ARP entry events
    • ARP table events
    • ARP HA events (on HA-capable platforms)
    • Interactions on an ARP interface
 
Step 4
show debugging


Example:

Router# show debugging

 

Lists the debugging options enabled on this router.

 
Step 5
no debug arp [vrf vrf-name | global] [arp-entry-event | arp-table-event | ha | interface-interaction]


Example:

Router# no debug arp static

 

(Optional) Disables debug trace for ARP packets.

  • When used with a keyword, this command disables debug trace for one of the following specific types of ARP events:
    • ARP entry events
    • ARP table events
    • ARP HA events (on HA-capable platforms)
    • Interactions on an ARP interface
 

Enabling an ARP Trap on the Number of Learned Entries on an Interface

Enable an ARP trap or threshold for the number of dynamically learned arp entries if network administrators are to be alerted when the number of ARP entries for an interface reaches a configured threshold. The alert will be in the form of interface-specific ARP syslog output.

If the number of ARP table entries for an interface reaches a high level (based on the number of nodes the router serves and the number of hosts on that interface), the cause might be an ARP DoS attack on the router through that interface. This condition is described in "ARP Table Size Monitoring per Interface".

Before You Begin

Determine the expected maximum number of entries for an interface. Such an estimate is typically based on the following information:

  • The number of nodes the router serves
  • The number of hosts on the interface

Depending on your network configuration, other factors such as whether proxy ARP is enabled can affect the number of ARP table entries for a given interface.


SUMMARY STEPS

1.    enable

2.    configure terminal

3.    interface type number

4.    arp log threshold entries entry-count

5.    end

6.    show running-config interface type number


DETAILED STEPS
  Command or Action Purpose
Step 1
enable


Example:

Router> enable

 

Enables privileged EXEC mode.

  • Enter your password if prompted.
 
Step 2
configure terminal


Example:

Router# configure terminal

 

Enters global configuration mode.

 
Step 3
interface type number


Example:

Router(config)# interface ethernet 0/0

 

Configures an interface type and enters interface configuration mode so that the specific interface can be configured.

 
Step 4
arp log threshold entries entry-count


Example:

Router(config-if)# arp log threshold entries 1000

 

Enables an ARP trap so that the ARP log is triggered when a specific number of dynamically learned entries is reached on the router interface.

 
Step 5
end


Example:

Router(config-if) end

 

Returns to privileged EXEC mode.

 
Step 6
show running-config interface type number


Example:

Router# show running-config interface ethernet 0/0

 

Displays information about the current operating configuration for the specified interface.

  • If an ARP trap is enabled for a given interface, the information for the interface command includes the arp log threshold entries command, followed by the threshold value.
 

Configuration Examples for Monitoring and Maintaining ARP Information

Setting the Maximum Limit for Learned ARP Table Entries Example

The following example displays how to set the maximum limit for the number of learned ARP table entries. A maximum limit of 512,000 learned ARP entries is set.

Router> enable
Router# configure terminal
Router(config)# ip arp entry learn 512000

Displaying the Maximum Limit for Learned ARP Table Entries Example

The following example displays the maximum limit for the number of learned ARP table entries after it has been set at the CLI:

Router# show arp summary
Total number of entries in the ARP table: 4.
Total number of Dynamic ARP entries: 0.
Total number of Incomplete ARP entries: 0.
Total number of Interface ARP entries: 3.
Total number of Static ARP entries: 1.
Total number of Alias ARP entries: 0.
Total number of Simple Application ARP entries: 0.
Total number of Application Alias ARP entries: 0.
Total number of Application Timer ARP entries: 0.
Maximum limit of Learn ARP entry : 512000.
Maximum configured Learn ARP entry limit : 512000.
Learn ARP Entry Threshold is 409600 and Permit Threshold is 486400.
Total number of Learn ARP entries: 0.
Interface              Entry Count
GigabitEthernet4/7               1
GigabitEthernet4/1.1             1
GigabitEthernet4/1               1
EOBC0/0 

The maximum limit is shown as being set to 512,000 (Maximum configured Learn ARP entry limit: 512000.). The allowed maximum limit for learned ARP table entries is 512,000 (Maximum limit of Learn ARP entry: 512000). A maximum limit greater than this figure cannot be set.

