To display all entries in the ARP cache, use this command without any arguments or keywords.
Entry Selection Options
You can to limit the scope of the command output by applying various combinations of the following ARP entry selection criteria:
- Entries under a specific VRF
- Entries in a specific ARP mode
- Entries for a specific host or entries for a specific network
- Entries associated with a specific router interface
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. 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-typeand interface-number arguments in the show arp command.
Detailed Output Format
To include additional details about each ARP entry displayed, use this command with the detail keyword. When this display option is used, the following additional information is included:
- Mode-specific details (such as entry update time)
- Subblocks (if any)
ARP Adjacency Notification
If Cisco Express Forwarding (CEF) 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). CEF 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.
- To verify that IPv4 CEF is running, use the show ip cef command.
- To verify that an adjacency exists for a connected device, that the adjacency is valid, and that the MAC header rewrite string is correct, use the show adjacency command.
The ARP table information is one of the sources for CEF 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 CEF adjacency via the adjacency database. If the synchronization succeeds, IP ARP adjacency is said to be “installed”; if the synchronization fails, IP ARP adjacency is said to have been “withdrawn.”
Attachment to an outbound interface occurs only for ARP entries in the following modes: alias, dynamic, static, Application Simple, and Application Timer.
To display detailed information about any ARP adjacency notification that may have occurred, use the show arp command with the detail keyword. You can use this information to supplement the information available through ARP/CEF adjacency debug trace. To enable debug trace for ARP/CEF adjacency interactions, use the debug arp command with the adjacency keyword.
ARP Cache Administration
To refresh all entries for the specified interface (or all interfaces) or to refresh all entries of the specified address (or all addresses) in the specified VRF table (or in the global VRF table), use the clear arp-cache command.
To enable debugging output for ARP transactions, use the debug arp command.
The following is sample output from the show arp command with no optional keywords or arguments specified:
Router# show arp Protocol Address Age (min) Hardware Addr Type InterfaceInternet 192.0.2.112 120 0000.a710.4baf ARPA Ethernet3AppleTalk 4028.5 29 0000.0c01.0e56 SNAP Ethernet2Internet 192.0.2.114 105 0000.a710.859b ARPA Ethernet3AppleTalk 4028.9 - 0000.0c02.a03c SNAP Ethernet2Internet 192.0.2.121 42 0000.a710.68cd ARPA Ethernet3Internet 192.0.2.9 - 0000.3080.6fd4 SNAP TokenRing0AppleTalk 4036.9 - 0000.3080.6fd4 SNAP TokenRing0Internet 192.0.2.9 - 0000.0c01.7bbd SNAP Fddi0
The table below describes the fields shown in the display.
Table 1 show arp Field Descriptions
Protocol for network address in the Address field.
The network address that corresponds to the Hardware Address.
Age in minutes of the cache entry. A hyphen (-) means the address is local.
LAN hardware address of a MAC address that corresponds to the network address.
Indicates the encapsulation type the Cisco IOS software is using for the network address in this entry. Possible values include:
- ARPA--For Ethernet interfaces.
- SAP--For Hewlett-Packard interfaces.
- SMDS--For Switched Multimegabit Data Service (SMDS) interfaces.
- SNAP--For FDDI and Token Ring interfaces.
- SRP-A--For Switch Route Processor, side A (SRP-A) interfaces.
- SRP-B--For Switch Route Processor, side B (SRP-B) interfaces.
Indicates the interface associated with this network address.
When this command is used to display dynamic ARP entries, the display information includes the time of the last update and the amount of time before the next scheduled refresh is to occur. The following is sample output from the show arp command for the dynamic ARP entry at network address 192.0.2.1:
Router# show arp 192.0.2.1 detail ARP entry for 192.0.2.1, link type IP. Alias, last updated 13323 minutes ago. Encap type is ARPA, hardware address is 1234.1234.1234, 6 bytes long. ARP subblocks: * Static ARP Subblock Floating entry. Entry is complete, attached to GigabitEthernet1/1. * IP ARP Adjacency Adjacency (for 192.0.2.1 on GigabitEthernet1/1) was installed.
When this command is used to display floating static ARP entries, the display information includes the associated interface, if any.The following is sample output from the show arp command for the floating static ARP entry at network address 192.0.2.2 whose intended interface is down:
Router# show arp 192.0.2.2 detail ARP entry for 192.0.2.2, link type IP. Alias, last updated 13327 minutes ago. Encap type is ARPA, hardware address is 1234.1234.1234, 6 bytes long. ARP subblocks: * Static ARP Subblock Floating entry. Entry is incomplete. * IP ARP Adjacency Adjacency (for 192.0.2.2 on GigabitEthernet1/1) was withdrawn.
The following is sample detailed output from the show arp command for the Application Alias ARP entry at network address 192.0.2.3:
Router# show arp 192.0.2.3 detail ARP entry for 192.0.2.3, link type IP. Application Alias, via Ethernet2/2, last updated 0 minute ago. Created by "HSRP". Encap type is ARPA, hardware address is 0000.0c07.ac02, 6 bytes long. ARP subblocks: * Application Alias ARP Subblock * HSRP ARP Application entry for application HSRP.
The following is sample detailed output from the show arp command for all dynamic ARP entries:
Router# show arp dynamic detail ARP entry for 192.0.2.4, link type IP. Dynamic, via Ethernet2/1, last updated 0 minute ago. Encap type is ARPA, hardware address is 0000.0000.0014, 6 bytes long. ARP subblocks: * Dynamic ARP Subblock Entry will be refreshed in 0 minute and 1 second. It has 1 chance to be refreshed before it is purged. Entry is complete. * IP ARP Adjacency Adjacency (for 192.0.2.4 on Ethernet2/1) was installed.