-
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
lease
To configure the duration of the lease for an IP address that is assigned from a Cisco IOS Dynamic Host Configuration Protocol (DHCP) server to a DHCP client, use the lease command in DHCP pool configuration mode. To restore the default value, use the no form of this command.
lease {days [hours [minutes]] | infinite}
no lease
Syntax Description
Defaults
1 day
Command Modes
DHCP pool configuration
Command History
Examples
The following example shows a 1-day lease:
lease 1The following example shows a 1-hour lease:
lease 0 1The following example shows a 1-minute lease:
lease 0 0 1The following example shows an infinite (unlimited) lease:
lease infiniteRelated Commands
ip dhcp pool
Configures a DHCP address pool on a Cisco IOS DHCP server and enters DHCP pool configuration mode.
logging server-arp
To enable the sending of Address Resolution Protocol (ARP) requests for syslog server address during system initialization bootup, use the logging server-arp command in global configuration mode. To disable the sending of ARP requests for syslog server addresses, use the no form of this command.
logging server-arp
no logging server-arp
Syntax Description
This command has no arguments or keywords.
Command Default
This command is disabled by default.
Command Modes
Global configuration.
Command History
12.3
This command was introduced.
12.3(4)T
This command was integrated into Cisco IOS Release 12.3(4)T.
12.3(5)B
This command was integrated into Cisco IOS Release 12.3(5)B.
Usage Guidelines
The logging server-arp global configuration command allows the sending of ARP requests for syslog server address during system initialization bootup.
When this CLI command is configured and saved to the startup configuration file, the system will send an ARP request for remote syslog server address before sending out the first syslog message.
The command should only be used when the remote syslog server is in the same subnet as the system router sending the ARP request.
Note
Use this command even if a static ARP has been configured with the syslog server address.
Examples
The following example shows how to enable an ARP request for syslog server addresses:
Router# configure terminalRouter(config)# logging server-arpRouter(config)# exitThe following example shows how to disable an ARP request for syslog server addresses:
Router# configure terminalRouter(config)# no logging server-arpRouter(config)# exitRelated Commands
arp (global)
Adds a permanent entry in the Address Resolution Protocol (ARP) cache, use the arp command in global configuration mode.
maxconns (server farm)
To limit the number of active connections to the real server, use the maxconns command in SLB server farm configuration mode. To restore the default of 4294967295, use the no form of this command.
maxconns maximum-number [sticky-override]
no maxconns
Syntax Description
Defaults
The default maximum number of simultaneous active connections on the real server is 4294967295.
Command Modes
SLB server farm configuration
Command History
Examples
The following example limits the real server to a maximum of 1000 simultaneous active connections:
Router(config)# ip slb serverfarm PUBLICRouter(config-slb-sfarm)# real 10.10.1.1Router(config-slb-real)# maxconns 1000Related Commands
nat
To configure IOS SLB Network Address Translation (NAT) and specify a NAT mode, use the nat SLB server farm configuration command. To remove a NAT configuration, use the no form of this command.
nat server
no nat server
Syntax Description
server
Specifies that the destination address in load-balanced packets sent to the real server is the address of the real server chosen by the server farm load-balancing algorithm.
Defaults
No IOS SLB NAT is configured.
Command Modes
SLB server farm configuration
Command History
12.1(1)E
This command was introduced.
12.1(5)T
This command was integrated into Cisco IOS Release 12.1(5)T.
Usage Guidelines
The no nat command is allowed only if the virtual server was removed from service with the no inservice command.
Examples
The following example changes to IOS SLB server farm configuration mode and configures NAT mode as server address translation on the server farm named FARM2:
ip slb serverfarm FARM2nat serverRelated Commands
netbios-name-server
To configure NetBIOS Windows Internet Naming Service (WINS) name servers that are available to Microsoft Dynamic Host Configuration Protocol (DHCP) clients, use the netbios-name-server command in DHCP pool configuration. To remove the NetBIOS name server list, use the no form of this command.
netbios-name-server address [address2...address8]
no netbios-name-server
Syntax Description
Command Modes
DHCP pool configuration
Command History
Usage Guidelines
One IP address is required, although you can specify up to eight addresses in one command line. Servers are listed in order of preference (address1 is the most preferred server, address2 is the next most preferred server, and so on).
Examples
The following example specifies the IP address of a NetBIOS name server available to the client:
netbios-name-server 10.12.1.90Related Commands
netbios-node-type
To configure the NetBIOS node type for Microsoft Dynamic Host Configuration Protocol (DHCP) clients, use the netbios-node-type command in DHCP pool configuration mode. To remove the NetBIOS node type, use the no form of this command.
netbios-node-type type
no netbios-node-type
Syntax Description
type
Specifies the NetBIOS node type. Valid types are:
•
•
•
•
Command Modes
DHCP pool configuration
Command History
Usage Guidelines
The recommended type is h-node (hybrid).
