-
Cisco IOS IP Command Reference, Volume 1 of 4: Addressing and Services, Release 12.3
-
Introduction
-
IP Addressing and Services Commands: A through C
-
IP Addressing and Services Commands: D through H
-
IP Addressing and Services Commands: idle through ip local-proxy-arp
-
IP Addressing and Services Commands: ip mask-reply through ip web-cache
-
IP Addressing and Services Commands: L through R
-
IP Addressing and Services Commands: serverfarm through show ip nat
-
IP Addressing and Services Commands: show ip nhrp through synguard
-
IP Addressing and Services Commands: T through W
-
Table Of Contents
inservice (server farm virtual server)
ip dhcp-client default-router distance
ip dhcp limited-broadcast-address
ip dhcp relay information check
ip dhcp relay information option
ip dhcp relay information policy
ip dhcp relay information trusted
ip dhcp relay information trust-all
ip drp authentication key-chain
ip forward-protocol spanning-tree
ip forward-protocol turbo-flood
ip icmp rate-limit unreachable
idle
To specify the minimum amount of time for which IOS SLB maintains connection information in the absence of packet activity, use the idle command in virtual server configuration mode. To restore the default idle duration value, use the no form of this command.
idle duration
no idle
Syntax Description
duration
Idle connection timer duration (in seconds). Valid values range from 10 to 65535. The default is 3600 seconds (1 hour).
Defaults
The default duration is 3600 seconds.
Command Modes
SLB virtual server configuration
Command History
12.0(7)XE
This command was introduced.
12.1(5)T
This command was integrated into Cisco IOS Release 12.1(5)T.
Usage Guidelines
TCP connections that do not send flows or keepalives before the idle timer expires are assumed to be inactive and are reset (RST).
If you are configuring an idle timer for HTTP flows, choose a low number such as 120 seconds as a starting point. A low number ensures that the IOS SLB connection database maintains a manageable size if problems at the server, client, or network result in a large number of connections. However, do not choose a value under 60 seconds; such a low value can reduce the efficiency of the IOS SLB feature.
Examples
The following example instructs the IOS SLB feature to maintain connection information for an idle connection for 120 seconds:
ip slb vserver PUBLIC_HTTPidle 120Related Commands
show ip slb vservers
Displays information about the virtual servers.
virtual
Configures the virtual server attributes.
import all
To import Dynamic Host Configuration Protocol (DHCP) option parameters into the DHCP Server database, use the import all command in DHCP pool configuration mode. To disable this feature, use the no form of this command.
import all
no import all
Syntax Description
This command has no arguments or keywords.
Defaults
Disabled
Command Modes
DHCP pool configuration
Command History
Usage Guidelines
When the no import all command is used, the Cisco IOS DHCP Server deletes all "imported" option parameters that were added to the specified pool in the server database. Manually configured DHCP option parameters override imported DHCP option parameters.
Imported option parameters are not part of the router configuration and are not saved in NVRAM.
Examples
The following example allows the importing of all DHCP options for a pool named pool1:
ip dhcp pool pool1network 172.16.0.0 /16import allRelated Commands
inservice (real server)
To enable the real server for use by the IOS SLB feature, use the inservice SLB real server configuration command. To remove the real server from service, use the no form of this command.
inservice
no inservice
Syntax Description
This command has no arguments or keywords.
Defaults
If you do not specify the inservice command, the real server is defined to IOS SLB but is not used.
Command Modes
SLB real server configuration
Command History
12.0(7)XE
This command was introduced.
12.1(5)T
This command was integrated into Cisco IOS Release 12.1(5)T.
Examples
The following example enables the real server for use by the IOS SLB feature:
ip slb serverfarm PUBLICreal 10.10.1.1inserviceRelated Commands
real
Identifies a real server.
show ip slb reals
Displays information about the real servers.
show ip slb serverfarms
Displays information about the server farm configuration.
inservice (server farm virtual server)
To enable the virtual server for use by IOS Server Load Balancing (IOS SLB), use the inservice command in SLB server farm virtual server configuration mode. To remove the virtual server from service, use the no form of this command.
inservice [standby group-name]
no inservice [standby group-name]
Syntax Description
Defaults
The virtual server is defined to IOS SLB but is not used.
Command Modes
SLB server farm virtual server configuration
Command History
Usage Guidelines
When you use the no form of this command to remove a virtual server from service, the virtual server acquiesces gracefully. No new connections are assigned, and existing connections are allowed to complete.
Examples
In the following example, the virtual server is enabled for use by the IOS SLB feature:
Router(config)# ip slb vserver PUBLIC_HTTPRouter(config-slb-vserver)# inserviceRelated Commands
show ip slb vservers
Displays information about the virtual servers.
virtual
Configures the virtual server attributes.
inservice (virtual server)
To enable the virtual server for use by the IOS SLB feature, use the inservice SLB virtual server configuration command. To remove the virtual server from service, use the no form of this command.
inservice [standby group-name]
no inservice [standby group-name]
Syntax Description
standby
(Optional) Configures the Hot Standby Router Protocol (HSRP) standby virtual server.
group-name
(Optional) Specifies the HSRP group name with which the IOS SLB virtual server is associated.
Defaults
If you do not specify the inservice command, the virtual server is defined to IOS SLB but is not used.
Command Modes
SLB virtual server configuration
Command History
12.0(7)XE
This command was introduced.
12.1(1)E
The standby keyword and group-name argument were added.
12.1(5)T
This command was integrated into Cisco IOS Release 12.1(5)T.
Examples
The following example enables the real server for use by the IOS SLB feature:
ip slb vserver PUBLIC_HTTPinserviceRelated Commands
show ip slb vservers
Displays information about the virtual servers.
virtual
Configures the virtual server attributes.
ip access-group
To control access to an interface, use the ip access-group command in interface configuration mode. To remove the specified access group, use the no form of this command.
ip access-group {access-list-number | access-list-name}{in | out}
no ip access-group {access-list-number | access-list-name}{in | out}
Syntax Description
Defaults
No access list is applied to the interface.
Command Modes
Interface configuration
Command History
10.0
This command was introduced.
11.2
The access-list-name argument was added.
Usage Guidelines
Access lists are applied on either outbound or inbound interfaces. For standard inbound access lists, after receiving a packet, the Cisco IOS software checks the source address of the packet against the access list. For extended access lists, the router also checks the destination access list. If the access list permits the address, the software continues to process the packet. If the access list rejects the address, the software discards the packet and returns an ICMP host unreachable message.
For standard outbound access lists, after receiving and routing a packet to a controlled interface, the software checks the source address of the packet against the access list. For extended access lists, the router also checks the destination access list. If the access list permits the address, the software sends the packet. If the access list rejects the address, the software discards the packet and returns an ICMP host unreachable message.
If the specified access list does not exist, all packets are passed.
When you enable outbound access lists, you automatically disable autonomous switching for that interface. When you enable input access lists on any CBus or CxBus interface, you automatically disable autonomous switching for all interfaces (with one exception—an SSE configured with simple access lists can still switch packets, on output only).
Examples
The following example applies list 101 on packets outbound from Ethernet interface 0:
interface ethernet 0ip access-group 101 outRelated Commands
ip access-list resequence
ip access-list resequence access-list-name starting-sequence-number increment
Syntax Description
Defaults
Disabled
Command Modes
Global configuration
Command History
12.2(14)S
This command was introduced.
12.2(15)T
This command was integrated into Cisco IOS Release 12.2(15)T.
Usage Guidelines
This command allows the permit and deny entries of a specified access list to be resequenced with an initial sequence number value determined by the starting-sequence-number argument, and continuing in increments determined by the increment argument. If the highest sequence number exceeds the maximum possible sequence number, then no sequencing occurs.
For backward compatibility with previous releases, if entries with no sequence numbers are applied, the first entry is assigned a sequence number of 10, and successive entries are incremented by 10. The maximum sequence number is 2147483647. If the generated sequence number exceeds this maximum number, the following message is displayed:
Exceeded maximum sequence number.If the user enters an entry without a sequence number, it is assigned a sequence number that is 10 greater than the last sequence number in that access list and is placed at the end of the list.
If the user enters an entry that matches an already existing entry (except for the sequence number), then no changes are made.
If the user enters a sequence number that is already present, the following error message is generated:
Duplicate sequence number.If a new access list is entered from global configuration mode, then sequence numbers for that access list are generated automatically.
Distributed support is provided so that the sequence numbers of entries in the Route Processor (RP) and line card (LC) are in synchronization at all times.
Sequence numbers are not nvgened. That is, the sequence numbers themselves are not saved. In the event that the system is reloaded, the configured sequence numbers revert to the default sequence starting number and increment.
This command works with named standard and extended IP access lists. Because the name of an access list can be designated as a number, numbers are acceptable as names as long as they are entered in named access list configuration mode.