Additional References

Related Documents

Related Topic

Document Title

Cisco IOS commands

Cisco IOS Master Commands List, All Releases

ARP commands: complete command syntax, command mode, command history, defaults, usage guidelines, and examples

Cisco IOS IP Addressing Services Command Reference

IP addressing tasks

"Configuring IPv4 Addresses" module

ARP configuration tasks

"Configuring Address Resolution Protocol Options" module

Standards

Standard

Title

No new or modified standards are supported by this feature, and support for existing standards has not been modified by this feature.

--

MIBs

MIB

MIBs Link

No new or modified MIBs are supported by this feature, and support for existing MIBs has not been modified by this feature.

To locate and download MIBs for selected platforms, Cisco IOS releases, and feature sets, use Cisco MIB Locator found at the following URL:

http://www.cisco.com/go/mibs

RFCs

RFC

Title

RFC 1812

Requirements for IP Version 4 Routers

Technical Assistance

Description

Link

The Cisco Support and Documentation website provides online resources to download documentation, software, and tools. Use these resources to install and configure the software and to troubleshoot and resolve technical issues with Cisco products and technologies. Access to most tools on the Cisco Support and Documentation website requires a Cisco.com user ID and password.

http://www.cisco.com/cisco/web/support/index.html

Feature Information for Monitoring and Maintaining ARP Information

The following table provides release information about the feature or features described in this module. This table lists only the software release that introduced support for a given feature in a given software release train. Unless noted otherwise, subsequent releases of that software release train also support that feature.

Use Cisco Feature Navigator to find information about platform support and Cisco software image support. To access Cisco Feature Navigator, go to www.cisco.com/go/cfn. An account on Cisco.com is not required.

Table 1 Feature Information for Monitoring and Maintaining ARP Information

Feature Name

Releases

Feature Information

ARP Rewrite

12.4(11)T

12.2(31)SB2

12.2(33)SRB

12.2(33)SRD3

12.2(33)SRE

15.1(1)SY

This feature introduces the following ARP support enhancements:

  • New ARP table entry types to support the attachment of application-specific data within individual entries
  • Enabling of ARP debug trace for specific ARP events
  • Filtering of ARP debug trace on a per-interface or per-access list basis
  • Displaying or refreshing of dynamically learned ARP table entries based on various selection criteria
  • Displaying or resetting of ARP HA status and statistics for HA-capable platforms
  • Displaying of ARP/Cisco Express Forwarding adjacency notification status
  • Enabling the ARP log if a specific number of dynamically learned entries is reached on a particular router interface

This feature is not marketed and hence, will not appear in the Feature Navigator.

In 12.2(33)SRD3, support for setting the maximum limit for learned ARP table entries on the Cisco 7600 platform was added.

This feature was integrated into Cisco IOS Release 12.2(33)SRE.

The following commands were introduced or modified: arp log threshold entries, clear arp-cache counters ha, clear arp-cache, debug arp, ip arp entry learn, show arp, show arp application, show arp ha, show arp summary.

Glossary

ACL --access control list. A list kept by routers to control access to or from the router for a number of services (for example, to prevent packets with a certain IP address from leaving a particular interface on the router).

active RP --The RP that controls the system, runs the routing protocols, and presents the system management interface.

adjacency --A relationship formed between selected neighboring routers and end nodes for the purpose of exchanging routing information. Adjacency is based upon the use of a common media segment by the routers and nodes involved.

ARP --Address Resolution Protocol. Internet protocol used to map an IP address to a MAC address. Used to obtain the physical address when only the logical address is known. Defined in RFC 826.

ARPA --Advanced Research Projects Agency. Research and development organization that is part of the Department of Defense (DoD). ARPA is responsible for numerous technological advances in communications and networking. ARPA evolved into DARPA, and then back into ARPA again (in 1994).

