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Because most hosts support dynamic address resolution, you generally do not need to specify static ARP cache entries. If you do need to define them, you can do so globally. Doing this task installs a permanent entry in the ARP cache. The access server uses this entry to translate bit IP addresses into bit hardware addresses.
Optionally, you can specify that the access server respond to ARP requests as if it were the owner of the specified IP address, and you also have the option of specifying an ARP entry timeout period when you define ARP entries.
The following two tables list the tasks to provide dynamic mapping between IP addresses and media address. Specify that the access server respond to ARP requests as if it were the owner of the specified IP address. You can change this encapsulation method to SNAP or HP Probe, as required by your network, to control the interface-specific handling of IP address resolution into bit Ethernet hardware addresses. You must explicitly configure all interfaces for Probe that will use Probe.
To specify the ARP encapsulation type, perform the following task in interface configuration mode:. The access server uses proxy ARP, as defined in RFC , to help hosts with no knowledge of routing determine the media addresses of hosts on other networks or subnets. For example, if the access server receives an ARP request for a host that is not on the same network as the ARP request sender, and if the access server has the best route to that host, then the access server sends an ARP reply packet giving its own local data link address.
The host that sent the ARP request then sends its packets to the access server, which forwards them to the intended host. Proxy ARP is enabled by default. To disable proxy ARP, perform the following task in interface configuration mode, as necessary, for your network:. Each unique IP address can have a host name associated with it. The access server maintains a cache of host name-to-address mappings for use by the EXEC connect , telnet , ping and related Telnet support operations.
This cache speeds the process of converting names to addresses. IP defines a naming scheme that allows a device to be identified by its location in the IP. This is a hierarchical naming scheme that provides for domains. Domain names are pieced together with periods. For example, Cisco Systems is a commercial organization that the IP identifies by a com domain name, so its domain name is cisco. To keep track of domain names, IP has defined the concept of a name server whose job it is to hold a cache, or database, of names mapped to IP addresses.
To map domain names to IP addresses, you must first identify the host names, then specify a name server, and enable the Domain Name System DNS , the Internet's global naming scheme that uniquely identifies network devices. You do these by performing the following tasks:. The access server maintains a table of host names and their corresponding addresses, also called host name-to-address mapping.
Higher-layer protocols such as Telnet use host names to identify network devices hosts. The access server and other network devices must be able to associate host names with IP addresses to communicate with other IP devices. Host names and IP addresses can be associated with one another through static or dynamic means. Manually assigning host names to addresses is useful when dynamic mapping is not available. To assign host names to addresses, perform the following task in global configuration mode:.
You can specify a default domain name that the access server software will use to complete domain name requests. You can specify either a single domain name or a list of domain names. Any IP host name that does not contain a domain name will have the domain name you specify appended to it before being added to the host table. To specify a domain name or names, perform either of the following tasks in global configuration mode:.
Define a default domain name that the access server will use to complete unqualified host names. To specify one or more hosts up to six that can function as a name server to supply name information for the Domain Name System DNS , perform the following task in global configuration mode:. If your network devices require connectivity with devices in networks for which you do not control name assignment, you can assign device names that uniquely identify your devices within the entire internetwork.
The Internet's global naming scheme, the DNS, accomplishes this task. This service is enabled by default. Tasks associated with HP Probe Proxy are shown in the following two tables. To configure HP Probe Proxy, perform the following task in interface configuration mode:. Enter the host name of an HP host for which the access server is acting as a proxy into the host table. See the " IP Configuration Examples " section at the end of this chapter for an example of configuring HP hosts on a network segment.
In such configurations, packets might have had to make several hops over the NBMA network before arriving at the exit access server the access server nearest the destination network. In addition, such NBMA networks whether partially or fully meshed have typically required tedious static configurations. These systems can then directly communicate without using an intermediate hop, which reduces traffic.
The NBMA network can be considered anonbroadcast network either because it technically does not support broadcasting for example, an X. Assume that the switches have virtual circuit connections represented by hops 1, 2, and 3 of the figure. Other address resolution methods can be in use while NHRP is deployed.
NHRP uses a virtual private network, which is a virtual Layer 3 network that is built on top of an actual Layer 3 network. The topology you can use over the virtual private network can be largely independent of the underlying network, and the protocols you run over it can be completely independent of it. A host or access server that is not an NHRP speaker must be configured with the identity of the Next Hop server that serves it. Each Next Hop server serves a set of destination hosts, which might or might not be directly connected to the NBMA network.
The table is created from information gleaned from NHRP register packets, extracted from NHRP request or reply packets that traverse the Next Hop server as they are forwarded, or through other means such as ARP and preconfigured tables. NHRP supports two modes of operation: server mode and fabric mode. Hosts attached directly to the NBMA network have no knowledge of whether NHRP is deployed in server or fabric mode and host configuration is the same in each case.
In practice, a host's default access server should also be its Next Hop server. A Next Hop server that serves a destination must lie along the routed path to that destination. In practice, this means that all egress access servers must double as Next Hop servers serving the destinations beyond them, and that hosts on the NBMA network are served by access servers that double as Next Hop servers.
This might occur in networks that have access servers that do not support NHRP or networks that have many directly attached hosts and relatively few access servers. Server mode requires static configuration of Next Hop server identity in the client stations hosts or access servers.