Examples
The following example resequences an access list named kmd1. The starting sequence number is 100, and the increment value is 5:
Router(config)# ip access-list resequence kmd1 100 5Related Commands
deny (IP)
Sets conditions under which a packet does not pass a named IP access list.
permit (IP)
Sets conditions under which a packet passes a named IP access list.
ip access-list
To define an IP access list by name, use the ip access-list global configuration command. To remove a named IP access list, use the no form of this command.
ip access-list {standard | extended} access-list-name
no ip access-list {standard | extended} access-list-name
Syntax Description
Defaults
No named IP access list is defined.
Command Modes
Global configuration
Command History
Usage Guidelines
Use this command to configure a named IP access list as opposed to a numbered IP access list. This command will place the router in access-list configuration mode, where you must define the denied or permitted access conditions with the deny and permit commands.
Specifying the standard or extended keyword with the ip access-list command determines the prompt you get when you enter access-list configuration mode.
Use the ip access-group command to apply the access list to an interface.
Named access lists are not compatible with Cisco IOS releases prior to Release 11.2.
Examples
The following example defines a standard access list named Internetfilter:
ip access-list standard Internetfilterpermit 192.5.34.0 0.0.0.255permit 128.88.0.0 0.0.255.255permit 36.0.0.0 0.255.255.255! (Note: all other access implicitly denied)Related Commands
ip accounting
To enable IP accounting on an interface, use the ip accounting command in interface configuration mode. To disable IP accounting, use the no form of this command.
ip accounting [access-violations] [output-packets]
no ip accounting [access-violations] [output-packets]
Syntax Description
Defaults
Disabled
Command Modes
Interface configuration
Command History
10.0
This command was introduced.
10.3
The access-violations keyword was added.
Usage Guidelines
The ip accounting command records the number of bytes (IP header and data) and packets switched through the system on a source and destination IP address basis. Only transit IP traffic is measured and only on an outbound basis; traffic generated by the router access server or terminating in this device is not included in the accounting statistics. Traffic coming from a remote site and transiting through a router is also recorded.
If you specify the access-violations keyword, the ip accounting command provides information identifying IP traffic that fails IP access lists. Identifying IP source addresses that violate IP access lists alerts you to possible attempts to breach security. The data might also indicate that you should verify IP access list configurations.
To receive a logging message on the console when an extended access list entry denies a packet access (to log violations), you must include the log keyword in the access-list (IP extended) or access-list (IP standard) command.
Statistics are accurate even if IP fast switching or IP access lists are being used on the interface.
IP accounting disables autonomous switching, SSE switching, and distributed switching (dCEF) on the interface. IP accounting will cause packets to be switched on the Route Switch Processor (RSP) instead of the Versatile Interface Processor (VIP), which can cause performance degradation.
Examples
The following example enables IP accounting on Ethernet interface 0:
interface ethernet 0ip accountingRelated Commands
ip accounting-list
To define filters to control the hosts for which IP accounting information is kept, use the ip accounting-list command in global configuration mode. To remove a filter definition, use the no form of this command.
ip accounting-list ip-address wildcard
no ip accounting-list ip-address wildcard
Syntax Description
ip-address
IP address in dotted decimal format.
wildcard
Wildcard bits to be applied to the ip-address argument.
Defaults
No filters are defined.
Command Modes
Global configuration
Command History
Usage Guidelines
The wildcard argument is a 32-bit quantity written in dotted-decimal format. Address bits corresponding to wildcard bits set to 1 are ignored in comparisons; address bits corresponding to wildcard bits set to zero are used in comparisons.
Examples
The following example adds all hosts with IP addresses beginning with 192.31 to the list of hosts for which accounting information will be kept:
ip accounting-list 192.31.0.0 0.0.255.255Related Commands
ip accounting mac-address
To enable IP accounting on a LAN interface based on the source and destination MAC address, use the ip accounting mac-address command in interface configuration mode. To disable IP accounting based on the source and destination MAC address, use the no form of this command.
ip accounting mac-address {input | output]
no ip accounting mac-address {input | output]
Syntax Description
input
Performs accounting based on the source MAC address on received packets.
output
Performs accounting based on the destination MAC address on transmitted packets.
Defaults
Disabled
Command Modes
Interface configuration
Command History
Usage Guidelines
This feature is supported on Ethernet, Fast Ethernet, and FDDI interfaces.
To display the MAC accounting information, use the show interface mac EXEC command.
MAC address accounting provides accounting information for IP traffic based on the source and destination MAC address on LAN interfaces. This calculates the total packet and byte counts for a LAN interface that receives or sends IP packets to or from a unique MAC address. It also records a timestamp for the last packet received or sent. With MAC address accounting, you can determine how much traffic is being sent to and/or received from various peers at NAPS/peering points.
Examples
The following example enables IP accounting based on the source and destination MAC address for received and transmitted packets:
interface ethernet 4/0/0ip accounting mac-address inputip accounting mac-address outputRelated Commands
show interface mac
Displays MAC accounting information for interfaces configured for MAC accounting.
ip accounting precedence
To enable IP accounting on any interface based on IP precedence, use the ip accounting precedence command in interface configuration mode. To disable IP accounting based on IP precedence, use the no form of this command.
ip accounting precedence {input | output]
no ip accounting precedence {input | output]
Syntax Description
input
Performs accounting based on IP precedence on received packets.
output
Performs accounting based on IP precedence on transmitted packets.
Defaults
Disabled
Command Modes
Interface configuration
Command History
Usage Guidelines
To display IP precedence accounting information, use the show interface precedence EXEC command.
The precedence accounting feature provides accounting information for IP traffic, summarized by IP precedence values. This feature calculates the total packet and byte counts for an interface that receives or sends IP packets and sorts the results based on IP precedence. This feature is supported on all interfaces and subinterfaces and supports CEF, dCEF, flow, and optimum switching.
Examples
The following example enables IP accounting based on IP precedence for received and transmitted packets:
interface ethernet 4/0/0ip accounting precedence inputip accounting precedence outputRelated Commands
show interface precedence
Displays precedence accounting information for an interface configured for precedence accounting.
ip accounting-threshold
To set the maximum number of accounting entries to be created, use the ip accounting-threshold command in global configuration mode. To restore the default number of entries, use the no form of this command.
ip accounting-threshold threshold
no ip accounting-threshold threshold
Syntax Description
threshold
Maximum number of entries (source and destination address pairs) that the Cisco IOS software accumulates.
Defaults
The default maximum number of accounting entries is 512 entries.
Command Modes
Global configuration
Command History
Usage Guidelines
The accounting threshold defines the maximum number of entries (source and destination address pairs) that the software accumulates, preventing IP accounting from possibly consuming all available free memory. This level of memory consumption could occur in a router that is switching traffic for many hosts. Overflows will be recorded; see the monitoring commands for display formats.
The default accounting threshold of 512 entries results in a maximum table size of 12,928 bytes. Active and checkpointed tables can reach this size independently.
Examples
The following example sets the IP accounting threshold to 500 entries:
ip accounting-threshold 500Related Commands
ip accounting-transits
To control the number of transit records that are stored in the IP accounting database, use the ip accounting-transits command in global configuration mode. To return to the default number of records, use the no form of this command.
ip accounting-transits count
no ip accounting-transits
Syntax Description
Defaults
The default number of transit records that are stored in the IP accounting database is 0.
Command Modes
Global configuration
Command History
Usage Guidelines
Transit entries are those that do not match any of the filters specified by ip accounting-list global configuration commands. If no filters are defined, no transit entries are possible.
To maintain accurate accounting totals, the Cisco IOS software maintains two accounting databases: an active and a checkpointed database.
Examples
The following example specifies that no more than 100 transit records are stored:
ip accounting-transits 100Related Commands
ip address dhcp
To acquire an IP address on an interface from the Dynamic Host Configuration Protocol (DHCP), use the ip address dhcp command in interface configuration mode. To unconfigure any address that was acquired, use the no form of this command.
ip address dhcp [client-id interface-name] [hostname host-name]
no ip address dhcp [client-id interface-name] [hostname host-name]
Syntax Description
Defaults
The host name is the globally configured host name of the router.
The client identifier is an ASCII value.Command Modes
Interface configuration
Command History
Usage Guidelines
The ip address dhcp command allows any interface to dynamically learn its IP address by using the DHCP protocol. It is especially useful on Ethernet interfaces that dynamically connect to an internet service provider (ISP). Once assigned a dynamic address, the interface can be used with the Port Address Translation (PAT) of Cisco IOS Network Address Translation (NAT) to provide Internet access to a privately addressed network attached to the router.
The ip address dhcp command also works with ATM point-to-point interfaces and will accept any encapsulation type. However, for ATM multipoint interfaces you must specify Inverse ARP via the protocol ip inarp interface configuration command and use only the aa15snap encapsulation type.
Some ISPs require that the DHCPDISCOVER message have a specific host name and client identifier that is the MAC address of the interface. The most typical usage of the ip address dhcp client-id interface-name hostname host-name command is when interface-name is the Ethernet interface where the command is configured and host-name is the host name provided by the ISP.
