ip forward-protocol-优快云博客

本文链接：https://blog.youkuaiyun.com/blakegao/article/details/20555325

本文档介绍了如何使用`ip forward-protocol`命令来指定转发特定类型的广播包，特别是与UDP和默认端口相关的应用，如DHCP、TFTP等。配置助手地址可以实现UDP广播和转发，但不恰当的设置可能对网络有害。

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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, Routing Information Protocol (RIP)] may be hazardous to your network.

If you use the ip forward-protocol command, specifying only 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. Because 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.

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 packets (ports 67 and 68)
TACACS service (port 49)
IEN-116 Name Service (port 42)

Note

If UDP port 68 is used as the destination port number, it is not forwarded by default.

Examples

The following example defines a helper address and uses the ip forward-protocol command. Using the udp keyword without specifying any port numbers will allow forwarding of UDP packets on the default ports.

Router(config)# ip forward-protocol udp
Router(config)# interface ethernet 1
Router(config-if)# ip helper-address 10.24.42.2

http://www.cisco.com/c/en/us/td/docs/ios-xml/ios/ipapp/command/iap-cr-book/iap-i1.html#wp1776761080