The client station must be configured with the IP address of one or more Next Hop servers, and there must be a path to that Next Hop server either directly, in which case the Next Hop server's NBMA address must be known, or indirectly, through an access server whose NBMA address is known. If there are multiple Next Hop servers, they must be configured with each others' addresses, the identities of the destinations they each serve, and a logical NBMA network identifier.
This static configuration requirement, which might also involve authentication, tends to limit the number of Next Hop servers. If the NBMA network offers a group addressing or multicast feature, the client station can be configured with a group address assigned to the group of Next Hop servers. The client might then submit NHRP requests to the group address, eliciting a response from one or more Next Hop servers, depending on the response strategy selected.
The servers can also be configured with the group or multicast address of their peers, and a Next Hop server might use this address to forward NHRP requests that its peers cannot satisfy. This might elicit a response to the Next Hop server from one or more Next Hop servers, depending on the response strategy. The purpose of using group addressing or a similar multicast mechanism in this scenario is to eliminate the need to preconfigure each Next Hop server in a logical NBMA network with both the individual identities of other Next Hop servers and the destinations they serve.
It reduces the number of Next Hop servers that might be used to process an NHRP request in those configurations where Next Hop servers either respond or forward via the multicast, only two Next Hop servers would be traversed and allows the Next Hop server that serves the NHRP request originator to cache next hop information associated with the reply.
To configure NHRP, perform the tasks described in the following sections. The first task is required, the remainder are optional. To enable NHRP for an interface on an access server, perform the following task in interface configuration mode. Alternatively, the station should be configured with a means of acquiring those addresses, that is, the group address that can be used to reach the Next Hop servers.
A third possibility is that the Next Hop servers can be physically located on the stations's default or peer access servers, so their IP addresses can be obtained from the station's IP forwarding table. If the station is attached to several link layer networks including logical NBMA networks , the station should also be configured to receive routing information from its Next Hop servers and peer access servers so that it can determine which IP networks are reachable through which link layer networks.
To configure static IP-to-NBMA address mapping on a station host or access server , perform the following task in interface configuration mode:. A Next Hop server is configured with its own identity, a set of IP address prefixes that correspond to the IP addresses of the stations it serves, a logical NBMA network identifier, and in the case of server mode, the identities of other Next Hop servers in the same logical NBMA network.
If a served station is attached to several link layer networks, the Next Hop server might also need to be configured to advertise routing information to such stations. If a Next Hop server acts as an egress access server for stations connected to link layer networks other than the NBMA network, the Next Hop server must also be configured to exchange routing information between the NBMA network and these other link layer networks.
In all cases, routing information is exchanged using conventional intradomain or interdomain routing protocols. To statically configure a Next Hop server, perform the following task in interface configuration mode:. To configure multiple networks that the Next Hop server serves, repeat the ip nhrp nhs command with the same Next Hop server address, but different IP network addresses.
To configure additional Next Hop servers, repeat the ip nhrp nhs command. Configuring an authentication string ensures that only access servers configured with the same string can intercommunicate using NHRP. Therefore, if the authentication scheme is to be used, the same string must be configured in all access servers that are configured for NHRP on a fabric.
To specify the authentication string for NHRP on an interface, perform the following task in interface configuration mode:. To define an access list, perform one of the following tasks in global configuration mode:. Then apply the IP access list to the interface by performing the following task in interface configuration mode:. The Route Record options contain the network and link layer addresses of all intermediate Next Hop servers between source and destination in the forward direction and between destination and source in the reverse direction.
By default, forward record options and reverse record options are included in NHRP request and reply packets. To suppress the use of these options, perform the following task in interface configuration mode:. The Next Hop server uses the primary IP address of the specified interface.
In this context, "advertised" means how long the access server tells other access servers to keep the addresses it is providing in NHRP responses. The default length of time for each response is seconds 2 hours. To change the length of time, perform the following task in interface configuration mode:. You can enable a generic route encapsulation GRE tunnel to operate in multipoint fashion. A tunnel network of multipoint tunnel interfaces can be thought of as an NBMA network.
To configure the tunnel, perform the following tasks in interface configuration mode:. The tunnel key should correspond to the NHRP network identifier specified in the ip nhrp network-id command. The NHRP cache can contain static entries caused by statically configured addresses and dynamic entries caused by the access server learning addresses from NHRP packets.
To clear static entries, use the no ip nhrp map command. Every access server ships with IP routing automatically enabled. If you choose to set up the access server to bridge rather than route IP datagrams, you must disable IP routing. To disable IP routing, perform the following task in global configuration mode:.
When IP routing is disabled, the access server will act as an IP end host for IP packets destined for or sourced by it, whether or not bridging is enabled for those IP packets not destined for the access server.
To reenable IP routing, use the ip routing command. The access server software provides three methods by which the access server can learn about routes to other networks when IP routing is disabled and the access server is acting as an IP host:. When IP routing is disabled, the default gateway feature and the router discovery client are enabled, and proxy ARP is disabled. When IP routing is enabled, the default gateway feature is disabled and you can configure proxy ARP and the router discovery servers.
The most common method of learning about other routes is by using proxy ARP. Proxy ARP, defined in RFC , enables an Ethernet host with no knowledge of routing to communicate with hosts on other networks or subnets.
Such a host assumes that all hosts are on the same local Ethernet and that it can use ARP to determine their hardware addresses. Under proxy ARP, if an access server receives an ARP Request for a host that is not on the same network as the ARP Request sender, the access server evaluates whether it has the best route to that host.