A client identifier (DHCP option 61) can be a hexadecimal or an ASCII value. By default, the client identifier is an ASCII value. The client-id interface option overrides the default and forces the use of the hexadecimal MAC address of the named interface.
Note
Between Cisco IOS Releases 12.1(3)T and 12.2(3), the client-id optional keyword allowed the change of the fixed ASCII value for the client identifier. After Release 12.2(3), the optional client-id keyword forced the use of the hexadecimal MAC address of the named interface as the client identifier.
If a Cisco router is configured to obtain its IP address from a DHCP server, it sends a DHCPDISCOVER message to provide information about itself to the DHCP server on the network.
If you use the ip address dhcp command with or without any of the optional keywords, the DHCP option 12 field (host name option) is included in the DISCOVER message. By default, the host name specified in option 12 will be the globally configured host name of the router. However, you can use the ip address dhcp hostname host-name command to place a different name in the DHCP option 12 field than the globally configured host name of the router.
The no ip address dhcp command unconfigures any IP address that was acquired, thus sending a DHCPRELEASE message.
You might need to experiment with different configurations to determine the one required by your DHCP server. Table 1 shows the possible configuration methods and the information placed in the DISCOVER message for each method.
Examples
In the examples that follow, the command ip address dhcp is entered for the Ethernet interface 1. The DISCOVER message sent by a router configured as shown in the following example would contain "cisco- mac-address -Eth1" in the client-ID field, and the value fresno in the option 12 field.
hostname fresno!interface Ethernet 1ip address dhcpThe DISCOVER message sent by a router configured as shown in the following example would contain "cisco- mac-address -Eth1" in the client-ID field, and the value sanfran in the option 12 field.
hostname fresno!interface Ethernet 1ip address dhcp hostname sanfranThe DISCOVER message sent by a router configured as shown in the following example would contain the MAC address of the Ethernet 1 interface in the client-id field, and the value fresno in the option 12 field.
hostname fresno!interface Ethernet 1ip address dhcp client-id Ethernet 1The DISCOVER message sent by a router configured as shown in the following example would contain the MAC address of the Ethernet 1 interface in the client-id field, and the value sanfran in the option 12 field.
hostname fresno!interface Ethernet 1ip address dhcp client-id Ethernet 1 hostname sanfranRelated Commands
ip dhcp pool
Configures a DHCP address pool on a Cisco IOS DHCP Server and enters DHCP pool configuration mode.
ip address pool (DHCP)
To enable the IP address of an interface to be automatically configured when a DHCP pool is populated with a subnet from IP Control Protocol (IPCP) negotiation, use the ip address pool command in interface configuration mode. To disable autoconfiguring of the IP address of the interface, use the no form of this command.
ip address pool name
no ip address pool
Syntax Description
name
Name of the DHCP pool. The IP address of the interface will be automatically configured from the DHCP pool specified in name.
Defaults
IP address pooling is disabled.
Command Modes
Interface configuration
Command History
Usage Guidelines
Use this command to automatically configure the IP address of a LAN interface when there are DHCP clients on the attached LAN that should be serviced by the DHCP pool on the router. The DHCP pool obtains its subnet dynamically through IPCP subnet negotiation.
Examples
The following example specifies that the IP address of Ethernet interface 2 will be automatically configured from the address pool named abc:
ip dhcp pool abcimport allorigin ipcp!interface Ethernet 2ip address pool abcRelated Commands
show ip interface
Displays the usability status of interfaces configured for IP.
ip address
To set a primary or secondary IP address for an interface, use the ip address interface configuration command. To remove an IP address or disable IP processing, use the no form of this command.
ip address ip-address mask [secondary]
no ip address ip-address mask [secondary]
Syntax Description
Defaults
No IP address is defined for the interface.
Command Modes
Interface configuration
Command History
Usage Guidelines
An interface can have one primary IP address and multiple secondary IP addresses. Packets generated by the Cisco IOS software always use the primary IP address. Therefore, all routers and access servers on a segment should share the same primary network number.
Hosts can determine subnet masks using the Internet Control Message Protocol (ICMP) mask request message. Routers respond to this request with an ICMP mask reply message.
You can disable IP processing on a particular interface by removing its IP address with the no ip address command. If the software detects another host using one of its IP addresses, it will print an error message on the console.
The optional secondary keyword allows you to specify an unlimited number of secondary addresses. Secondary addresses are treated like primary addresses, except the system never generates datagrams other than routing updates with secondary source addresses. IP broadcasts and Address Resolution Protocol (ARP) requests are handled properly, as are interface routes in the IP routing table.
Secondary IP addresses can be used in a variety of situations. The following are the most common applications:
•
•
•
Note
Note
To transparently bridge IP on an interface, you must perform the following two tasks:
•
•
To concurrently route and transparently bridge IP on an interface, see the bridge crb command.
Examples
In the following example, 131.108.1.27 is the primary address and 192.31.7.17 and 192.31.8.17 are secondary addresses for Ethernet interface 0:
interface ethernet 0ip address 131.108.1.27 255.255.255.0ip address 192.31.7.17 255.255.255.0 secondaryip address 192.31.8.17 255.255.255.0 secondaryRelated Commands
ip broadcast-address
To define a broadcast address for an interface, use the ip broadcast-address interface configuration command. To restore the default IP broadcast address, use the no form of this command.
ip broadcast-address [ip-address]
no ip broadcast-address [ip-address]
Syntax Description
Defaults
Default address: 255.255.255.255 (all ones)
Command Modes
Interface configuration
Command History
Examples
The following example specifies an IP broadcast address of 0.0.0.0:
ip broadcast-address 0.0.0.0ip casa
To configure the router to function as a forwarding agent, use the ip casa global configuration command. To disable the forwarding agent, use the no form of this command.
ip casa control-address igmp-address
no ip casa
Syntax Description
Defaults
No default behavior or values.
Command Modes
Global configuration
Command History
Examples
The following example specifies the Internet address (10.10.4.1) and IGMP address (224.0.1.2) for the forwarding agent:
ip-casa 10.10.4.1 224.0.1.2Related Commands
forwarding-agent
Specifies the port on which the forwarding agent will listen for wildcard and fixed affinities.
ip cef traffic-statistics
To change the time interval that controls when Next Hop Resolution Protocol (NHRP) will set up or tear down a switched virtual circuit (SVC), use the ip cef traffic-statistics global configuration command. To restore the default values, use the no form of this command.
ip cef traffic-statistics [load-interval seconds] [update-rate seconds]
no ip cef traffic-statistics
Syntax Description
Defaults
load-interval: 30 seconds
update-rate: 10 seconds
Command Modes
Global configuration
Command History
Usage Guidelines
The ip nhrp trigger-svc command sets the threshold by which NHRP sets up and tears down a connection. The threshold is the CEF traffic load statistics. The thresholds in the ip nhrp trigger-svc command are measured during a sampling interval of 30 seconds, by default. To change that interval over which that threshold is determined, use the load-interval seconds option of the ip cef traffic-statistics command.
When NHRP is configured on a CEF switching node with a Versatile Interface Processor (VIP2) adapter, you must make sure the update-rate keyword is set to 5 seconds.
Other Cisco IOS features could also use the ip cef traffic-statistics command; this NHRP feature relies on it.
Examples
In the following example, the triggering and teardown thresholds are calculated based on an average over 120 seconds:
ip cef traffic-statistics load-interval 120Related Commands
ip nhrp trigger-svc
Configures when NHRP will set up and tear down an SVC based on aggregate traffic rates.
ip classless
At times the router might receive packets destined for a subnet of a network that has no network default route. To have the Cisco IOS software forward such packets to the best supernet route possible, use the ip classless global configuration command. To disable this feature, use the no form of this command.
ip classless
no ip classless
Syntax Description
This command has no arguments or keywords.
Defaults
Enabled
Command Modes
Global configuration
Command History
10.0
This command was introduced.
11.3
The default behavior changed from disabled to enabled.
Usage Guidelines
This command allows the software to forward packets that are destined for unrecognized subnets of directly connected networks. The packets are forwarded to the best supernet route.
When this feature is disabled, the Cisco IOS software discards the packets when a router receives packets for a subnet that numerically falls within its subnetwork addressing scheme, no such subnet number is in the routing table, and there is no network default route.
Note
Examples
The following example prevents the software from forwarding packets destined for an unrecognized subnet to the best supernet possible:
no ip classlessip default-gateway
To define a default gateway (router) when IP routing is disabled, use the ip default-gateway global configuration command. To disable this function, use the no form of this command.
ip default-gateway ip-address
no ip default-gateway ip-address
Syntax Description
Defaults
Disabled
Command Modes
Global configuration
Command History
Usage Guidelines
The Cisco IOS software sends any packets that need the assistance of a gateway to the address you specify. If another gateway has a better route to the requested host, the default gateway sends an Internet Control Message Protocol (ICMP) redirect message back. The ICMP redirect message indicates which local router the Cisco IOS software should use.
