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ip forward-protocol question

When I use the ip helper-address it enables by default the forwarding few protocols.

I would like to disable the onw I don;t want + enable other not included in the original config.

How can I see these?

I can find any show command telling me what protocols are already forwarded...

I know I can find them on the Cisco doc cd, but I'm wondering if there is any other way to verify what I'm doing.

Thanks to read!


Re: ip forward-protocol question


ip helper address

this comand sets the helper address to address. The helper address is the ip address of the host that can answer the UDP requests from other hosts. There router sees the requests broadcast on LAN interface and forwards them to the helper address(its actully generate the unicast packet as router not allow to pass the broadcast). A helper is perticularly useful for the DHCP request... the helper address command forwards packets for the BOOTP, DHCP,DNS,TFTP,TACACS, TIME and NetBIOS name...

in short it will convert the broadcast request for server in to unicast...

ip forward protocol

this command is allow you to forward any additional UDP ports that you want to forwarded automatically to the thelper IP address...

you can use the show run command... in which you will find the services which are configure by your self using the above both the commands...

you can disable it just by adding "no" in front of the command no ip helper-address a.b.c.d

hope this help you

rate this post if it helps



Hall of Fame Super Gold

Re: ip forward-protocol question

Devang has correctly identified the ip helper-address command as the command that takes certain UDP broadcasts and propagates a unicast (or possibly a directed broadcast) packet and the ip forward-protocol command as the command that can control what additional UDP broadcasts will be forwarded. It may not be obvious in his response that the ip forward-protocol command can also be used to disable forwarding of the standard protocols. For example you might want to forward DHCP and DNS but not the TFTP packets. You could achieve this by using the command:

no ip forward-protocol udp tftp




Re: ip forward-protocol question

I think, you can try " show ip nbar port-map " command, this will give you list of tcp and udp ports

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-minu mathur

when you use ip helpe address method, then following UDP ports are by defaul open

1) TFTP-69

2) DNS-53

3) Time-37

4) Netbios name services-137

5)Netbious datagram services-138

6)BOOTP server-67

7)Bootp client-68

8) Tacacs-49

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Community Member

Re: ip forward-protocol question

Any show command telling me what protocol are forwarded?

I know I can take the initial list (enabled by the ip helper-address, take off the one I've used the no forward-protocol for and figure out the answer...

But is there any "decent" way to double check what I'm doing?

Thanks! :-)

Hall of Fame Super Gold

Re: ip forward-protocol question


I do not know of any command which will show what ports are being forwarded.



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