Examples
The following example specifies the client's NetBIOS type as hybrid:
netbios node-type h-nodeRelated Commands
network (DHCP)
To configure the subnet number and mask for a Dynamic Host Configuration Protocol (DHCP) address pool on a Cisco IOS DHCP server, use the network command in DHCP pool configuration mode. To remove the subnet number and mask, use the no form of this command.
network network-number [mask | prefix-length]
no network
Syntax Description
Defaults
No default behavior or values.
Command Modes
DHCP pool configuration
Command History
Usage Guidelines
This command is valid for DHCP subnetwork address pools only. If the mask or prefix length is not specified, the class A, B, or C natural mask is used. The DHCP Server assumes that all host addresses are available. The system administrator can exclude subsets of the address space by using the ip dhcp excluded-address command.
You cannot configure manual bindings within the same pool that is configured with the network command.
Examples
The following example configures 172.16.0.0/16 as the subnetwork number and mask of the DHCP pool:
network 172.16.0.0/16Related Commands
next-server
To configure the next server in the boot process of a Dynamic Host Configuration Protocol (DHCP) client, use the next-server command in DHCP pool configuration. To remove the boot server list, use the no form of this command.
next-server address [address2...address8]
no next-server address
Syntax Description
Defaults
If the next-server command is not used to configure a boot server list, the DHCP Server uses inbound interface helper addresses as boot servers.
Command Modes
DHCP pool configuration
Command History
Usage Guidelines
You can specify up to eight servers in the list. Servers are listed in order of preference (address1 is the most preferred server, address2 is the next most preferred server, and so on).
Examples
The following example specifies 10.12.1.99 as the IP address of the next server in the boot process:
next-server 10.12.1.99Related Commands
no ip gratuitous-arps
To disable the transmission of gratuitous Address Resolution Protocol (ARP) messages for an address in a local pool, use the no ip gratuitous-arps command in global configuration mode.
no ip gratuitous-arps
Syntax Description
This command has no keywords or arguments.
Defaults
Disabled
Command Modes
Global configuration
Command History
Usage Guidelines
A Cisco router will send out a gratuitous ARP message when a client connects and negotiates an address over a PPP connection. This transmission occurs even when the client receives the address from a local address pool.
Examples
The following example disables gratuitous arp messages from being sent:
no ip gratuitous-arpsoption
To configure Cisco IOS Dynamic Host Configuration Protocol (DHCP) server options, use the option command in DHCP pool configuration mode. To remove the options, use the no form of this command.
option code [instance number] {ascii string | hex string | ip address}
no option code [instance number]
Syntax Description
Defaults
The default instance number is 0.
Command Modes
DHCP pool configuration
Command History
Usage Guidelines
DHCP provides a framework for passing configuration information to hosts on a TCP/IP network. Configuration parameters and other control information are carried in tagged data items that are stored in the options field of the DHCP message. The data items themselves are also called options. The current set of DHCP options are documented in RFC 2131, Dynamic Host Configuration Protocol.
Examples
The following example configures DHCP option 19, which specifies whether the client should configure its IP layer for packet forwarding. A value of 0 means disable IP forwarding; a value of 1 means enable IP forwarding. IP forwarding is enabled in the following example:
option 19 hex 01The following example configures DHCP option 72, which specifies the World Wide Web servers for DHCP clients. World Wide Web servers 172.16.3.252 and 172.16.3.253 are configured in the following example:
option 72 ip 172.16.3.252 172.16.3.253Related Commands
ip dhcp pool
Configures a DHCP address pool on a Cisco IOS DHCP server and enters DHCP pool configuration mode.
origin
To configure an address pool as an on-demand address pool (ODAP), use the origin command in DHCP pool configuration mode. To disable the ODAP, use the no form of this command.
origin {dhcp | aaa | ipcp} [subnet size initial size [autogrow size]]
no origin {dhcp | aaa | ipcp} [subnet size initial size [autogrow size]]
Syntax Description
Defaults
The default size value is /0.
The valid range for the size value is /0, and /4 to /30.
Command Modes
DHCP pool configuration
Command History
Usage Guidelines
If you do not configure the pool as an autogrow pool, the pool will not request additional subnets if one subnet is already in the pool.