If the access server does have the best route, it sends an ARP Reply packet giving its own Ethernet hardware address. The host that sent the ARP Request then sends its packets to the access server, which forwards them to the intended host. The software treats all networks as if they are local and performs ARP requests for every IP address.
This feature is enabled by default. Proxy ARP works as long as other access servers support it. Many other access servers, especially host-based routing software, do not support it. Another method for locating routes is to define a default router or gateway. The software sends all nonlocal packets to this access server functioning as a router, which either routes them appropriately or sends an Internet Control Message Protocol ICMP redirect message back to the access server, telling it of a better route.
The ICMP redirect message indicates which local access server the host should use. The software caches the redirect messages and routes each packet thereafter as efficiently as possible. The limitations of this method are that there is no means of detecting when the default access server has crashed or is unavailable, and no method of picking another access server if one of these events should occur.
To set up a default gateway for a host, perform the following task in global configuration mode:. To display the address of the default gateway, use the show ip redirects EXEC command. You can configure these protocols in any combination. When possible, use GDP or IRDP because they allow each access server to specify both a priority and the time after which an access server should be assumed down if no further packets are received. Access servers discovered using IGRP are assigned an arbitrary priority of Access servers discovered through RIP are assigned a priority of For IGRP and RIP, the software attempts to measure the time between updates and will assume that the access server is down if no updates are received for 2.
Each access server discovered becomes a candidate for the default access server. The list of candidates is scanned and a new highest priority access server is selected when any of the following events occur:. In this case, the server flushes the ARP cache and the ICMP redirect cache and picks a new default access server in an attempt to find a successful route to the destination. To configure the access server discovery feature using the GDP routing protocol, perform the following task in interface configuration mode:.
To configure the access server discovery feature using the IRDP routing protocol, perform the following task in interface configuration mode:. To configure the access server discovery feature using the RIP routing protocol, perform the following task in interface configuration mode:. To configure the access server discovery feature using the IGRP routing protocol, perform the following task in interface configuration mode:. To transparently bridge IP on an interface, perform the following tasks beginning in global configuration mode:.
You can route IP on some interfaces and transparently bridge it on other interfaces simultaneously. To enable concurrent routing and bridging for the router, perform the following task in global configuration mode:. At this point in the configuration process, you can configure one or more of the many routing protocols based on your individual network needs.
Routing protocols provide topology information of an internetwork. If you want to continue to perform basic IP configuration tasks, continue reading the following sections. A broadcast is a data packet destined for all hosts on a particular physical network. Network hosts recognize broadcasts by special addresses. Broadcasts are heavily used by some protocols, including several important Internet protocols. Control of broadcast messages is an essential part of the IP network administrator's job.
Our access servers support two kinds of broadcasting: directed broadcasting and flooding. A directed broadcast is a packet sent to a specific network or series of networks, and a flooded broadcast packet is sent to every network. A directed broadcast address includes the network or subnet fields. Several early IP implementations do not use the current broadcast address standard.
Instead, they use the old standard, which calls for all zeros instead of all ones to indicate broadcast addresses. Many of these implementations do not recognize an all-ones broadcast address and fail to respond to the broadcast correctly. Others forward all-ones broadcasts, which causes a serious network overload known as a broadcast storm.
Routers provide some protection from broadcast storms by limiting their extent to the local cable. Bridges including intelligent bridges , because they are Layer 2 devices, forward broadcasts to all network segments, thus propagating all broadcast storms. The best solution to the broadcast storm problem is to use a single broadcast address scheme on a network. Most modern IP implementations allow the network manager to set the address to be used as the broadcast address.
Many implementations, including the one on our access server, can accept and interpret all possible forms of broadcast addresses. The current broadcast address standard provides specific addressing schemes for forwarding broadcasts. Perform the tasks in the following sections to enable these schemes:. See the " IP Configuration Examples " section at the end of this chapter for broadcasting configuration examples. To enable forwarding of directed broadcasts on an interface where the broadcast becomes a physical broadcast, perform one of the tasks that follow.
By default, this feature is enabled only for those protocols configured using the ip forward-protocol global configuration command. You can specify an access list to control which broadcasts are forwarded. When an access list is specified, only those IP packets permitted by the access list are eligible to be translated from directed broadcasts to physical broadcasts. Perform either of the following tasks in interface configuration mode as required for your network:.
Disable directed broadcast-to-physical broadcast translation on an interface. Network hosts occasionally use UDP broadcasts to determine address, configuration, and name information. If such a host is on a network segment that does not include a server, UDP broadcasts are normally not forwarded. You can remedy this situation by configuring the interface of your access server to forward certain classes of broadcasts to a helper address.
You can have more than one helper address per interface. You can specify multiple UDP protocols. By default, both UDP and ND forwarding are enabled if a helper address has been defined for an interface. The description for the ip forward-protocol command in the Router Products Command Reference publication lists the ports that are forwarded by default if you do not specify any UDP ports.
This means that the access server is now compatible with DHCP clients. To enable forwarding and to specify the destination address, perform the following task in interface configuration mode:. Enable forwarding and specify the destination address for forwarding UDP broadcast packets, including BootP.
To specify which protocols will be forwarded, perform the following task in global configuration mode:. See the "IP Configuration Examples" section in this publication for an example of how to configure helper addresses. The access server supports IP broadcasts on both local- and wide-area networks.
