Cathayschool Blog

Enabling the DNS

admin — Sun, 01 Apr 2012 09:28:48 +0000

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 global naming scheme of the Internet, the DNS, accomplishes this task. This service is enabled by default.

To re-enable DNS if it has been disabled, use the following command in global configuration mode:

Specifying a Name Server

admin — Sun, 01 Apr 2012 09:28:21 +0000

To specify one or more hosts (up to six) that can function as a name server to supply name information for the DNS, use the following command in global configuration mode:

Specifying the Domain Name

admin — Sun, 01 Apr 2012 09:27:43 +0000

You can specify a default domain name that the Cisco IOS 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, use either of the following commands in global configuration mode:

Assigning Host Names to IP Addresses

admin — Sun, 01 Apr 2012 09:26:47 +0000

The Cisco IOS software maintains a table of host names and their corresponding addresses, also called a host name-to-address mapping. Higher-layer protocols such as Telnet use host names to identify network devices (hosts). The router 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, use the following command in global configuration mode:

Mapping Host Names to IP Addresses

admin — Sun, 01 Apr 2012 09:26:04 +0000

Each unique IP address can have an associated host name. The Cisco IOS software maintains a cache of host name-to-address mappings for use by the connect, telnet, and ping EXEC commands, 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 (.) as the delimiting characters. For example, Cisco is a commercial organization that the IP identifies by a com domain name, so its domain name is cisco.com. A specific device in this domain, the File Transfer Protocol (FTP) system, for example, is identified as ftp.cisco.com.

To keep track of domain names, IP has defined the concept of a name server, whose job 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 Naming System (DNS), the global naming scheme of the Internet that uniquely identifies network devices. These tasks are described in the following sections:

?Assigning Host Names to IP Addresses

?Specifying the Domain Name

?Specifying a Name Server

?Enabling the DNS

?Using the DNS to Discover ISO CLNS Addresses

Configuring Local-Area Mobility

admin — Sun, 01 Apr 2012 09:25:22 +0000

Local-area mobility provides the ability to relocate IP hosts within a limited area without reassigning host IP addresses and without changes to the host software. Local-area mobility is supported on Ethernet, Token Ring, and FDDI interfaces only.

To create a mobility area with only one router, use the following commands in the interface configuration mode:

To create larger mobility areas, you must first redistribute the mobile routes into your Interior Gateway Protocol (IGP). The IGP must support host routes. You can use Enhanced Interior Gateway Routing Protocol (IGRP), Open Shortest Path First (OSPF), IS-IS, or RIPv2. To redistribute the mobile routes into your existing IGP configuration, use the following commands in configuration mode:

Mobile routes will always be preferred over a subnet boundary or summarized route because they are more specific. It is important to ensure that configured or redistributed static routes do not include any host routes for the potentially mobile hosts; otherwise, a longest match could come up with two routes and cause ambiguity. Mobile routes will be seen as external routes to the configured routing protocol, even within a summarization area; therefore, they will not be properly summarized by default. This is the case even when these routes are advertised at a summarization boundary, if mobile hosts are not on their home subnet.

Enabling Proxy ARP

admin — Sun, 01 Apr 2012 09:24:43 +0000

The Cisco IOS software uses proxy ARP (as defined in RFC 1027) to help hosts with no knowledge of routing determine the media addresses of hosts on other networks or subnets. For example, if the router receives an ARP request for a host that is not on the same interface as the ARP request sender, and if the router has all of its routes to that host through other interfaces, then it generates a proxy ARP reply packet giving its own local data-link address. The host that sent the ARP request then sends its packets to the router, which forwards them to the intended host. Proxy ARP is enabled by default.

To enable proxy ARP if it has been disabled, use the following command in interface configuration mode (as needed) for your network:

Setting ARP Encapsulations

admin — Sun, 01 Apr 2012 09:23:32 +0000

By default, standard Ethernet-style ARP encapsulation (represented by the arpa keyword) is enabled on the IP interface. 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 48-bit Ethernet hardware addresses.