Examples
The following example defines the router on IP address 192.31.7.18 as the default router:
ip default-gateway 192.31.7.18Related Commands
ip dhcp aaa default username
To specify the default user name for non-VRF address pools that have been configured to obtain subnets through AAA, use the ip dhcp aaa default username command in global configuration mode. To disable this functionality, use the no form of this command.
ip dhcp aaa default username name
no ip dhcp aaa default username name
Syntax Description
Defaults
No default behavior or values.
Command Modes
Global configuration
Command History
12.2(8)T
This command was introduced.
12.2(15)T
The behavior for when the USERNAME attribute is sent in the AAA request was changed.
Usage Guidelines
Address pools that are configured with the vrf and origin aaa DHCP pool configuration commands will set the USERNAME attribute in the AAA request to the specified VRF name. If the VPN ID as specified in RFC 2685 is configured for the VRF, the VPN ID will be sent instead.
Address pools that are not configured with the vrf command but are configured with the origin aaa command, will set the USERNAME attribute in the AAA request to the specified name in the ip dhcp aaa default username command.
Use the debug aaa attribute command to verify the value of the USERNAME attribute in the subnet request to the AAA server.
In Cisco IOS Release 12.2(8)T, if this command is not configured, no AAA subnet request from non-VRF ODAPs will be sent.
In Cisco IOS Release 12.2(15)T, if the DHCP pool is not configured with VRF and the ip dhcp aaa default username command is not configured, the AAA request will still be sent with the USERNAME attribute set to the DHCP pool name.
This command is not needed if all ODAPs on the VHG/PE are VRF-associated.
Examples
The following example sets the USERNAME attribute in the AAA request to green:
ip dhcp aaa default username greenRelated Commands
ip dhcp bootp ignore
To allow the Cisco IOS DHCP server to selectively ignore and not reply to received Bootstrap Protocol (BOOTP) request packets, use the ip dhcp bootp ignore command in global configuration mode. To return to the default behavior, use the no form of this command.
ip dhcp bootp ignore
no ip dhcp bootp ignore
Syntax Description
This command has no arguments or keywords.
Defaults
Command Modes
Global configuration
Command History
Usage Guidelines
The Cisco IOS software can forward these ignored BOOTP request packets to another DHCP server if the ip helper-address interface configuration command is configured on the incoming interface. If the ip helper-address command is not configured, the router will drop the received BOOTP request.
Examples
The following example shows that the router will ignore received BOOTP requests:
hostname Router!ip subnet-zero!ip dhcp bootp ignoreRelated Commands
ip helper-address
Forwards UDP broadcasts, including BOOTP, received on an interface.
ip dhcp-client broadcast-flag
To configure the Cisco IOS Dynamic Host Configuration (DHCP) client to set the broadcast flag, use the ip dhcp-client broadcast-flag command in global configuration mode. To disable this feature, use the no form of this command.
ip dhcp-client broadcast-flag
no dhcp-client broadcast-flag
Syntax Description
This command has no arguments or keywords.
Defaults
The broadcast flag is on.
Command Modes
Global configuration
Command History
Usage Guidelines
Use this command to set the broadcast flag to 1 or 0 in the DHCP packet header when the DHCP client sends a discover requesting an IP address. The DHCP Server listens to this broadcast flag and broadcasts the reply packet if the flag is set to 1.
If you enter no ip dhcp-client broadcast-flag, the broadcast flag is set to 0 and the DHCP Server unicasts the reply packets to the client with the offered IP address.
The Cisco IOS DHCP client can receive both broadcast and unicast offers from the DHCP Server.
Examples
The following example sets the broadcast flag on:
Router(config)# ip dhcp-client broadcast-flagRelated Commands
ip address dhcp
Acquires an IP address on an interface via DHCP.
service dhcp
Enables DHCP server and relay functions.
ip dhcp-client default-router distance
To configure a default DHCP administrative distance for clients, use the ip dhcp-client default-router distance command in global configuration mode. To return to the default of 254, use the no form of this command.
ip dhcp-client default-router distance value
no ip dhcp-client default-router distance value
Syntax Description
distance
DHCP administrative distance. The value argument sets the default distance. The range is from 1 to 255.
Defaults
254
Command Modes
Global configuration
Command History
12.2
This command was introduced.
12.2(11)T
This command was integrated into Cisco IOS Release 12.2(11)T.
12.2(18)S
This command was integrated into Cisco IOS Release 12.2(18)S.
Examples
The following example shows how to configure the default admininstrative distance to be 25:
ip dhcp-client default-router distance 25Related Commands
ip dhcp conflict logging
To enable conflict logging on a Cisco IOS Dynamic Host Configuration Protocol (DHCP) server, use the ip dhcp conflict logging command in global configuration mode. To disable conflict logging, use the no form of this command.
ip dhcp conflict logging
no ip dhcp conflict logging
Syntax Description
This command has no arguments or keywords.
Defaults
Conflict logging is enabled.
Command Modes
Global configuration
Command History
Usage Guidelines
We recommend using a DHCP server database agent to store automatic bindings. If you decide not to use a DHCP Server database agent to store automatic bindings, use the no ip dhcp conflict logging command to disable the recording of address conflicts. By default, the Cisco IOS DHCP server records DHCP address conflicts in a log file.
Examples
The following example disables the recording of DHCP address conflicts:
no ip dhcp conflict loggingRelated Commands
ip dhcp database
To configure a Cisco IOS Dynamic Host Configuration Protocol (DHCP) server and relay agent to save automatic bindings on a remote host called a database agent, use the ip dhcp database command in global configuration mode. To remove the database agent, use the no form of this command.
ip dhcp database url [timeout seconds | write-delay seconds]
no ip dhcp database url
Syntax Description
Defaults
DHCP waits 300 seconds for both a write delay and a timeout.
Command Modes
Global configuration
Command History
Usage Guidelines
The administrator may configure multiple database agents. Bindings are transferred by using FTP, Trivial File Transport Protocol (TFTP), or remote copy protocol (rcp).
The DHCP relay agent can save route information to the same database agents to ensure recovery after reloads.
Examples
The following example specifies the DHCP database transfer timeout value as 80 seconds:
ip dhcp database ftp://user:password@172.16.1.1/router-dhcp timeout 80The following example specifies the DHCP database update delay value as 100 seconds:
ip dhcp database tftp://172.16.1.1/router-dhcp write-delay 100Related Commands
show ip dhcp database
Displays Cisco IOS DHCP Server database agent information.
ip dhcp excluded-address
To specify IP addresses that a Cisco IOS Dynamic Host Configuration Protocol (DHCP) Server should not assign to DHCP clients, use the ip dhcp excluded-address command in global configuration mode. To remove the excluded IP addresses, use the no form of this command.
ip dhcp excluded-address low-address [high-address]
no ip dhcp excluded-address low-address [high-address]
Syntax Description
low-address
The excluded IP address, or first IP address in an excluded address range.
high-address
(Optional) The last IP address in the excluded address range.
Defaults
All IP pool addresses are assignable.
Command Modes
Global configuration
Command History
Usage Guidelines
The DHCP Server assumes that all pool addresses may be assigned to clients. Use this command to exclude a single IP address or a range of IP addresses.
Examples
The following example configures an excluded IP address range from 172.16.1.100 through 172.16.1.199:
ip dhcp excluded-address 172.16.1.100 172.16.1.199Related Commands
ip dhcp limited-broadcast-address
To override a configured network broadcast and have the DHCP server and relay agent send an all networks, all nodes broadcast to a DHCP client, use the ip dhcp limited-broadcast-address global configuration command. To disable this functionality, use the no form of this command.
ip dhcp limited-broadcast-address
no ip dhcp limited-broadcast-address
Syntax Description
This command has no arguments or keywords.
Defaults
Default broadcast address: 255.255.255.255 (all ones)
Command Modes
Global configuration
Command History
Usage Guidelines
When a DHCP client sets the broadcast bit in the DHCP packet, the DHCP server and relay agent send DHCP messages to clients using the all ones broadcast address (255.255.255.255). If the ip broadcast-address interface configuration command has been configured to send a network broadcast, the all ones broadcast set by DHCP is overridden. To remedy this situation, use the ip dhcp limited-broadcast-address command to ensure that a configured network broadcast does not override the default DHCP behavior.
Some DHCP clients can only accept an all ones broadcast and may not be able to acquire a DHCP address unless this command is configured on the router interface connected to the client.
Examples
The following example configures DHCP to override any network broadcast:
ip dhcp limited-broadcast-addressRelated Commands
ip dhcp ping packets
To specify the number of packets a Cisco IOS Dynamic Host Configuration Protocol (DHCP) server sends to a pool address as part of a ping operation, use the ip dhcp ping packets command in global configuration mode. To prevent the server from pinging pool addresses, use the no form of this command. To return the number of ping packets sent to the default value, use the default form of this command.
ip dhcp ping packets number
no ip dhcp ping packets
default ip dhcp ping packets
Syntax Description
number
The number of ping packets that are sent before the address is assigned to a requesting client. The default value is two packets.