There are several ways to indicate an IP broadcast address. Currently, the most popular way the default is an address consisting of all ones Our access servers also can receive and understand any form of IP broadcast. To set the access server's IP broadcast address, perform the following task in interface configuration mode:. If the access server does not have nonvolatile memory, and you need to specify the broadcast address to use before the access server has been configured, you have to change the IP broadcast address by setting jumpers in the processor configuration register.
Setting bit 10 causes the access server to use all zeros. Bit 10 interacts with bit 14, which controls the network and subnet portions of the broadcast address. Setting bit 14 causes the access server to include the network and subnet portions of its address in the broadcast address. Some access server platforms allow the configuration register to be set through the software; see the "Loading System Images and Configuration Files" chapter for details.
For other access server platforms, the configuration register can only be changed through hardware; see the appropriate hardware installation and maintenance manual for your system. The IP suite offers a number of services that control and manage IP connections.
ICMP messages are sent by access servers to hosts or other access servers when a problem is discovered with the Internet header. If the access server receives a nonbroadcast packet destined for itself that uses an unknown protocol, it sends an ICMP Protocol Unreachable message back to the source. Similarly, if the access server receives a packet that it is unable to deliver to the ultimate destination because it knows of no route to the destination address, it sends an ICMP Host Unreachable message to the source.
You can disable this service by performing the following task in interface configuration mode:. Routes sometimes can become less than optimal. For example, it is possible for the access server to be forced to resend a packet through the same interface on which it was received.
If this happens, the access server sends an ICMP Redirect message to the packet's originator telling it that it is on a subnet directly connected to the access server, and that it must forward the packet to another system on the same subnet. It does so because the originating host presumably could have sent that packet to the next hop without involving the access server at all.
The Redirect message instructs the sender to remove the access server from the route and substitute a specified device representing a more direct path. You can disable the sending of ICMP Redirect messages by performing the following task in interface configuration mode:. IP Path MTU Discovery allows a host to dynamically discover and cope with differences in the maximum allowable maximum transmission unit MTU size of the various links along the path.
Sometimes an access server is unable to forward a datagram because it requires fragmentation the packet is larger than the MTU you set for the interface with the ip mtu command , but the "Don't fragment" DF bit is set. The access server sends a message to the sending host, alerting it to the problem. The host must fragment packets for the destination so that they fit the smallest packet size of all the links along the path.
This technique is shown in Figure Figure shows an attempt to send IP packets over a network where the MTU in the first access server is set to bytes, but then reaches an access server where the MTU is set to bytes. If the datagram's "Don't fragment" bit is set, the datagram is dropped because the byte access server is unable to forward it. All packets larger than bytes are dropped in this case. IP Path MTU Discovery is also useful when a connection is first being established and the sender has no information at all about the intervening links.
If an end host does not support IP Path MTU Discovery, an access server will have no mechanism available to avoid fragmenting datagrams generated by the end host. Because the CTR card does not support the switching of frames larger than bytes, some interoperability problems may occur if CTR cards are intermixed with other Token Ring cards on the same network. You can minimize this by setting lower and the same IP maximum packet sizes for all devices on the network with the ip mtu interface command.
All interfaces have a default MTU packet size. However, the reverse is not true; changing the IP MTU value has no effect on the value for the mtu interface configuration command. Also, all devices on a physical medium must have the same protocol MTU in order to operate. To set the MTU packet size for a specified interface, perform the following task in interface configuration mode:. Occasionally, network devices need to know the subnet mask for a particular subnetwork in the internetwork.
The access server examines IP header options on every packet. If the access server finds a packet with one of these options enabled, it performs the appropriate action. If it finds a packet with an invalid option, it sends an ICMP Parameter Problem message to the source of the packet and discards the packet. IP provides an option called source routing, which allows the source IP host to specify a route through the IP network. You specify source routing as an option in the IP header.
If source routing is specified, the access server forwards the packet according to the specified source route. You use this feature when you want to force a packet to take a certain route through the network. The default is to perform source routing. You can disable IP source-route header options by performing the following task in global configuration mode:. Cause the access server to discard any IP datagram containing a source-route option.
Packet filtering helps control packet movement through the network. Such control can help limit network traffic and restrict network use by certain users or devices. To permit or deny packets from crossing specified access server interfaces, we provide access lists. See the " IP Configuration Examples " section at the end of this chapter for examples of configuring access lists.
An access list is a sequential collection of permit and deny conditions that apply to IP addresses. The access server tests addresses against the conditions in an access list one by one. The first match determines whether the access server accepts or rejects the address. Because the access server stops testing conditions after the first match, the order of the conditions is critical.
If no conditions match, the access server rejects the address. Step 1 Create an access list by specifying an access list number and access conditions. Step 2 Apply the access list to interfaces or terminal lines. To create a standard access list, perform one of the following tasks in global configuration mode:. Define a standard IP access list using an abbreviation for the source and source mask of 0. To create an extended access list, perform one of the following tasks in global configuration mode:.
Define an extended IP access list using an abbreviation for a source and source wildcard of 0. Define an extended IP access list using an abbreviation for a source and source wildcard of source 0. After an access list is created initially, any subsequent additions possibly entered from the terminal are placed at the end of the list.
In other words, you cannot selectively add or remove access list command lines from a specific access list. Note Keep in mind when making the standard and extended access list that by default, the end of the access list contains an implicit deny statement for everything if it did not find a match before reaching the end.