Use the dhcp keyword to obtain subnets from DHCP, the aaa keyword to obtain subnets from the AAA server, and the ipcp keyword to obtain subnets from IPCP negotiation. If you expect that the utilization of the pool may grow over time, use the autogrow size option. If a pool has been configured with the autogrow size option, ensure that the source server is capable of providing more than one subnet to the same pool. Even though the Cisco IOS software specifies the requested subnet size, it can accept any offered subnet size from the source server.
In the Cisco IOS 12.2(8)T release, the origin command supports only VRF-associated pools. Work is in progress to support both VRF and non-VRF pools.
Examples
The following example configures an address pool named green to use DHCP as the subnet allocation protocol with an initial subnet size of 24 and an autogrow subnet size of 24:
ip dhcp pool greenvrf greenorigin dhcp subnet size initial /24 autogrow /24utilization mark high 80utilization mark low 20Related Commands
permit (IP)
To set conditions to allow a packet to pass a named IP access list, use the permit command in access list configuration mode. To remove a permit condition from an access list, use the no form of this command.
[sequence-number] permit source [source-wildcard]
[sequence-number] permit protocol source source-wildcard destination destination-wildcard [precedence precedence] [tos tos] [log] [time-range time-range-name] [fragments]
no sequence-number
no permit source [source-wildcard]
no permit protocol source source-wildcard destination destination-wildcard [precedence precedence] [tos tos] [log] [time-range time-range-name] [fragments]
Internet Control Message Protocol (ICMP)
[sequence-number] permit icmp source source-wildcard destination destination-wildcard [icmp-type [icmp-code] | icmp-message] [precedence precedence] [tos tos] [log] [time-range time-range-name] [fragments]
Internet Group Management Protocol (IGMP)
[sequence-number] permit igmp source source-wildcard destination destination-wildcard [igmp-type] [precedence precedence] [tos tos] [log] [time-range time-range-name] [fragments]
Transmission Control Protocol (TCP)
[sequence-number] permit tcp source source-wildcard [operator [port]] destination destination-wildcard [operator [port]] [established] [precedence precedence] [tos tos] [log] [time-range time-range-name] [fragments]
User Datagram Protocol UDP)
[sequence-number] permit udp source source-wildcard [operator [port]] destination destination-wildcard [operator [port]] [precedence precedence] [tos tos] [log] [time-range time-range-name] [fragments]
Syntax Description
sequence-number
(Optional) Sequence number assigned to the permit statement, causing the system to insert the statement in that numbered position in the access list.
source
Number of the network or host from which the packet is being sent. There are three alternative ways to specify the source:
•
•
•
source-wildcard
(Optional) Wildcard bits to be applied to source. There are three alternative ways to specify the source wildcard:
•
•
•
protocol
Name or number of an Internet protocol. It can be one of the keywords eigrp, gre, icmp, igmp, ip, ipinip, nos, ospf, tcp, or udp, or an integer in the range from 0 to 255 representing an Internet protocol number. To match any Internet protocol (including ICMP, TCP, and UDP), use the ip keyword. Some protocols allow further qualifiers described later.
destination
Number of the network or host to which the packet is being sent. There are three alternative ways to specify the destination:
•
•
•
destination-wildcard
Wildcard bits to be applied to the destination. There are three alternative ways to specify the destination wildcard:
•
•
•
precedence precedence
(Optional) Packets can be filtered by precedence level, as specified by a number from 0 to 7 or by name as listed in the section "Usage Guidelines."
tos tos
(Optional) Packets can be filtered by type of service (ToS) level, as specified by a number from 0 to 15, or by name as listed in the section "Usage Guidelines" of the access-list (IP extended) command.
log
(Optional) Causes an informational logging message about the packet that matches the entry to be sent to the console. (The level of messages logged to the console is controlled by the logging console command.)
The message includes the access list number, whether the packet was permitted or denied; the protocol, whether it was TCP, UDP, ICMP or a number; and, if appropriate, the source and destination addresses and source and destination port numbers. The message is generated for the first packet that matches, and then at 5-minute intervals, including the number of packets permitted or denied in the prior 5-minute interval.
Use the ip access-list log-update command to generate logging messages when the number of matches reaches a configurable threshold (rather than waiting for a 5-minute interval). See the ip access-list log-update command for more information.
The logging facility may drop some logging message packets if there are too many to be handled or if there is more than one logging message to be handled in 1 second. This behavior prevents the router from crashing due to too many logging packets. Therefore, the logging facility should not be used as a billing tool or an accurate source of the number of matches to an access list.