Defaults
Two packets
Command Modes
Global configuration
Command History
Usage Guidelines
The DHCP server pings a pool address before assigning the address to a requesting client. If the ping is unanswered, the DHCP server assumes (with a high probability) that the address is not in use and assigns the address to the requesting client.
Setting the number argument to a value of 0 completely turns off DHCP server ping operation .
Examples
The following example specifies five ping attempts by the DHCP server before ceasing any further ping attempts:
ip dhcp ping packets 5Related Commands
ip dhcp ping timeout
To specify how long a Cisco IOS Dynamic Host Configuration Protocol (DHCP) Server waits for a ping reply from an address pool, use the ip dhcp ping timeout command in global configuration mode. To restore the default number of milliseconds (500) of the timeout, use the no form of this command.
ip dhcp ping timeout milliseconds
no ip dhcp ping timeout
Syntax Description
Defaults
500 milliseconds
Command Modes
Global configuration
Command History
Usage Guidelines
This command specifies how long to wait for a ping reply (in milliseconds).
Examples
The following example specifies that the DHCP Server will wait 800 milliseconds for a ping reply before considering the ping a failure:
ip dhcp ping timeout 800Related Commands
ip dhcp pool
To configure a Dynamic Host Configuration Protocol (DHCP) address pool on a Cisco IOS DHCP server and enter DHCP pool configuration mode, use the ip dhcp pool command in global configuration mode. To remove the address pool, use the no form of this command.
ip dhcp pool name
no ip dhcp pool name
Syntax Description
name
Name of the pool. Can either be a symbolic string (such as engineering) or an integer (such as 0).
Defaults
DHCP address pools are not configured.
Command Modes
Global configuration
Command History
Usage Guidelines
During execution of this command, the configuration mode changes to DHCP pool configuration mode, which is identified by the (config-dhcp)# prompt. In this mode, the administrator can configure pool parameters, like the IP subnet number and default router list.
Examples
The following example configures pool1 as the DHCP address pool:
ip dhcp pool pool1Related Commands
ip dhcp relay information check
To configure a Cisco IOS Dynamic Host Configuration Protocol (DHCP) Server to validate the relay agent information option in forwarded BOOTREPLY messages, use the ip dhcp relay information check global configuration command. To disable an information check, use the no form of this command.
ip dhcp relay information check
no ip dhcp relay information check
Syntax Description
This command has no arguments or keywords.
Defaults
The DHCP server checks relay information. Invalid messages are dropped.
Command Modes
Global configuration
Command History
Usage Guidelines
This command is used by cable access router termination systems. By default, DHCP checks relay information. Invalid messages are dropped.
Examples
The following example configures the DHCP Server to check that the relay agent information option in forwarded BOOTREPLY messages is valid:
ip dhcp relay information checkRelated Commands
ip dhcp relay information option
To enable the system to insert the Dynamic Host Configuration Protocol (DHCP) relay agent information option in forwarded BOOTREQUEST messages to a Cisco IOS DHCP server, use the ip dhcp relay information option command in global configuration mode. To disable inserting relay information into forwarded BOOTREQUEST messages, use the no form of this command.
ip dhcp relay information option [vpn]
no ip dhcp relay information option [vpn]
Syntax Description
Defaults
The DHCP server does not insert relay information.
Command Modes
Global configuration
Command History
12.0(1)T
This command was introduced.
12.2(4)B
The vpn keyword was added.
12.2(8)T
This command was integrated into Cisco IOS Release 12.2(8)T.
Usage Guidelines
This command is used by cable access router termination systems. This functionality enables a DHCP server to identify the user (cable access router) sending the request and initiate appropriate action based on this information. By default, DHCP does not insert relay information.
The ip dhcp relay information option command automatically adds the circuit identifier suboption and the remote ID suboption to the DHCP relay agent information option (also called option 82).
The vpn optional keyword should be used only when the DHCP server allocates addresses based on VPN identification suboptions.
The ip dhcp relay information option vpn command adds the following VPN-related suboptions into the relay agent information option when DHCP broadcasts are forwarded by the relay agent from clients to a DHCP server:
•
•
•
After these suboptions are successfully added, the gateway address is set to the outgoing interface of the router toward the DHCP server IP address configured using the ip helper-address interface configuration command.
If only the ip dhcp relay information option vpn command is configured, the VPN identifier, subnet selection, and server identifier override suboptions are added to the relay information option. Note that the circuit identifier suboption and the remote ID suboption are not added to the relay information option. However, if both the ip dhcp relay information option command and the ip dhcp relay information option vpn command are configured, all five suboptions are added to the relay agent information option.
When the packets are returned from the DHCP server, option 82 is removed before the reply is forwarded to the client.
Even if the vpn option is specified, the VPN suboptions are added only to those DHCP or BOOTP broadcasts picked up by the interface configured with a VRF name or VPN ID.
For clients from unnumbered ATM or serial interfaces, when this command is enabled, the VPN identifier suboption will contain the VRF name of the unnumbered interface.
Subnet selection and server identifier override suboptions are added from the IP address of the interface that the unnumbered interface is configured to borrow its IP address from. The client host route will be added on the respective VRF routing tables.
If the ip dhcp smart-relay global configuration command is enabled, then the server identifier override and subnet selection suboptions will use the secondary IP address of the incoming interface when the same client retransmits more than three DHCP DISCOVER packets (for both numbered and unnumbered interfaces).
Examples
The following example configures a DHCP server to insert the DHCP relay agent information option, including VPN suboptions, in forwarded BOOTREQUEST messages. In this example, the circuit identifier suboption and the remote ID suboption are not included in the relay information option:
ip dhcp relay information option vpnThe following example configures a DHCP server to insert the DHCP relay agent information option, including VPN suboptions, the circuit identifier suboption, and the remote ID suboption, in forwarded BOOTREQUEST messages:
ip dhcp relay information option vpnip dhcp relay information optionRelated Commands
ip dhcp relay information policy
To configure the information reforwarding policy for a Dynamic Host Configuration Protocol (DHCP) relay agent (what a relay agent should do if a message already contains relay information), use the
ip dhcp relay information policy command in global configuration . To restore the default relay information policy, use the no form of this command.ip dhcp relay information policy {drop | keep | replace}
no ip dhcp relay information policy
Syntax Description
Defaults
The DHCP server replaces existing relay information.
Command Modes
Global configuration
Command History
Usage Guidelines
This command is used by cable access router termination systems. A DHCP relay agent may receive a message from another DHCP relay agent that already contains relay information. By default, the relay information from the previous relay agent is replaced.
Examples
The following examples configure a DHCP relay agent to drop messages with existing relay information, keep existing information, and replace existing information:
ip dhcp relay information policy dropip dhcp relay information policy keepip dhcp relay information policy replaceRelated Commands
ip dhcp relay information trusted
To configure an interface as a trusted source of the Dynamic Host Configuration Protocol (DHCP) relay agent information option, use the ip dhcp relay information trusted command in interface configuration mode. To restore the interface to the default behavior, use the no form of the command.
ip dhcp relay information trusted
no ip dhcp relay information trusted
Syntax Description
This command has no arguments or keywords.
Defaults
All interfaces on the router are considered untrusted.
Command Modes
Interface configuration
Command History
Usage Guidelines
By default, if the gateway address is set to all zeros in the DHCP packet and the relay information option is already present in the packet, the Cisco IOS DHCP relay agent will discard the packet. If the ip dhcp relay information trusted command is configured on an interface, the Cisco IOS DHCP relay agent will not discard the packet even if the gateway address is set to all zeros. Instead, the received DHCPDISCOVER or DHCPREQUEST messages will be forwarded to the addresses configured by the ip helper-address command as in normal DHCP relay operation.
Examples
In the following example, interface Ethernet 1 is configured as a trusted source for the relay agent information:
interface ethernet 1ip dhcp relay information trustedRelated Commands
ip dhcp relay information trust-all
To configure all interfaces on a router as trusted sources of the Dynamic Host Configuration Protocol (DHCP) relay agent information option, use the ip dhcp relay information trust-all command in global configuration mode. To restore the interfaces to their default behavior, use the no form of the command.
ip dhcp relay information trust-all
no ip dhcp relay information trust-all
Syntax Description
This command has no arguments or keywords.
Defaults
All interfaces on the router are considered untrusted.
Command Modes
Global configuration
Command History
Usage Guidelines
By default, if the gateway address is set to all zeros in the DHCP packet and the relay information option is already present in the packet, the Cisco IOS DHCP relay agent will discard the packet. If the ip dhcp relay information trust-all command is configured globally, the Cisco IOS DHCP relay agent will not discard the packet even if the gateway address is set to all zeros. Instead, the received DHCPDISCOVER or DHCPREQUEST messages will be forwarded to the addresses configured by the ip helper-address command as in normal DHCP relay operation.