Further, with standard access lists, if you omit the mask from an associated IP host address access list specification, 0. Refer to the " IP Configuration Examples " section at the end of this chapter for examples of implicit masks. After an access list is created, you can apply it to one or more interfaces. Access lists can be applied on either outbound or inbound interfaces.
The next two tables show how this task is accomplished for both terminal lines and network interfaces. Restrict incoming and outgoing connections between a particular virtual terminal line into a device and the addresses in an access list. For inbound access lists, after receiving a packet, the access server checks the source address of the packet against the access list.
If the access list permits the address, the access server continues to process the packet. If the access list rejects the address, the access server discards the packet and returns an ICMP Host Unreachable message. For outbound access lists, after receiving and routing a packet to a controlled interface, the access server checks the source address of the packet against the access list.
If the access list permits the address, the access server transmits the packet. When you apply an access list standard or extended that has not yet been defined to an interface, the access server will act as if the access list has not been applied to the interface and will accept all packets. Remember this behavior if you use undefined access lists as a means of security in your network. Note Set identical restrictions on all the virtual terminal lines, because a user can attempt to connect to any of them.
The Hot Standby Router Protocol provides high network availability because it routes IP traffic from hosts on Ethernet or Token Ring networks without relying on the availability of any single router. This feature is useful for hosts that do not support a router discovery protocol such as IRDP and do not have the functionality to switch to a new router when their selected router reloads or loses power.
Because existing TCP sessions can survive the failover , this protocol also provides a more transparent means of recovery for hosts that dynamically select a next hop for routing IP traffic. One of these routers is selected by the protocol to be the active router. The active router receives and routes packets destined for the group's MAC address. A new standby router is also selected at that time. Routers that are running the Hot Standby Router Protocol send and receive multicast UDP-based hello packets to detect router failure and to designate active and standby routers.
You can configure multiple Hot Standby groups on an interface, thereby making fuller use of the redundant routers. To do so, specify a group number for each Hot Standby command you configure for the interface. Note Token Ring interfaces allow up to three Hot Standby groups each. Note The Cisco series devices that use Lance Ethernet hardware do not support multiple Hot Standby groups on a single Ethernet interface. To enable the Hot Standby Router Protocol on an interface, perform the following task in interface configuration mode:.
To configure other Hot Standby group attributes that affect how the local router participates in the Hot Standby Router Protocol, perform one or more of the following tasks in interface configuration mode:. Configure the time between hello packets and the holdtime before other routers declare the active router to be down.
Set the router's Hot Standby priority, used in choosing the active router. Specify that, if the local router has priority over the current active router, the local router should attempt to take its place as the active router. Configure the interface to track other interfaces, so that if one of the other interfaces goes down, the router's Hot Standby priority is lowered. Select an authentication string to be carried in all Hot Standby Router Protocol messages.
IPSO is generally used to comply with the U. Government's DoD security policy. To enable IPSO and set security classifications on an interface, perform either of the following tasks in interface configuration mode:. Set an interface to the requested IPSO range of classifications and authorities.
Use the no ip security command to reset an interface to its default state. To specify how IP security options are processed, perform any of the following optional tasks in interface configuration mode:. Ensure that all packets leaving the access server on an interface contain a basic security option. Remove any basic security option that might be present on a packet leaving the access server through an interface.
Treat as valid any packets that have Reserved1 through Reserved4 security levels. In order to fully comply with IPSO, the default values for the minor keywords have become complex. To disable the transmission, use the no form of this command.
Gratuitous ARP messages for locally originated peer addresses are not sent by default. Gratuitous ARP messages are not sent out when the client receives the address from the local address pool. The non-local keyword was added and the default behavior of the command changed. The name of this command was changed from no ip gratuitous-arps to ip gratuitous-arps. A Cisco router will send out a gratuitous ARP message out of all interfaces when a client connects and negotiates an address over a PPP connection.
However, by default, gratuitous ARP messages are not sent out when the client receives the address from the local address pool. The ip gratuitous-arps non-local command option is the default form and is not saved in the running configuration. Cisco Series Router. To maximize the performance of the router, disable gratuitous ARP requests using the no ip gratuitous-arps command.
The following example enables the sending of gratuitous ARP messages if the transmission has been disabled:. If the hostname cache does not exist yet, it is automatically created. To remove a hostname-to-address mapping, use the no form of this command. Optional The view-name argument specifies the name of the DNS view whose hostname cache is to store the mappings.
Name of the host. The first character 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. Modem telephone number that is mapped to the IP host address for use in Cisco modem user interface mode. This argument is not relevant to the Split DNS feature. The default is Telnet port Optional Up to seven additional associated IP addresses, delimited by a single space. The ellipses in the syntax description are used to indicate a range of values.
Do not use ellipses when entering host IP addresses. The ip-address9 argument specifies an additional IP address to add to the hostname cache. The use of the optional additional keyword enables the addition of more than eight IP addresses to the hostname cache.
Optional Additional associated IP addresses, delimited by a single space. Optional Mail Exchange MX resource record settings for the host:. The lower this value, the higher the host is in priority. Range is from 0 to An MX record specifies how you want e-mail to be accepted for the domain specified in the hostname argument.
You can have several MX records for a single domain name, and they can be ranked in order of preference. Optional Name Server NS resource record setting for the host:. Machines that provide name service do not have to reside in the named domain.