Cisco Express Forwarding --A Layer 3 switching technology. Cisco Express Forwarding can also refer to central Cisco Express Forwarding mode, one of two modes of Cisco Express Forwarding operation. Cisco Express Forwarding enables a Route Processor to perform express forwarding. Distributed Cisco Express Forwarding is the other mode of Cisco Express Forwarding operation.

DHCP --Dynamic Host Configuration Protocol. Provides a mechanism for allocating IP addresses dynamically so that addresses can be reused when hosts no longer need them.

hop --Passage of a data packet between two network nodes (for example, between two routers).

IP --Internet Protocol. Network layer for the TCP/IP protocol suite. Internet Protocol version 4 is a connectionless, best-effort packet switching protocol. Defined in RFC 791.

IP datagram --Fundamental unit of information passed across the Internet. An IP datagram contains source and destination addresses along with data and a number of fields that define such things as the length of the datagram, the header checksum, and flags to indicate whether the datagram can be (or was) fragmented.

MAC --Media Access Control. Lower of the two sublayers of the data link layer defined by the IEEE. The MAC sublayer handles access to shared media, such as whether token passing or contention will be used.

MAC address --Media Access Control address. Standardized data link layer address that is required for every port or device that connects to a LAN. Also known as a hardware address, MAC-layer address, and physical address.

MiM --Man-in-the-Middle. A type of ARP attack performed by mimicking another device (for example, the default gateway) in the ARP packets sent to the attacked device so that the end station or router learns counterfeited device identities. This deception allows a malicious user to pose as an intermediary who can launch an ARP-spoofing attack.

proxy ARP --proxy Address Resolution Protocol. Variation of the ARP protocol in which an intermediate device (for example, a router) sends an ARP response on behalf of an end node to the requesting host. Proxy ARP can lessen bandwidth use on slow-speed WAN links. See also ARP.

RP --Route Processor. Processor module in the Cisco 7000 series routers that contains the CPU, system software, and most of the memory components that are used in the router. Sometimes called a supervisory processor.

SSO --stateful switchover. A method of providing redundancy and synchronization for many Cisco IOS applications and features. SSO is necessary for the Cisco IOS firewall to learn about the redundancy state of the network and to synchronize their internal application state with their redundant peers. SSO allows the active and standby routers to share firewall session state information so that each router has enough information to become the active router at any time.

standby RP --The RP that waits in case the active RP fails.

VPN --Virtual Private Network. Framework that consists of multiple peers transmitting private data securely to one another over an otherwise public infrastructure. A VPN protects inbound and outbound network traffic by using protocols that tunnel and encrypt all data at the IP level. This framework permits networks to extend beyond their local topology, while remote users are provided with the appearance and functionality of a direct network connection. A VPN enables IP traffic to travel securely over a public TCP/IP network by encrypting all traffic from one network to another. A VPN uses "tunneling" to encrypt all information at the IP level.

VRF --VPN routing and forwarding instance. A VRF consists of an IP routing table, a derived forwarding table, a set of interfaces that use the forwarding table, and a set of rules and routing protocols that determine what goes into the forwarding table. In general, a VRF includes the routing information that defines a customer VPN site that is attached to a PE router. Each VPN instantiated on the PE router has its own VRF.

Cisco and the Cisco logo are trademarks or registered trademarks of Cisco and/or its affiliates in the U.S. and other countries. To view a list of Cisco trademarks, go to this URL: www.cisco.com/go/trademarks. Third-party trademarks mentioned are the property of their respective owners. The use of the word partner does not imply a partnership relationship between Cisco and any other company. (1110R)

Any Internet Protocol (IP) addresses and phone numbers used in this document are not intended to be actual addresses and phone numbers. Any examples, command display output, network topology diagrams, and other figures included in the document are shown for illustrative purposes only. Any use of actual IP addresses or phone numbers in illustrative content is unintentional and coincidental.

© 2012 Cisco Systems, Inc. All rights reserved.