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CCNP SWITCH Portable Command Guide: Implementing Inter-VLAN Routing

  • Sample Chapter is provided courtesy of Cisco Press.
  • Date: Apr 6, 2010.

Configuring a DHCP Helper Address

NOTE

For the SVI line state to be up, at least one port in the VLAN must be up and forwarding. The switchport auto-state exclude command excludes a port from the SVI interface line-state up-or-down calculation.

Router

Router(config)#interface fastethernet 0/0

Moves to interface configuration mode.

Router(config-if)#ip helper-address 172.16.20.2

DHCP broadcasts will be forwarded as a unicast to this specific address rather than be dropped by the router.

Layer 3 Switch

Switch(config)#interface vlan 10

Moves to SVI configuration mode.

Switch(config-if)#ip helper-address 172.16.20.2

DHCP broadcasts will be forwarded as a unicast to this specific address rather than be dropped by the router.

NOTE

The ip helper-address command forwards broadcast packets as a unicast to eight different UDP ports by default:

  • TFTP (port 69)
  • DNS (port 53)
  • Time service (port 37)
  • NetBIOS name server (port 137)
  • NetBIOS datagram server (port 138)
  • Boot Protocol (BOOTP) client and server datagrams (ports 67 and 68)
  • TACACS service (port 49)
  • Host Name Service (port 42)

To close some of these ports, use the no ip forward-protocol udp x command at the global configuration prompt, where x is the port number you want to close. The following command stops the forwarding of broadcasts to port 49:

Router(config)#no ip forward-protocol udp 49

To open other UDP ports, use the ip forward-helper udp x command, where x is the port number you want to open:

Router(config)#ip forward-protocol udp 517
8. DHCP Client on a Cisco IOS Software Ethernet Interface | Next Section Previous Section

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