An NS record lists the name of the machine that provides domain service for the domain indicated by the hostname argument. For each domain you must have at least one NS record. NS records for a domain must exist in both the zone that delegates the domain and in the domain itself. Optional Server SRV resource record settings for the host:. The use of SRV records enables administrators to use several servers for a single domain, to move services from host to host with little difficulty, and to designate some hosts as primary servers for a service and others as backups.
Clients ask for a specific service or protocol for a specific domain and receive the names of any available servers. The mx keyword and the preference and mx-server-hostname arguments were added. The srv keyword and the priority , weight , port , and target arguments were added.
The ns keyword and the nameserver-hostname argument were added. The capability to map a modem telephone number to an IP host was added for the Cisco modem user interface feature. The view keyword and view-name argument were added. This command is integrated into Cisco IOS An IPv6 address can be specified for the ip-address argument, and the additional ip-address keyword-argument pair.
This command adds the specified hostname-to-IP address mappings as follows:. If the specified VRF does not exist yet, a warning is displayed and the entry is added to the hostname cache anyway. If the specified view does not exist yet, a warning is displayed and the entry is added to the hostname cache anyway. 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 the srv keyword and the priority , weight , port , and target arguments. To display the display the default domain name, the style of name lookup service, a list of name server hosts, and the cached list of hostnames and addresses specific to a particular DNS view or for all configured DNS views, use the show hosts command.
If a global or VRF-specific DNS hostname cache contains hostnames that are associated with multiple IP addresses, round-robin rotation of the returned addresses can be enabled on a DNS view-specific basis by using the domain round-robin command. The following example shows how to add three mapping entries to the global hostname cache and then remove one of those entries from the global hostname cache:. The following example shows how to add three mapping entries to the hostname cache for the DNS view user3 that is associated with the VRF vpn and then remove one of those entries from that hostname cache:.
Displays the default domain name, the style of name lookup service, a list of name server hosts, and the cached list of hostnames and addresses specific to a particular DNS view or for all configured DNS views. To disable the host list, use the no form of this command.
List of servers that will receive DDNS updates. Optional Identifies the virtual routing and forwarding VRF table. The vrf-name argument identifies the address pool to which the VRF is associated. The interface configuration overrides the global configuration. The following example shows how to configure a list of hosts:. To ensure that Internet hostnames comply with Section 2.
To remove the restriction on hostnames, use the no form of this command. This command is disabled by default, that is, characters that are not specified in Section 2. Section 2. A hostname is composed of one or more labels, separated by periods. Each label is composed of one or more of the following characters: letters A-Z, a-z , digits , and the hyphen -. No other characters are allowed. Alphabetic characters in hostnames can be either uppercase or lowercase, in any combination.
A hyphen cannot be the first character of any label. The most significant label also described as the top-level domain or TLD , that is, the group of characters that follow the final dot of the domain name, must contain at least one letter or hyphen, and must have least two characters. A hostname, including the periods, cannot have more than characters. However, hostnames should not exceed 63 characters because conforming applications might be unable to handle hostnames longer than that.
The following hostnames comply with Section 2. The following hostnames do not comply with Section 2. When the ip hostname strict command is configured on a router, any hostname configured on the router must comply with Section 2.
When the ip hostname strict command is not configured on a router, characters that are not specified in Section 2. The following example shows how to specify compliance with Section 2. Defines a default domain name to complete unqualified hostnames. Defines a static hostname-to-address mapping in the hostname cache. 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. This command was introduced on the Catalyst series switches. Use this feature only on subnets where hosts are intentionally prevented from communicating directly to the Catalyst series switch on which they are connected. The following example shows how to enable the local proxy ARP feature:. To enable local-area mobility, use the ip mobile arp command in interface configuration mode. To disable local-area mobility, use the no form of this command.
The default value is 5. Optional Hold time, in minutes. This is the length of time the software considers that a relocated host is present without receiving some type of ARP broadcast or unicast from the host. Normally, the hold time should be at least three times greater than the keepalive time.
Optional Indicates that you are applying an access list. This access list applies only to local-area mobility. Optional Number of a standard IP access list. The range is from 1 to Only hosts with addresses permitted by this access list are accepted for local-area mobility. Optional Name of an IP access list.
The name cannot contain a space or quotation mark, and must begin with an alphabetic character to avoid ambiguity with numbered access lists. VRF-awareness for local-area mobility is available in this release. To create larger mobility areas, you must first redistribute the mobile routes into your Interior Gateway Protocol IGP.
The IGP must support host routes. The mobile area must consist of a contiguous set of subnets. Using an access list to control the list of possible mobile nodes is strongly encouraged. Without an access list, misconfigured hosts can be mistaken for mobile nodes and disrupt normal operations. The following example shows how to configure local-area mobility on Ethernet interface To specify the address of one or more name servers to use for name and address resolution, use the ip name-server command in global configuration mode.
To remove the addresses specified, use the no form of this command. The following example shows how to specify IPv4 hosts Defines a default domain name to complete unqualified hostnames names without a dotted decimal domain name. To designate that traffic originating from or destined for the interface is subject to Network Address Translation NAT , to enable NAT logging, or to enable static IP address support, use the ip nat command in interface configuration mode.
To prevent the interface from being able to translate or log, use the no form of this command. Optional Indicates that the interface is connected to the inside network the network subject to NAT translation.
