ip forward-protocol

ip forward-protocol

To specify which protocols and ports the router forwards when forwarding broadcast packets, use the ip forward-protocol global configuration command. To remove a protocol or port, use the no form of this command.

ip forward-protocol {udp [ port] | nd | sdns}
no ip forward-protocol {udp [ port] | nd | sdns}

Syntax Description

udp	Forward User Datagram Protocol (UDP) datagrams. See the "Default" section below for a list of port numbers forwarded by default.
port	(Optional) Destination port that controls which UDP services are forwarded.
nd	Forward Network Disk (ND) datagrams. This protocol is used by older diskless Sun workstations.
sdns	Secure Data Network Service.

Default

If an IP helper address is defined, UDP forwarding is enabled on default ports. If UDP flooding is configured, UDP flooding is enabled on the default ports.

If a helper address is specified and UDP forwarding is enabled, broadcast packets destined to the following port numbers are forwarded by default:

• Trivial File Transfer Protocol (TFTP) (port 69)

• Domain Naming System (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)

Note    Using the ip directed-broadcast interface configuration command with the optional access-list-number argument overrides the behavior of the ip forward-protocol command.

Command Mode

Global configuration

Usage Guidelines

This command first appeared in Cisco IOS Release 10.0.

Enabling a helper address or UDP flooding on an interface causes the Cisco IOS software to forward particular broadcast packets. You can use the ip forward-protocol command to specify exactly which types of broadcast packets you would like to have forwarded. A number of commonly forwarded applications are enabled by default. Enabling forwarding for some ports (for example, RIP) may be hazardous to your network.

If you use the ip forward-protocol command, specifying just UDP, without the port, enables forwarding and flooding on the default ports.

One common application that requires helper addresses is Dynamic Host Configuration Protocol (DHCP). DHCP is defined in RFC 1531. DHCP protocol information is carried inside of BOOTP packets. To enable BOOTP broadcast forwarding for a set of clients, configure a helper address on the router interface closest to the client. The helper address should specify the address of the DHCP server. If you have multiple servers, you can configure one helper address for each server. Since BOOTP packets are forwarded by default, DHCP information can now be forwarded by the software. The DHCP server now receives broadcasts from the DHCP clients.

Example

The following example uses the ip forward-protocol command to specify forwarding of UDP port 3001 in addition to the default ports, and then defines a helper address:

ip forward-protocol udp 3001

!

interface ethernet 1

ip helper-address 131.120.1.0

Related Commands

ip directed-broadcast
ip forward-protocol spanning-tree
ip forward-protocol turbo-flood
ip helper-address