When you set HP Probe encapsulation, the Cisco IOS software uses the Probe protocol whenever it attempts to resolve an IEEE-802.3 or Ethernet local data-link address. The subset of Probe that performs address resolution is called Virtual Address Request and Reply. Using Probe, the router can communicate transparently with HP IEEE-802.3 hosts that use this type of data encapsulation. You must explicitly configure all interfaces for Probe that will use Probe.

To specify the ARP encapsulation type, use the following command in interface configuration mode:

Defining a Static ARP Cache

admin — Tue, 06 Mar 2012 07:03:04 +0000

ARP and other address resolution protocols provide a dynamic mapping between IP addresses and media addresses. Because most hosts support dynamic address resolution, generally you need not specify static ARP cache entries. If you must define them, you can do so globally. Performing this task installs a permanent entry in the ARP cache. The Cisco IOS software uses this entry to translate 32-bit IP addresses into 48-bit hardware addresses.

Optionally, you can specify that the software respond to ARP requests as if it were the owner of the specified IP address. In case you do not want the ARP entries to be permanent, you have the option of specifying an ARP entry timeout period when you define ARP entries.

The following two tables list the tasks to provide static mapping between IP addresses and a media address.

Use either of the following commands in global configuration mode to specify that the software respond to ARP requests:

Use the following command in interface configuration mode to set the length of time an ARP cache entry will stay in the cache:

To display the type of ARP being used on a particular interface and also display the ARP timeout value, use the show interfaces EXEC command. Use the show arp EXEC command to examine the contents of the ARP cache. Use the show ip arp EXEC command to show IP entries. To remove all nonstatic entries from the ARP cache, use the clear arp-cache privileged EXEC command.

Establishing Address Resolution

admin — Tue, 06 Mar 2012 07:01:52 +0000

A device in the IP can have both a local address (which uniquely identifies the device on its local segment or LAN) and a network address (which identifies the network to which the device belongs). The local address is more properly known as a data link address because it is contained in the data link layer (Layer? 2 of the OSI model) part of the packet header and is read by data-link devices (bridges and all device interfaces, for example). The more technically inclined person will refer to local addresses as MAC addresses, because the MAC sublayer within the data link layer processes addresses for the layer.

To communicate with a device on Ethernet, for example, the Cisco IOS software first must determine the 48-bit MAC or local data-link address of that device. The process of determining the local data-link address from an IP address is called address resolution. The process of determining the IP address from a local data-link address is called reverse address resolution.

The software uses three forms of address resolution: Address Resolution Protocol (ARP), proxy ARP, and Probe (similar to ARP). The software also uses the Reverse Address Resolution Protocol (RARP). ARP, proxy ARP, and RARP are defined in RFCs 826, 1027, and 903, respectively. Probe is a protocol developed by the Hewlett-Packard Company (HP) for use on IEEE-802.3 networks.

ARP is used to associate IP addresses with media or MAC addresses. Taking an IP address as input, ARP determines the associated media address. Once a media or MAC address is determined, the IP address or media address association is stored in an ARP cache for rapid retrieval. Then the IP datagram is encapsulated in a link-layer frame and sent over the network. Encapsulation of IP datagrams and ARP requests and replies on IEEE 802 networks other than Ethernet is specified by the Subnetwork Access Protocol (SNAP).

RARP works the same way as ARP, except that the RARP request packet requests an IP address instead of a local data-link address. Use of RARP requires a RARP server on the same network segment as the router interface. RARP often is used by diskless nodes that do not know their IP addresses when they boot. The Cisco IOS software attempts to use RARP if it does not know the IP address of an interface at startup. Also, Cisco routers can act as RARP servers by responding to RARP requests that they are able to answer. See the “Configure Additional File Transfer Functions” chapter in the Cisco IOS Configuration Fundamentals Configuration Guide to learn how to configure a router as a RARP server.

The tasks required to set address resolution are contained in the following sections:

?Defining a Static ARP Cache

?Setting ARP Encapsulations

?Enabling Proxy ARP

?Configuring Local-Area Mobility