Cisco® CCNP Route Exam Notes : ARP(address Resolution Protocol)

3. Layer3 Technologies

3.1 Identify IPv4 addressing and subnetting

3.1.b ARP

The ARP stands for the address resolution protocol is the telecommunication protocol. This ARP is mostly used to convert the IP address to the physical address like Ethernet address. When a packet is sent to another device within the same network out an interface, ARP is used if the layer 2 address of the destination is not known. The transmitting device will send an ARP request in order to figure out the address mapping. The ARP request contains the destination IP address and a destination MAC of FF:FF:FF:FF:FF. The destination MAC is flooded out all switchports (if connected to a switch). Normally ARP takes place within the same device when the traffic is within the same subnet. If traffic is destined to a different subnet, the ARP request will be sent by the router instead.

3.1.c DHCP relay and server

A DHCP relay agent is any host that forwards DHCP packets between clients and servers. Relay agents are used to forward requests and replies between clients and servers when they are not on the same physical subnet. Relay agent forwarding is distinct from the normal forwarding of an IP router, where IP datagrams are switched between networks somewhat transparently. By contrast, relay agents receive DHCP messages and then generate a new DHCP message to send out on another interface.

IOS contains a method for handing out IPv4 addresses while simultaneously maintaining a database which has a record for all the addresses that have been handed out. Automatic DHCP address allocation is typically based on an IP address, whether it be the gateway address or the incoming interface IP address. In some networks, it is necessary to use additional information to further determine which IP addresses to allocate. By using option 82, the Cisco IOS relay agent has long been able to include additional information about itself when forwarding client-originated DHCP packets to a DHCP server. The Cisco IOS DHCP server can also use option 82 as a means to provide additional information to properly allocate IP addresses to DHCP clients.

DHCP configuration commands

DHCP configuration commands

ip dhcp pool <poolname>

Configures a DHCP address pool on a Cisco IOS DHCP Server and enters DHCP pool configuration mode


Provides the IP address range for allocation to the dhcp client computers.


Assigns the domain name for the dhcp client. The domain name is used as a suffix for any domain requests sent out.


Specifies the IP address of a DNS server. One IP address is required, although you can specify up to eight addresses in one command line


Specifies the IP address of a router. One IP address is required, although you can specify up to eight addresses in one command line.

Lease 2

To configure the duration of the lease for an IP address that is assigned from a Cisco IOS DHCP Server to a DHCP client, use the lease DHCP pool configuration command. To restore the default value, use the no form of this command. The default lease period is 1 day.

IP helper addresses forward a client broadcast address (such as a DHCP or DNS request) to a unicast or directed broadcast address. Helper-address is required due to the fact that routers do not forward broadcasts. By defining a helper-address, a router will be able to forward a broadcast from a client to the desired server or network. There can be more than one helper-address on a network. The helper-address must be defined on the interface that receives the original client broadcast. The command ip helper-address, defined on interface e 0 of routerA will route the broadcasts originating at that interface to network resource at

ip helper-address command enable to route broadcasts from client computers. However, you may not want all that broadcast be routed through the network. To prevent certain broadcasts from being carried onto the network, use the command "no ip forward-protocol "

When you enable the IP Helper address, all traffic for the UDP ports are automatically forwarded to the address specified. To restrict the forwarded traffic, you can specify the restrictions by adding any of the no commands to your configuration at the Global Configuration mode.

UDP port 69 is used for TFTP (Trival File Transfer Protocol).

Ex: no ip forward-protocol udp 69

3.1.d DHCP protocol operations

A DHCP device which joins a LAN goes through a couple different steps before it can communicate with other devices on the network:

  • The device sends out DHCPDISCOVER frames to broadcast address
  • The DHCP server responds with a DHCPOFFER. The pool the server chooses depends on the GADDR which is received in original broadcast
  • The client then responds with a DHCPREQUEST broadcast letting the broadcast segment know what address it is being assigned
  • The DHCP server finally responds with a DHCPACK unicast packet, and the process is completed.

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