From: Brian McGahan (bmcgahan@internetworkexpert.com)
Date: Mon Dec 15 2003 - 21:25:06 GMT-3
Jonathan,
You misread my post; the point is that the 'ip helper-address'
command instructs the router to forward udp broadcast packets received on an
interface to the configured address if the port matches that which is
specified in the 'ip forward-protocol udp' command.
Since 'ip forward-protocol udp 67' is default, DHCP requests will be
forwarded when the 'ip helper-address' command is configured on the
interface. This is not to say that 'ip forward-protocol' command does not
relate to the 'ip helper-address' command. There is no "ip helper-address
list of ports" as you implied. Instead, the default ports forwarded by the
'ip helper-address' command are the default list of 'ip forward protocol
udp' ports as Chuck listed.
HTH,
Brian McGahan, CCIE #8593
bmcgahan@internetworkexpert.com
Internetwork Expert, Inc.
http://www.InternetworkExpert.com
Toll Free: 877-224-8987
Direct: 708-362-1418 (Outside the US and Canada)
> -----Original Message-----
> From: nobody@groupstudy.com [mailto:nobody@groupstudy.com] On Behalf Of
> Jonathan Hays
> Sent: Monday, December 15, 2003 4:23 PM
> To: ccielab@groupstudy.com
> Subject: RE: ip helper-address
>
> you wrote:
> -----Original Message-----
> From: nobody@groupstudy.com [mailto:nobody@groupstudy.com] On Behalf Of
> Brian McGahan
> Sent: Monday, December 15, 2003 3:52 PM
> To: 'Pun, Alec CL'; ccielab@groupstudy.com
> Subject: RE: ip helper-address
>
>
> Alec,
>
> Technically the "ip forward-protocol udp" command does need to
> be
> configured to forward dhcp requests with the "ip helper-address"
> command.
> It's just that "ip forward-protocol udp bootps" is default:
>
> Router(config)#ip forward-protocol udp 67
> Router(config)#do sh run | in forward-protocol
>
> Router(config)#no ip forward-protocol udp 67
> Router(config)#do sh run | in forward-protocol
> no ip forward-protocol udp bootps
>
> If you disable the port, as Chuck suggested, the packet cannot
> be
> forwarded.
>
>
> HTH,
>
> Brian McGahan, CCIE #8593
> bmcgahan@internetworkexpert.com
>
> Internetwork Expert, Inc.
> http://www.InternetworkExpert.com
> Toll Free: 877-224-8987
> Direct: 708-362-1418 (Outside the US and Canada)
>
> = = =
>
> Brian,
>
> Sorry, but I must respectfully question the need to configure the "ip
> forward-protocol udp 67" in addition to "ip helper-address" just to get
> DHCP proxy functions to work. I have set up this "DHCP proxy" feature
> for many clients using only the "ip helper-address" command on the
> ethernet interface, and I don't recall having needed the "ip
> forward-protocol udp 67" to get things working. In point of fact the "ip
> helper-address" command enables forwarding on udp 67 so what would be
> the point of configuring this command, unless you had previously entered
> "no ip forward-protocol udp 67" for some reason?
>
> Or are you trying to say that the "ip helper-address" command is
> actually a sort of macro that executes "ip forward-protocol udp 67" but
> it doesn't show up in the "sh run" output since it's the default?
>
> Please clarify,
>
> Jonathan
>
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