Optional Indicates that the interface is connected to the outside network. Optional Enables NAT logging translations. Optional Enables syslog for NAT logging translations. Traffic leaving or arriving at this interface is not subject to NAT. The allow-static-host keyword was added.
Only packets moving between inside and outside interfaces can be translated. You must specify at least one inside interface and outside interface for each border router where you intend to use NAT. The following example translates between inside hosts addressed from either the The following example enables static IP address support for the router at Clears dynamic NAT translations from the translation table.
Enables NAT of the inside destination address. Enables NAT of the inside source address. Enables NAT of the outside source address. Enables a port other than the default port. To enable flow cache entries in Network Address Translation NAT , use the ip nat create flow-entries command in global configuration mode. To disable flow cache entries in NAT, use the no form of this command.
Disabling flow cache entries will result in lesser performance as this functionality does multiple database searches to find the most specific translation to use. A session is also called a flow cache entry.
Instead of creating sessions, dynamic and static NAT translations can translate a packet from the binding or bindings, if both inside and outside bindings are available. A binding or a half entry is an association between a local IP address and a global IP address. Disabling flow cache entries for dynamic and static translations saves memory usage and provides more scalability for your NAT translations. The following example shows how to disable flow cache entries in a dynamic NAT configuration:.
The following example shows how to enable flow cache entries in a static CGN configuration:. Enables Network Address Translation on a virtual interface without inside or outside specification. To enable the Network Address Translation NAT of a globally unique outside host address to multiple inside host addresses, use the ip nat inside destination command in global configuration mode.
This command is primarily used to implement TCP load balancing by performing destination address rotary translation. To remove the dynamic association to a pool, use the no form of this command. Specifies the standard IP access list number. Packets with destination addresses that pass the access list are translated using global addresses from the named pool. Specifies the name of a standard IP access list. Specifies the name of the pool from which global IP addresses are allocated during dynamic translation.
Specifies the NAT redundancy operation. The mapping-id map-id keyword and argument combination was added. The redundancy redundancy-id keyword and argument pair was added. To implement TCP load balancing, you must configure NAT to use rotary pools as specified with the ip nat pool command and the rotary keyword. Packets from addresses that match the standard access list are translated using global addresses allocated from the pool named with the ip nat pool command.
The following example shows how to define a virtual address with connections that are distributed among a set of real hosts. The rotary pool defines the addresses of the real hosts. The access list defines the virtual address.
If a translation does not already exist, TCP packets from serial interface 0 the outside interface whose destination matches the access list are translated to an address from the rotary pool. Designates that traffic originating from or destined for the interface is subject to NAT. To enable Network Address Translation NAT of the inside source address, use the ip nat inside source command in global configuration mode.
To remove the static translation, or the dynamic association to a pool, use the no form of this command. Specifies the number of a standard IP access list. Packets with source addresses that pass the access list are dynamically translated using global addresses from the named pool. Specifies an interface for the global address. Interface type. For more information, use the question mark? For more information about the numbering syntax for your networking device, use the question mark?
Specifies the name of the pool from which global IP addresses are allocated dynamically. Optional Prohibits the translation of an embedded address or port in the payload. Optional Establishes NAT redundancy. Optional Enables the device to use one global address for many local addresses. When overloading is configured, the TCP or UDP port number of each inside host distinguishes between the multiple conversations using the same local IP address.
Optional Enables outside-to-inside initiated sessions to use route maps for destination-based NAT. Optional Specifies the port map to be associated for NAT. Local IP address assigned to a host on the inside network. The address could be randomly chosen, allocated from RFC , or obsolete. Globally unique IP address of an inside host as it appears to the outside network.
Optional Forcefully deletes an entry and its children from the configuration. Optional Prohibits an alias from being created for the global address. Specifies the local subnet translation. IP network mask to be used with subnet translations.
No NAT translation of inside source addresses occurs. This command was modified to include the ability to use route maps with static translations, and the route-map name keyword-argument pair was added. This command was modified to include static translation with Hot Standby Routing Protocol HSRP , and the redundancy group-name keyword-argument pair was added. This command was modified to enable the translation of the IP header address only, and the no-payload keyword was added.
The interface keyword was added for static translations. The vrf name keyword-argument pair was added. The reversible keyword was added. The oer keyword was added. The vrf name keyword-argument pair was removed from Cisco series routers. The rg-id argument and the mapping-id mapping-id keyword-argument pair were added. The reversible keyword was added to Static NAT configuration.
The optional keywords of the ip nat inside source command can be entered in any order. This command has two forms: the dynamic and the static address translation. The form with an access list establishes the dynamic translation. Packets that enter the device through the inside interface and packets sourced from the device are checked against the access list for possible NAT candidates.
The access list is used to specify which traffic is to be translated. Alternatively, the syntax form with the keyword static establishes a single static translation. When a session is initiated from outside with the source IP as the outside global address, the device is unable to determine the destination VRF of the packet.
When you configure NAT with a VRF-enabled interface address that acts as the global address, you must configure the ip nat inside source static no-alias command. If the no-alias keyword is not configured, Telnet to the VRF-enabled interface address fails. The following example shows how to translate between inside hosts addressed from either the The following example shows how to translate sessions from outside to inside networks:.
The following example shows how to configure the route map R1 to allow outside-to-inside translation for static NAT:. Configures an interface type and enters interface configuration mode. Defines an IP access list or object group access control list by name or number.
