Cisco CCNA Study Guide - Router Alley

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CCNA Study Guide v2.62 – Aaron Balchunas 278 Configuring a Cisco Router as a DHCP Server Cisco routers can be configured to function as DHCP servers. The first step is to create a DHCP pool: Router(config)# ip dhcp pool MYPOOL Router(dhcp-config)# network 192.168.1.0 255.255.255.0 The first command creates a dhcp pool named MYPOOL. The second command creates our DHCP scope, indicating the range of addresses to be leased. The above command indicates any address between 192.168.1.1 – 192.168.1.255 can be leased. Specific addresses can be excluded from being leased: Router(config)# ip dhcp excluded-address 192.168.1.1 Router(config)# ip dhcp excluded-address 192.168.1.5 192.168.1.10 The first command excludes only address 192.168.1.1. The second command excludes address 192.168.1.5 through 192.168.1.10. To specify DHCP options to be leased with the address: Router(config)# ip dhcp pool MYPOOL Router(dhcp-config)# default-router 192.168.1.1 Router(dhcp-config)# dns-server 192.168.1.5 Router(dhcp-config)# domain-name MYDOMAIN To specify the duration of the DHCP lease: Router(config)# ip dhcp pool MYPOOL Router(dhcp-config)# lease 1 12 The above changes the default lease from 8 days to 1 day, 12 hours. To view current DHCP leases: Router# show ip dhcp binding * * * All original material copyright © 2013 by Aaron Balchunas (aaron@routeralley.com), unless otherwise noted. All other material copyright © of their respective owners. This material may be copied and used freely, but may not be altered or sold without the expressed written consent of the owner of the above copyright. Updated material may be found at http://www.routeralley.com.
CCNA Study Guide v2.62 – Aaron Balchunas 279 IP Helper Address Recall that DHCP clients broadcast their DHCPDiscover packets, when searching for a DHCP server. What would happen if the DHCP server is on a different network, separated from the clients by a router? Routers, by default, will never forward a broadcast. Thus, in the above example, the client would never be able to reach the DHCP server to acquire its IP address. That is, unless the ip helper-address command is used: Router(config)# interface fa0 Router(config-if)# ip helper-address 10.1.1.5 Notice that the ip helper-address command is configured on the interface connecting to the DHCP client, pointing to the IP address of the DHCP server. When the client broadcasts its DHCPDiscover packet, the router will direct that broadcast to the DHCP server. And there was much rejoicing. By default, the ip helper-address command will forward the following UDP traffic: • TFTP (port 69) • DNS (port 53) • Time (port 37) • NetBIOS (ports 137-138) • ND (Network Disks – used by Sun workstations) • TACACS (port 49) • BOOTP/DHCP (ports 67-68) Customized UDP traffic can be specified using the following command: Router(config)# ip forward-protocol udp 107 Router(config)# no ip forward-protocol udp 69 (Reference: http://www.cisco.com/en/US/products/sw/iosswrel/ps5207/products_command_reference_chapter09186a0080238b72.html#wp1182972) * * * All original material copyright © 2013 by Aaron Balchunas (aaron@routeralley.com), unless otherwise noted. All other material copyright © of their respective owners. This material may be copied and used freely, but may not be altered or sold without the expressed written consent of the owner of the above copyright. Updated material may be found at http://www.routeralley.com.
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