当在接口使用ip helper-address命令，路由器会前传接收到的UDP广播报文到指定服务器地址
Lab_B(config)#interface f0/0
Lab_B(config-if)#ip helper-address 192.168.254.251
下表列出了前传的数据包类型
| Port or Protocol | Meaning | On by Default |
| <0–65535> | Port number (create your own) | |
| biff | Biff (mail notification, comsat, 512) | |
| bootpc | Bootstrap Protocol (BOOTP) client (68) | X |
| bootps | Bootstrap Protocol (BOOTP) server (67) | X |
| discard | Discard (9) | |
| dnsix | DNSIX security protocol auditing (195) | |
| domain | Domain Name Service (DNS) (53) | X |
| echo | Echo (7) | |
| isakmp | Internet Security Association and Key Management Protocol (ISAKMP) (500) | |
| mobile-ip | Mobile IP registration (434) | |
| nameserver | IEN116 name service (obsolete, 42) | |
| netbios-dgm | NetBios datagram service (138) | X |
| netbios-ns | NetBios name service (137) | X |
| netbios-ss | NetBios session service (139) | |
| ntp | Network Time Protocol (NTP) (123) | |
| pim-auto-rp | PIM Auto-RP (496) | |
| rip | Routing Information Protocol (RIP) (router, in.routed, 520) |
这些默认的前传类型，可以通过下面的命令关闭，只开启bootps UDP 67，来提高路由器的安全性
Lab_B(config)#no ip forward-protocol udp 69
Lab_B(config)#no ip forward-protocol udp 53
Lab_B(config)#no ip forward-protocol udp 37
Lab_B(config)#no ip forward-protocol udp 137
Lab_B(config)#no ip forward-protocol udp 138
Lab_B(config)#no ip forward-protocol udp 68
Lab_B(config)#no ip forward-protocol udp 49