Establishes static routes for a VRF instance. Associates a VRF instance with a diameter peer. Distributes any routes that have a destination network number address that is permitted by a standard access list, an extended access list, or a prefix list, or performs policy routing on packets.
Sets conditions in a named IP access list or object group access control list that will permit packets. Defines the conditions for redistributing routes from one routing protocol into another routing protocol, or enables policy routing. To enable the high-speed logging of Network Address Translation NAT translations by using a flow exporter, use the ip nat log translations flow-export command in global configuration mode.
To disable the logging of NAT translations by using a flow exporter, use the no form of this command. Specifies the flow exporter Version 9 format. Specifies the destination IPv4 address for which translations will be logged. Specifies the destination address for which translations will be logged. Name or IPv4 address of the destination. Local UDP port number.
Valid values are from 1 to Optional Logs only NAT binding translations. Optional Specifies the source interface for which translations will be logged. Optional Specifies the destination VRF for which translations will be logged.
The bind-only keyword was added. The volume of data that is logged for NAT bindings translations is significantly reduced when you enable the bind-only keyword. When you configure the ip nat log translations flow-export command without the bind-only keyword, translations for both NAT bindings and NAT sessions are logged.
Sessions are identified by the 5-tuple the source IP address, the destination IP address, the protocol, the source port, and the destination port information. Sessions are normally created and destroyed at a much faster rate than bindings and, as a result, configuring the bind-only keyword can significantly reduce the volume of translation logs. The bind-only keyword is most useful for dynamic NAT configurations without the overload configuration.
Thus, configuring the bind-only keyword is not very useful for PAT users. The following example shows how to enable translation logging for a specific destination and source interface:. To enable the high-speed logging of Network Address Translation NAT translations to the syslog, use the ip nat log translation syslog command in global configuration mode. To disable the logging of NAT translations, use the no form of this command.
When you configure the ip nat log translations syslog command without the bind-only keyword, translations for both NAT bindings and NAT sessions are logged. To enable Network Address Translation NAT of the outside source address, use the ip nat outside source command in global configuration mode. To remove the static entry or the dynamic association, use the no form of this command.
Packets with source addresses that pass the access list are translated using global addresses from the named pool. Specifies the name of the pool from which global IP addresses are allocated. Optional Adds a static route for the outside local address. Globally unique IP address assigned to a host on the outside network by its owner. The address was allocated from the globally routable network space. Local IP address of an outside host as it appears to the inside network.
Optional Prohibits an alias from being created for the local address. Optional Enables the NAT redundancy operation. Port number assigned to a host on the outside network by its owner. Port number of an outside host as it appears to the inside network. Sets up a single static network translation. Globally unique network address assigned to a host on the outside network by its owner.
The address is allocated from a globally routable network space. Local network address of an outside host as it appears to the inside network. The address is allocated from an address space that is routable on the inside network. Subnet mask for the networks that are translated.
No translation of source addresses coming from the outside to the inside network occurs. The optional keywords of the ip nat outside source command except for the vrf name keyword can be entered in any order. You can use NAT to translate inside addresses that overlap with outside addresses.
Use this command if your IP addresses in the stub network happen to be legitimate IP addresses belonging to another network, and you need to communicate with those hosts or devices. This command has two general forms: dynamic and static address translation.
The form with an access list establishes dynamic translation. Packets from addresses that match the standard access list are translated using global addresses allocated from the pool that is named by using the ip nat pool command.
Alternatively, the syntax form with the static keyword establishes a single static translation. The match-in-vrf keyword is supported with the ip nat outside source static command. The match-in-vrf keyword is not supported with the dynamic NAT configuration.
When you configure the ip nat outside source static command to add static routes for static outside local addresses, there is a delay in the translation of packets and packets are dropped. To avoid dropped packets, configure either the ip nat outside source static add-route command or the ip route command.
The following example shows how to translate between inside hosts addressed from the Further, packets from outside hosts addressed from the Clears dynamic NAT from the translation table. Skip to content Skip to search Skip to footer. Bias-Free Language. Bias-Free Language The documentation set for this product strives to use bias-free language.
Find Matches in This Book. Log in to Save Content. PDF - Complete Book 8. Updated: May 18, Chapter: ip dhcp-client network-discovery through ip nat sip-sbc. This command was introduced. Name of the DHCP server. Start of authority record parameters.
Authoritative name server. DNS mailbox of administrative contact. Enables the DNS server on a router. Name of a DNS view list. Refers to the unnamed DNS view. Name of an existing DNS view list. Selects an interface to configure. Associated host IP address. Optional Mail Exchange MX resource record settings for the host: preference --The order in which mailers select MX records when they attempt mail delivery to the host.
Optional Name Server NS resource record setting for the host: nameserver-hostname --The DNS name of the machine that provides domain service for the particular domain. Defines the hostname for a network server. Optional Sets local-area mobility timers. Defines a standard IP access list. Sets default metric values for OSPF. Sets default metric values for RIP. Specifies the list of networks for the BGP routing process.
Specifies a list of networks for the RIP routing process. Redistributes routes from one routing domain into another routing domain. Configures an OSPF routing process. IPv4 or IPv6 addresses of a name server. Optional IP addresses of additional name servers a maximum of six name servers. Support for IPv6 addresses was added.
Optional Enables NAT logging. Displays NAT statistics. Displays active NAT translations.