Examples
In the following example, all interfaces on the router are configured as a trusted source for relay agent information:
ip dhcp relay information trust-allRelated Commands
ip dhcp smart-relay
To allow the Cisco IOS Dynamic Host Configuration Protocol (DHCP) relay agent to switch the gateway address (giaddr field of a DHCP packet) to secondary addresses when there is no DHCPOFFER message from a DHCP server, use the ip dhcp smart-relay global configuration command. To disable this smart-relay functionality and restore the default behavior, use the no form of this command.
ip dhcp smart-relay
no ip dhcp smart-relay
Syntax Description
This command has no arguments or keywords.
Defaults
Disabled
Command Modes
Global configuration
Command History
Usage Guidelines
The DHCP relay agent attempts to forward the primary address as the gateway address three times. After three attempts and no response, the relay agent automatically switches to secondary addresses.
Examples
The following example enables the DHCP relay agent to automatically switch to secondary address pools:
ip dhcp smart-relayip directed-broadcast
To enable the translation of a directed broadcast to physical broadcasts, use the ip directed-broadcast interface configuration command. To disable this function, use the no form of this command.
ip directed-broadcast [access-list-number] | [extended access-list-number]
no ip directed-broadcast [access-list-number] | [extended access-list-number]
Syntax Description
Defaults
Disabled; all IP directed broadcasts are dropped.
Command Modes
Interface configuration
Command History
10.0
This command was introduced.
12.0
The default behavior changed to directed broadcasts being dropped.
Usage Guidelines
An IP directed broadcast is an IP packet whose destination address is a valid broadcast address for some IP subnet, but which originates from a node that is not itself part of that destination subnet.
A router that is not directly connected to its destination subnet forwards an IP directed broadcast in the same way it would forward unicast IP packets destined to a host on that subnet. When a directed broadcast packet reaches a router that is directly connected to its destination subnet, that packet is "exploded" as a broadcast on the destination subnet. The destination address in the IP header of the packet is rewritten to the configured IP broadcast address for the subnet, and the packet is sent as a link-layer broadcast.
The ip directed-broadcast interface command controls the explosion of directed broadcasts when they reach their target subnets. The command affects only the final transmission of the directed broadcast on its ultimate destination subnet. It does not affect the transit unicast routing of IP directed broadcasts.
If directed broadcast is enabled for an interface, incoming IP packets whose addresses identify them as directed broadcasts intended for the subnet to which that interface is attached will be exploded as broadcasts on that subnet. If an access list has been configured with the ip directed-broadcast command, only directed broadcasts that are permitted by the access list in question will be forwarded; all other directed broadcasts destined for the interface subnet will be dropped.
If the no ip directed-broadcast command has been configured for an interface, directed broadcasts destined for the subnet to which that interface is attached will be dropped, rather than being broadcast.
Note
Examples
The following example enables forwarding of IP directed broadcasts on Ethernet interface 0:
interface ethernet 0ip directed-broadcastRelated Commands
ip forward-protocol
Specifies which protocols and ports the router forwards when forwarding broadcast packets.
ip dns primary
To configure the router as authoritative for a zone, use the ip dns primary command in global configuration mode. To configure the router as nonauthoritative for a zone, use the no form of this command.
ip dns primary domain-name soa primary-server-name mailbox-name [refresh-interval [retry-interval [expire-ttl [minimum-ttl]]]]
no ip dns primary domain-name
Syntax Description
Command Default
No authority record parameters are configured for the DNS name server, so queries to the DNS server for locally defined hosts will not receive authoritative responses from this server.
Command Modes
Global configuration
Command History
Usage Guidelines
Use this command to configure the router as an authoritative name server for the host table, or zone file, of a DNS domain. The primary name server name and a DNS mailbox name are required authority record parameters. Optionally, you can override the default values for the polling refresh interval, the refresh retry interval, the authority expire time, and the minimum TTL for zone information.
To display the authoritative name server configuration for the router, use the show ip dns primary command.
Examples
The following example shows how to configure the router as the primary DNS server authoritative for the example.com domain, or zone:
Router(config)# ip dns primary example.com soa ns1.example.com mb1.example.com 10800 900 5184000 172800In the above example, the DNS domain name of the router is ns1.example.com, and the administrative contact for this zone is mb1@example.com. The refresh time is 3 hours, the refresh retry time is 15 minutes, the authority expire time is 60 days, and the minimum TTL is 2 days.
Related Commands
ip domain list
To define a list of default domain names to complete unqualified host names, use the ip domain list command in global configuration mode. To delete a name from a list, use the no form of this command.
ip domain list name
no ip domain list name
Syntax Description
name
Domain name. Do not include the initial period that separates an unqualified name from the domain name.
Defaults
No domain names are defined.
Command Modes
Global configuration
Command History
10.0
This command was introduced.
12.2
The syntax of the command changed from ip domain-list to ip domain list.
Usage Guidelines
If there is no domain list, the domain name that you specified with the ip domain name global configuration command is used. If there is a domain list, the default domain name is not used. The ip domain list command is similar to the ip domain name command, except that with the ip domain list command you can define a list of domains, each to be tried in turn.
The Cisco IOS software will still accept the previous version of the command, ip domain-list.
Examples
The following example adds several domain names to a list:
ip domain list company.comip domain list school.eduThe following example adds a name to and then deletes a name from the list:
ip domain list school.eduno ip domain list school.eduRelated Commands
ip domain lookup
To enable the IP Domain Naming System (DNS)-based host name-to-address translation, use the ip domain lookup command in global configuration mode. To disable the DNS, use the no form of this command.
ip domain lookup
no ip domain lookup
Syntax Description
This command has no arguments or keywords.
Defaults
Enabled
Command Modes
Global configuration
Command History
10.0
This command was introduced.
12.2
The syntax of the command changed from ip domain-lookup to ip domain lookup.
Usage Guidelines
The Cisco IOS software will still accept the previous version of the command, which is ip domain-lookup.
Examples
The following example enables the IP DNS-based host name-to-address translation:
ip domain lookupRelated Commands
ip domain name
To define a default domain name that the Cisco IOS software uses to complete unqualified host names (names without a dotted-decimal domain name), use the ip domain name command in global configuration mode. To disable use of the Domain Name System (DNS), use the no form of this command.
ip domain name name
no ip domain name name
Syntax Description
name
Default domain name used to complete unqualified host names. Do not include the initial period that separates an unqualified name from the domain name.
Defaults
Enabled
Command Modes
Global configuration
Command History
10.0
This command was introduced.
12.2
The syntax of the command changed from ip domain-name to ip domain name.
Usage Guidelines
Any IP host name that does not contain a domain name (that is, any name without a dot) will have the dot and cisco.com appended to it before being added to the host table.
The Cisco IOS software will still accept the previous version of the command, which is ip domain-name.
Examples
The following example defines cisco.com as the default domain name:
ip domain name cisco.comRelated Commands
ip domain retry
To specify the number of times to retry sending Domain Name System (DNS) queries, use the ip domain retry command in global configuration mode. To return to the default behavior, use the no form of this command.
ip domain retry number
no ip domain retry number
Syntax Description
number
Number of times to retry sending a DNS query to the DNS server. The range is from 0 to 100; the default is 2.
Defaults
number: 2 times
Command Modes
Global configuration
Command History
Usage Guidelines
If the ip domain retry command is not configured, the Cisco IOS software will only send DNS queries out twice.
Examples
The following example shows how to configure the router to send out 10 DNS queries before giving up:
ip domain retry 10Related Commands
ip domain round-robin
To enable round-robin functionality on DNS servers, use the ip domain round-robin command in global configuration mode. To disable round-robin functionality, use the no form of the command.
ip domain round-robin
no ip domain round-robin
Syntax Description
This command has no arguments or keywords.
Defaults
Round robin is not enabled.
Command Modes
Global configuration
Command History
Usage Guidelines
In a multiple server configuration without the DNS round-robin functionality, the first host server/IP address is used for the whole time to live (TTL) of the cache, and uses the second and third only in the event of host failure. This behavior presents a problem when a high volume of users all arrive at the first host during the TTL time. The network access server (NAS) then sends out a DNS query; the DNS servers reply with a list of the configured IP addresses to the NAS. The NAS then caches these IP addresses for a given time (for example, five minutes). All users that dial in during the five minute TTL time will land on one host, the first IP address in the list.
In a multiple server configuration with the DNS round-robin functionality, the DNS server returns the IP address of all hosts to rotate between the cache of host names. During the TTL of the cache, users are distributed among the hosts. This functionality distributes calls across the configured hosts and reduces the amount of DNS queries.
Examples
The following example allows a Telnet to www.company.com to connect to each of the three IP addresses specified in the following order: the first time the Telnet command is given, it would connect to 10.0.0.1; the second time the command is given, it would connect to 10.1.0.1; and the third time the command is given, it would connect to 10.2.0.1. In each case, the other two addresses would also be tried if the first one failed; this is the normal operation of the Telnet command.