If you enable CEF and then create an access list that uses the log keyword, the packets that match the access list are not CEF switched. They are fast switched. Logging disables CEF.
time-range time-range-name
(Optional) Name of the time range that applies to this permit statement. The name of the time range and its restrictions are specified by the time-range and absolute or periodic commands, respectively.
fragments
(Optional) The access list entry applies to noninitial fragments of packets; the fragment is either permitted or denied accordingly. For more details about the fragments keyword, see the "Access List Processing of Fragments" and "Fragments and Policy Routing" sections in the "Usage Guidelines" section.
icmp-type
(Optional) ICMP packets can be filtered by ICMP message type. The type is a number from 0 to 255.
icmp-code
(Optional) ICMP packets that are filtered by ICMP message type can also be filtered by the ICMP message code. The code is a number from 0 to 255.
icmp-message
(Optional) ICMP packets can be filtered by an ICMP message type name or ICMP message type and code name. The possible names are found in the section "Usage Guidelines" of the access-list (IP extended) command.
igmp-type
(Optional) IGMP packets can be filtered by IGMP message type or message name. A message type is a number from 0 to 15. IGMP message names are listed in the section "Usage Guidelines" of the access-list (IP extended) command.
operator
(Optional) Compares source or destination ports. Possible operands include lt (less than), gt (greater than), eq (equal), neq (not equal), and range (inclusive range).
If the operator is positioned after the source and source-wildcard, it must match the source port.
If the operator is positioned after the destination and destination-wildcard, it must match the destination port.
The range operator requires two port numbers. All other operators require one port number.
port
(Optional) The decimal number or name of a TCP or UDP port. A port number is a number from 0 to 65535. TCP and UDP port names are listed in the section "Usage Guidelines" of the access-list (IP extended) command.
TCP port names can only be used when filtering TCP. UDP port names can only be used when filtering UDP.
established
(Optional) For the TCP protocol only: Indicates an established connection. A match occurs if the TCP datagram has the ACK or RST bits set. The nonmatching case is that of the initial TCP datagram to form a connection.
Defaults
There are no specific conditions under which a packet passes the named access list.
Command Modes
Access list configuration
Command History
Usage Guidelines
Use this command following the ip access-list command to define the conditions under which a packet passes the access list.
The time-range option allows you to identify a time range by name. The time-range, absolute, and periodic commands specify when this permit statement is in effect.
Access List Processing of Fragments
The behavior of access-list entries regarding the use or lack of the fragments keyword can be summarized as follows:
Be aware that you should not simply add the fragments keyword to every access list entry because the first fragment of the IP packet is considered a nonfragment and is treated independently of the subsequent fragments. An initial fragment will not match an access list permit or deny entry that contains the fragments keyword, the packet is compared to the next access list entry, and so on, until it is either permitted or denied by an access list entry that does not contain the fragments keyword. Therefore, you may need two access list entries for every deny entry. The first deny entry of the pair will not include the fragments keyword, and applies to the initial fragment. The second deny entry of the pair will include the fragments keyword and applies to the subsequent fragments. In the cases where there are multiple deny access list entries for the same host but with different Layer 4 ports, a single deny access-list entry with the fragments keyword for that host is all that needs to be added. Thus all the fragments of a packet are handled in the same manner by the access list.
Packet fragments of IP datagrams are considered individual packets and each counts individually as a packet in access list accounting and access list violation counts.
Note
Fragments and Policy Routing
Fragmentation and the fragment control feature affect policy routing if the policy routing is based on the match ip address command and the access list had entries that match on Layer 4 through 7 information. It is possible that noninitial fragments pass the access list and are policy routed, even if the first fragment was not policy routed or the reverse.
By using the fragments keyword in access list entries as described earlier, a better match between the action taken for initial and noninitial fragments can be made and it is more likely policy routing will occur as intended.
Examples
The following example sets conditions for a standard access list named Internetfilter:
ip access-list standard Internetfilterdeny 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)The following example permits Telnet traffic on Mondays, Tuesdays, and Fridays from 9:00 a.m. to 5:00 p.m.:
time-range testingperiodic Monday Tuesday Friday 9:00 to 17:00!ip access-list extended legalpermit tcp any any eq telnet time-range testing!interface ethernet 0ip access-group legal inThe following example shows how to add an entry to an existing access list:
Router# show access-listStandard IP access list 12 permit 10.4.0.0, wildcard bits 0.0.255.2555 permit 10.0.0.0, wildcard bits 0.0.255.25510 permit 10.0.0.0, wildcard bits 0.0.255.25520 permit 10.0.0.0, wildcard bits 0.0.255.255Router(config)# ip access-list standard 1Router(config-std-nacl)# 15 permit 5.5.5.5 0.0.255.255The following examples shows how the entry with the sequence number of 20 is removed from the access list:
Router(config)# ip access-list standard 1Router(config-std-nacl)# no 20Router# show access-listStandard IP access list 110 permit 0.0.0.0, wildcard bits 0.0.0.25530 permit 0.0.0.0, wildcard bits 0.0.0.25540 permit 0.4.0.0, wildcard bits 0.0.0.255The following examples shows how, if a user tries to enter an entry that is a duplicate of an entry already on the list, no changes occur. The entry that the user is trying to add is a duplicate of the entry already in the access list with a sequence number of 20.
