6-1 IP Addressing and Resolution | Cisco Field Manual[c] Router Configuration

The Address Resolution Protocol (ARP) is used to resolve a host's 48-bit MAC address when the IP address is known. ARP is defined in RFC 826. ARP entries are added to a router cache as requests and replies are received. The entries in the ARP cache are aged out after a configurable time.

To resolve a host's IP address from its MAC address, the Reverse Address Resolution Protocol (RARP) is used. RARP is defined in RFC 903. A router acts as a RARP server for addresses present in its ARP cache.
Proxy ARP is used to provide address resolution for hosts that reside on networks other than the ARP requester. If a router has a route to the unresolved host on another interface, it generates an ARP reply on behalf of that host. The ARP requester then receives the router's MAC address in the ARP reply.
Domain Name System (DNS) is used to resolve IP addresses and fully qualified domain names .
Router/gateway discovery methods are used to assist a router or hosts that need to locate a router. This can occur when IP routing is disabled on a router and routes to other networks need to be discovered .

Configuration

Assign IP addresses to interfaces.
1. (Optional) Assign an IP address:
```
 (interface)  ip address   ip-address mask  [  secondary  ] 
```
  One primary IP address and mask are assigned to the interface. If the optional secondary flag is added, an additional secondary IP address is assigned to the interface. Secondary addresses are useful to add support for additional logical subnets on a single interface.
  
  -OR-
2. (Optional) Allow IP over a serial or tunnel interface without an IP address:
```
 (interface)  ip unnumbered   type number  
```
  The IP address of the interface specified by type and number is used for packets generated on the interface. The IP address is also used to determine whether routing updates from routing protocols should be sent over the unnumbered interface.
  
  NOTE
  
  Before an unnumbered interface can be configured, the interface that will be used to host the IP address must be configured, enabled, and active. Only the following encapsulations can be used with IP unnumbered: HDLC, PPP, LAPB, Frame Relay, SLIP, and tunnels.
Configure ARP features.
1. (Optional) Define a static ARP entry:
```
 (global)  arp   ip-address MAC-address  {  arpa   snap  } [  alias  ] 
```
  Normally, ARP resolution is handled dynamically. In cases where a static entry is necessary, this command allows the router to resolve the ip-address and MAC-address values. The ARP type should be set to arpa for Ethernet interfaces and snap for FDDI and Token Ring interfaces. The alias keyword can be used to cause the router to answer a matching ARP request as if it were the owner of the IP address. Normally, this is done through proxy ARP (enabled by default) if the router has a route to the ARP request's destination address. However, the alias keyword can be used to supply a proxy ARP reply even if the destination is on an unreachable or nonexistent network.
2. (Optional) Configure the ARP cache timeout interval:
```
 (interface)  arp timeout   seconds  
```
  Each ARP entry remains in the ARP cache for the specified number of seconds (the default is 14400 seconds, or 4 hours). A value of 0 keeps entries indefinitely. The timeout value is configured on a per-interface basis.
3. (Optional) Clear ARP entries manually:
```
 (exec)  clear arp-cache  
```
  All dynamic ARP cache entries are cleared when this command is executed.
Configure DNS features for the router.
1. (Optional) Configure static IP host mappings (no DNS server):
```
 (global)  ip host   name  [  tcp-port-number  ]  address1  [  address2...   address8  ] 
```
  A static mapping is defined in the router between name and from one to eight IP addresses. A TCP port number can also be defined to be used for Telnet sessions from the router to the host (the default is port 23).
2. (Optional) Configure the default domain name:
```
 (global)  ip domain-name   domain  
```
  Host names that do not include a domain name have the domain name appended before being used by the router.
3. (Optional) Define a name server for the router:
```
 (global)  ip name-server   address1  [  address2...   address6  ] 
```
  A list of one to six name server IP addresses can be configured. These name servers are queried by the router when DNS lookups are required for executing a Cisco IOS Software command (that is, opening a Telnet session to a host name) or for command output (that is, traceroute).
4. (Optional) Use DNS resolution for the router:
```
 (global) [  no  ]  ip domain-lookup  
```
  By default, DNS lookup is enabled so that the router requests name resolution from the name servers specified with the ip name-server command. If DNS resolution is not needed, you can disable it with the no keyword.
(Optional) Configure a default gateway:
```
 (global)  ip default-gateway   ip-address  
```
When IP routing is disabled on a router, all packets destined for nonlocal networks can be forwarded to a default gateway.
(Optional) Configure ICMP Router Discovery Protocol (IRDP).
1. Enable IRDP on an interface:
```
 (interface)  ip irdp  
```
  The router processes IRDP on the interface, either as a host (IRDP packets are received) or as a client (IRDP packets are sent). IRDP advertisements are sent as broadcasts.
2. (Optional) Define the IRDP hold time:
```
 (interface)  ip irdp holdtime   seconds  
```
  IRDP advertisements are considered valid for a period of seconds.
3. (Optional) Set the IRDP preference:
```
 (interface)  ip irdp preference   number  
```
  Advertisements from the router can be preferred over other routers, with a higher preference number. The preference value can be from 2 ³¹ to 2 ³¹ (the default is 0).
4. (Optional) Enable IRDP multicast mode:
```
 (interface)  ip irdp multicast  
```
  IRDP advertisements are sent using multicast address 224.0.0.1 rather than as broadcasts. This mode makes IRDP compatible with Sun Solaris hosts.

Example

Both primary and secondary IP addresses are configured on the Ethernet 0 interface. Serial 1 is configured as an unnumbered interface, using Ethernet 0. A static ARP entry is configured for a single host. The router is also configured to look up host names using two DNS servers:

  interface ethernet 0   ip address 192.168.16.1 255.255.255.0   ip address 192.168.17.1 255.255.255.0 secondary   interface serial 1   ip unnumbered Ethernet 0   arp 192.168.16.217 0050.0492.6e77 arpa   ip domain example.com   ip name-server 192.168.3.5 192.168.3.6   ip domain-lookup