Router(config)# ip host www.company.com 10.0.0.1 10.1.0.1 10.2.0.1Router(config)# ip domain round-robinip domain timeout
To specify the amount of time to wait for a response to a DNS query, use the ip domain timeout command in global configuration mode. To return to the default behavior, use the no form of this command.
ip domain timeout seconds
no ip domain timeout seconds
Syntax Description
seconds
Time, in seconds, to wait for a response to a DNS query. The range is from 0 to 3600; the default is 3.
Defaults
seconds: 3 seconds
Command Modes
Global configuration
Command History
Usage Guidelines
If the ip domain timeout command is not configured, the Cisco IOS software will only wait 3 seconds for a response to a DNS query.
Examples
The following example shows how to configure the router to wait 50 seonds for a response to a DNS query:
ip domain timeout 50Related Commands
ip drp access-group
To control the sources of Director Response Protocol (DRP) queries to the DRP Server Agent, use the ip drp access-group command in global configuration mode. To remove the access list, use the no form of this command.
ip drp access-group access-list-number
no ip drp access-group access-list-number
Syntax Description
access-list-number
Number of a standard IP access list in the range from 1 to 99 or from 1300 to 1999.
Defaults
The DRP Server Agent will answer all queries.
Command Modes
Global configuration
Command History
Usage Guidelines
This command applies an access list to the interface, thereby controlling which devices can send queries to the DRP Server Agent.
If both an authentication key chain and an access group have been specified, both security measures must permit access before a request is processed.
Examples
The following example configures access list 1, which permits only queries from the host at 33.45.12.4:
access-list 1 permit 33.45.12.4ip drp access-group 1Related Commands
ip drp authentication key-chain
To configure authentication on the Director Response Protocol (DRP) Server Agent for DistributedDirector, use the ip drp authentication key-chain command in global configuration mode. To remove the key chain, use the no form of this command.
ip drp authentication key-chain name-of-chain
no ip drp authentication key-chain name-of-chain
Syntax Description
Defaults
No authentication is configured for the DRP Server Agent.
Command Modes
Global configuration
Command History
Usage Guidelines
When a key chain and key are configured, the key is used to authenticate all DRP requests and responses. The active key on the DRP Server Agent must match the active key on the primary agent. Use the key and key-string commands to configure the key.
Examples
The following example configures a key chain named ddchain:
ip drp authentication key-chain ddchainRelated Commands
ip drp server
To enable the Director Response Protocol (DRP) Server Agent that works with DistributedDirector, use the ip drp server command in global configuration mode. To disable the DRP Server Agent, use the no form of this command.
ip drp server
no ip drp server
Syntax Description
This command has no arguments or keywords.
Defaults
Disabled
Command Modes
Global configuration
Command History
Examples
The following example enables the DRP Server Agent:
ip drp serverRelated Commands
ip forward-protocol
To specify which protocols and ports the router forwards when forwarding broadcast packets, use the ip forward-protocol command in global configuration mode. To remove a protocol or port, use the no form of this command.
ip forward-protocol {udp [port] | nd | sdns}
no ip forward-protocol {udp [port | nd | sdns}
Syntax Description
Defaults
Enabled
Command Modes
Global configuration
Command History
Usage Guidelines
Enabling a helper address or UDP flooding on an interface causes the Cisco IOS software to forward particular broadcast packets. You can use the ip forward-protocol command to specify exactly which types of broadcast packets you would like to have forwarded. A number of commonly forwarded applications are enabled by default. Enabling forwarding for some ports [for example, Routing Information Protocol (RIP)] may be hazardous to your network.
If you use the ip forward-protocol command, specifying only UDP without the port enables forwarding and flooding on the default ports.
One common application that requires helper addresses is Dynamic Host Configuration Protocol (DHCP). DHCP is defined in RFC 1531. DHCP protocol information is carried inside of BOOTP packets. To enable BOOTP broadcast forwarding for a set of clients, configure a helper address on the router interface closest to the client. The helper address should specify the address of the DHCP server. If you have multiple servers, you can configure one helper address for each server. Because BOOTP packets are forwarded by default, DHCP information can now be forwarded by the software. The DHCP server now receives broadcasts from the DHCP clients.
If an IP helper address is defined, UDP forwarding is enabled on default ports. If UDP flooding is configured, UDP flooding is enabled on the default ports.
If a helper address is specified and UDP forwarding is enabled, broadcast packets destined to the following port numbers are forwarded by default:
•
•
•
•
•
•
•
•
Note
Examples
The following example defines a helper address and uses the ip forward-protocol command. Using the udp keyword without specifying any port numbers will allow forwarding of UDP packets on the default ports.
ip forward-protocol udpinterface ethernet 1ip helper-address 10.24.42.2
ip forward-protocol spanning-tree
To permit IP broadcasts to be flooded throughout the internetwork in a controlled fashion, use the ip forward-protocol spanning-tree command in global configuration mode. To disable the flooding of IP broadcasts, use the no form of this command.
ip forward-protocol spanning-tree [any-local-broadcast]
no ip forward-protocol spanning-tree [any-local-broadcast]
Syntax Description
Defaults
Disabled
Command Modes
Global configuration
Command History
Usage Guidelines
A packet must meet the following criteria to be considered for flooding:
•
•
•
•
•
A flooded UDP datagram is given the destination address specified by the ip broadcast-address interface configuration command on the output interface. The destination address can be set to any desired address. Thus, the destination address may change as the datagram propagates through the network. The source address is never changed. The TTL value is decremented.
After a decision has been made to send the datagram out on an interface (and the destination address possibly changed), the datagram is handed to the normal IP output routines and is therefore subject to access lists, if they are present on the output interface.
The ip forward-protocol spanning-tree command uses the database created by the bridging Spanning-Tree Protocol. Therefore, the transparent bridging option must be in the routing software, and bridging must be configured on each interface that is to participate in the flooding in order to support this capability.
If an interface does not have bridging configured, it still will be able to receive broadcasts, but it will never forward broadcasts received on that interface. Also, it will never use that interface to send broadcasts received on a different interface.
If no actual bridging is desired, you can configure a type-code bridging filter that will deny all packet types from being bridged. Refer to the "Configuring Transparent Bridging" chapter in the Cisco IOS Bridging and IBM Networking Configuration Guide for more information about using access lists to filter bridged traffic. The spanning-tree database is still available to the IP forwarding code to use for the flooding.
The spanning-tree-based flooding mechanism forwards packets whose contents are all ones (255.255.255.255), all zeros (0.0.0.0), and, if subnetting is enabled, all networks (131.108.255.255 as an example in the network number 131.108.0.0). This mechanism also forward packets whose contents are the zeros version of the all-networks broadcast when subnetting is enabled (for example, 131.108.0.0).
This command is an extension of the ip helper-address interface configuration command, in that the same packets that may be subject to the helper address and forwarded to a single network can now be flooded. Only one copy of the packet will be put on each network segment.
Examples
The following example permits IP broadcasts to be flooded through the internetwork in a controlled fashion:
ip forward-protocol spanning-treeRelated Commands
ip forward-protocol turbo-flood
To speed up flooding of User Datagram Protocol (UDP) datagrams using the spanning-tree algorithm, use the ip forward-protocol turbo-flood command in global configuration mode. To disable this feature, use the no form of this command.
ip forward-protocol turbo-flood
no ip forward-protocol turbo-flood
Syntax Description
This command has no arguments or keywords.
Defaults
Disabled
Command Modes
Global configuration
Command History
Usage Guidelines
Used in conjunction with the ip forward-protocol spanning-tree global configuration command, this feature is supported over Advanced Research Projects Agency (ARPA)-encapsulated Ethernets, FDDI, and High-Level Data Link Control (HDLC) encapsulated serials, but is not supported on Token Rings. As long as the Token Rings and the non-HDLC serials are not part of the bridge group being used for UDP flooding, turbo flooding will behave normally.
Examples
The following is an example of a two-port router using this command:
ip forward-protocol turbo-floodip forward-protocol spanning-tree!interface ethernet 0ip address 128.9.1.1bridge-group 1!interface ethernet 1ip address 128.9.1.2bridge-group 1!bridge 1 protocol decRelated Commands
ip helper-address
To enable the forwarding of User Datagram Protocol (UDP) broadcasts, including BOOTP, received on an interface, use the ip helper-address command in interface configuration mode. To disable the forwarding of broadcast packets to specific addresses, use the no form of this command.
ip helper-address [vrf name | global] address [redundancy vrg-name]
no ip helper-address [vrf name | global] address [redundancy vrg-name]
Syntax Description
Defaults
Disabled
Command Modes
Interface configuration
Command History
Usage Guidelines
Combined with the ip forward-protocol global configuration command, the ip helper-address command allows you to control which broadcast packets and which protocols are forwarded.