Router# show access-list 101Extended IP access list 10110 permit ip host 3.3.3.3 host 45.5.5.3420 permit icmp any any30 permit ip host 65.34.2.2 host 43.2.54.240 permit ip host 45.3.4.31 host 34.3.32.3 logRouter(config)# ip access-list extended 101Router(config-ext-nacl)# 100 permit icmp any anyRouter(config-ext-nacl)# endRouter# show access-list 101Extended IP access list 10110 permit ip host 3.3.3.3 host 45.5.5.3420 permit icmp any any30 permit ip host 65.34.2.2 host 43.2.54.240 permit ip host 45.3.4.31 host 34.3.32.3 logThe following example shows what occurs if a user tries to enter a new entry with a sequence number of 20 when an entry with a sequence number of 20 is already in the list. An error message appears, and no change is made to the access list.
Router# show access-list 101Extended IP access list 10110 permit ip host 3.3.3.3 host 45.5.5.3420 permit icmp any any30 permit ip host 65.34.2.2 host 43.2.54.240 permit ip host 45.3.4.31 host 34.3.32.3 logRouter(config)# ip access-list extended 101Router(config-ext-nacl)# 20 permit udp host 1.1.1.1 host 2.2.2.2Duplicate sequence number.Router(config-ext-nacl)# endRouter# show access-list 101Extended IP access list 10110 permit ip host 3.3.3.3 host 45.5.5.3420 permit icmp any any30 permit ip host 65.34.2.2 host 43.2.54.240 permit ip host 45.3.4.31 host 34.3.32.3 logRelated Commands
predictor
To specify the load-balancing algorithm for selecting a real server in the server farm, use the predictor command in SLB server farm configuration mode. To restore the default load-balancing algorithm of weighted round robin, use the no form of this command.
predictor [roundrobin | leastconns]
no predictor
Syntax Description
Defaults
The default predictor is weighted round robin.
Command Modes
SLB server farm 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 specifies the weighted least connections algorithm:
ip slb serverfarm PUBLICpredictor leastconnsRelated Commands
real
To identify a real server as a member of a server farm, use the real command in SLB server farm configuration mode. To remove the real server from the IOS SLB configuration, use the no form of this command.
real ip-address
no real ip-address
Syntax Description
Defaults
No default behavior or values.
Command Modes
SLB server farm 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 identifies a real server as a member of the server farm:
ip slb serverfarm PUBLICreal 10.1.1.1Related Commands
reassign
To specify the threshold of consecutive unanswered synchronizations that, if exceeded, results in an attempted connection to a different real server, use the reassign command in SLB real server configuration mode. To restore the default reassignment threshold, use the no form of this command.
reassign threshold
no reassign
Syntax Description
Defaults
The default threshold is three SYNs.
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 sets the threshold of unanswered SYNs to 2:
ip slb serverfarm PUBLICreal 10.10.1.1reassign 2Related 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.
remark
To write a helpful comment (remark) for an entry in a named IP access list, use the remark command in access list configuration command. To remove the remark, use the no form of this command.
remark remark
no remark remark
Syntax Description
Defaults
The access list entries have no remarks.
Command Modes
Standard named or extended named access list configuration
Command History
Usage Guidelines
The remark can be up to 100 characters long; anything longer is truncated.
If you want to write a comment about an entry in a numbered IP access list, use the access-list remark command.
Examples
In the following example, the Jones subnet is not allowed to use outbound Telnet:
ip access-list extended telnettingremark Do not allow Jones subnet to telnet outdeny tcp host 171.69.2.88 any eq telnetRelated Commands
retry (real server)
To specify how long to wait before a new connection is attempted to a failed server, use the retry command in SLB real server configuration mode. To restore the default retry value, use the no form of this command.
retry retry-value
no retry
Syntax Description
Defaults
The retry-value default is 60 seconds.
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 specifies that 120 seconds must elapse after the detection of a server failure before a new connection is attempted:
ip slb serverfarm PUBLICreal 10.10.1.1retry 120Related 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.