One common application that requires helper addresses is Dynamic Host Configuration Protocol (DHCP), which is defined in RFC 1531. To enable BOOTP or DHCP broadcast forwarding for a set of clients, configure a helper address on the router interface connected to the client. The helper address should specify the address of the BOOTP or DHCP server. If you have multiple servers, you can configure one helper address for each server.
All of the following conditions must be met in order for a UDP or IP packet to be helpered by the ip helper-address command:
•
•
•
•
•
If the DHCP server resides in a Virtual Private Network (VPN) or global space that is different from the interface VPN, then the vrf name or global option allows you to specify the name of the VRF or global space in which the DHCP server resides.
The ip helper-address vrf name address option uses the address associated with the VRF name regardless of the VRF of the incoming interface. If the ip helper-address vrf name address command is configured and later the vrf is deleted from the configuration, then all IP helper addresses associated with that VRF name will be removed from the interface configuration.
If the ip helper-address address command is already configured on an interface with no VRF name configured, and later the interface is configured with the ip helper-address vrf name address command, then the previously configured ip helper-address address is considered to be global.
Note
Examples
The following example defines an address that acts as a helper address:
interface ethernet 1ip helper-address 121.24.43.2The following example defines an address that acts as a helper address and is associated with the VRF named red:
interface ethernet 1/0ip helper-address vrf red 121.25.44.2The following example defines an address that acts as a helper address and is associated with the VRG named shop:
interface ethernet 1/0ip helper-address 121.25.45.2 redundancy shopRelated Commands
ip forward-protocol
Specifies which protocols and ports the router forwards when forwarding broadcast packets.
ip host
To define a static host name-to-address mapping in the host cache, use the ip host command in global configuration mode. To remove the host name-to-address mapping, use the no form of this command.
ip host {name | tmodem-telephone-number} [tcp-port-number] {address1 [address2...address8] | [mx preference mx-server-hostname | ns nameserver-hostname | srv priority weight port target]}
no ip host {name | tmodem-telephone-number} [tcp-port-number] {address1 [address2...address8] | [mx preference mx-server-hostname | ns nameserver-hostname | srv priority weight port target]}
Syntax Description
Defaults
Disabled
Command Modes
Global configuration
Command History
Usage Guidelines
The first character of the hostname can be either a letter or a number. If you use a number, the types of operations you can perform (such as ping) are limited.
If the hostname cache does not exist yet, it is automatically created.
To specify the machine that provides domain service for the domain, use the ns keyword and the nameserver-hostname argument
To specify where the mail for the host is to be sent, use the mx keyword and the preference and mx-server-hostname arguments.
To specify a host that offers a service in the domain, use thhe srv keyword and the priority, weight, port, and target arguments.
Examples
The following example defines two static mappings:
ip host host2 10.168.7.18ip host host3 10.2.0.2 192.168.7.33The following example shows how to map modem telephone number 555-1234 to IP host address 10.1.5.5 for the Cisco modem user interface mode:
ip host t5551234 10.1.5.5Related Commands
ip icmp rate-limit unreachable
To have the Cisco IOS software limit the rate at which Internet Control Message Protocol (ICMP) destination unreachable messages are generated, use the ip icmp rate-limit unreachable command in global configuration mode. To remove the rate limit, use the no form of this command.
ip icmp rate-limit unreachable [df] milliseconds
no ip icmp rate-limit unreachable [df]
Syntax Description
Defaults
The default value is one ICMP destination unreachable message per 500 milliseconds.
Command Modes
Global configuration
Command History
Usage Guidelines
The no ip icmp rate-limit unreachable command turns off the previously configured rate limit. To re-set the rate limit to its default value, use the default ip icmp rate-limit unreachable command.
The Cisco IOS software maintains two timers: one for general destination unreachable messages and one for DF destination unreachable messages. Both share the same time limits and defaults. If the df option is not configured, the ip icmp rate-limit unreachable command sets the time values for DF destination unreachable messages. If the df option is configured, its time values remain independent from those of general destination unreachable messages.
Examples
The following example sets the rate of the ICMP destination unreachable message to one message every 10 milliseconds:
ip icmp rate-limit unreachable 10The following example turns off the previously configured rate limit:
no ip icmp rate-limit unreachableThe following example sets the rate limit back to the default:
default ip icmp rate-limit unreachableip icmp redirect
To control the type of Internet Control Message Protocol (ICMP) redirect message that is sent by the Cisco IOS software, use the ip icmp redirect command in global configuration mode. To set the value back to the default, use the no form of this command.
ip icmp redirect [host | subnet]
no ip icmp redirect [host | subnet]
Syntax Description
Defaults
The router will send ICMP subnet redirect messages.
Because the ip icmp redirect subnet command is the default, the command will not be displayed in the configuration.
Command Modes
Global configuration
Command History
Usage Guidelines
An ICMP redirect message can be generated by a router when a packet is received and transmitted on the same interface. In this situation, the router will forward the original packet and send a ICMP redirect message back to the sender of the original packet. This behavior allows the sender to bypass the router and forward future packets directly to the destination (or a router closer to the destination).
There are two types of ICMP redirect messages: redirect for a host address or redirect for an entire subnet.
The ip icmp redirect command determines the type of ICMP redirects sent by the system and is configured on a per system basis. Some hosts do not understand ICMP subnet redirects and need the router to send out ICMP host redirects. Use the ip icmp redirect host command to have the router send out ICMP host redirects. Use the ip icmp redirect subnet command to set the value back to the default, which is to send subnet redirects.
To prevent the router from sending ICMP redirects, use the no ip redirects interface configuration command.
Examples
The following example enables the router to send out ICMP host redirects:
ip icmp redirect hostsThe following example sets the value back to the default, which is subnet redirects:
ip icmp redirect subnetRelated Commands
ip information-reply
To have the Cisco IOS software send Internet Control Message Protocol (ICMP) information replies, use the ip information-reply command in interface configuration mode. To disable this function, use the no form of this command.
ip information-reply
no information-reply
Syntax Description
This command has no arguments or keywords.
Defaults
Disabled
Command Modes
Interface configuration
Command History
Usage Guidelines
The ability for the Cisco IOS software to respond to ICMP information request messages with an ICMP information reply message is disabled by default. Use this command to allow the software to send ICMP information reply messages.
Examples
The following example enables the sending of ICMP information reply messages on Ethernet interface 0:
interface ethernet 0ip address 131.108.1.0 255.255.255.0ip information-replyip irdp
To enable ICMP Router Discovery Protocol (IRDP) processing on an interface, use the ip irdp interface configuration command. To disable IRDP routing, use the no form of this command.
ip irdp [multicast | holdtime seconds | maxadvertinterval seconds | minadvertinterval seconds | preference number | address address [number]]
no ip irdp
Syntax Description
Defaults
Disabled
When enabled, IRDP uses these defaults:
•
•
•
•
Command Modes
Interface configuration
Command History
Usage Guidelines
If you change the maxadvertinterval value, the other two values also change, so it is important to change the maxadvertinterval value before changing either the holdtime or minadvertinterval values.
The ip irdp multicast command allows for compatibility with Sun Microsystems Solaris, which requires IRDP packets to be sent out as multicasts. Many implementations cannot receive these multicasts; ensure end-host ability before using this command.
Examples
The following example sets the various IRDP processes:
! enable irdp on interface Ethernet 0interface ethernet 0 ip irdp ! send IRDP advertisements to the multicast addressip irdp multicast ! increase router preference from 100 to 50ip irdp preference 50 ! set maximum time between advertisements to 400 secsip irdp maxadvertinterval 400 ! set minimum time between advertisements to 100 secsip irdp minadvertinterval 100 ! advertisements are good for 6000 secondsip irdp holdtime 6000 ! proxy-advertise 131.108.14.5 with default router preferenceip irdp address 131.108.14.5 ! proxy-advertise 131.108.14.6 with preference of 50ip irdp address 131.108.14.6 50Related Commands
ip local-proxy-arp
To enable the local proxy Address Resolution Protocol (ARP) feature, use the ip local-proxy-arp command in interface configuration mode. To disable this feature, use the no form of this command.
ip local-proxy-arp
no ip local-proxy-arp
Syntax Description
This command has no arguments or keywords.
Defaults
This command is not enabled by default.
Command Modes
Interface configuration
Command History
Usage Guidelines
The local proxy ARP feature allows the Multilayer Switching Feature Card (MSFC) to respond to ARP requests for IP addresses within a subnet where normally no routing is required. With the local proxy ARP feature enabled, the MSFC responds to all ARP requests for IP addresses within the subnet and forwards all traffic between hosts in the subnet. Use this feature only on subnets where hosts are intentionally prevented from communicating directly to the Catalyst 6500 series switch on which they are connected.
Before the local proxy ARP feature can be used, the IP proxy ARP feature must be enabled. The IP proxy ARP feature is enabled by default.
Internet Control Message Protocol (ICMP) redirects are disabled on interfaces where the local proxy ARP feature is enabled.
Examples
The following example shows how to enable the local proxy ARP feature:
Router(config-if)# ip local-proxy-arp
