From: Joe Smith (j333smith@hotmail.com)
Date: Thu Dec 30 2004 - 00:56:12 GMT-3
Tim,
The DHCP request is coming from R4 and therefore is two hops away. If the
TTL is one when it reaches the R5 isdn interface, and R5 does its job and
subtracts one, the TTL reaches zero and is never forwarded to R1. This is
supported by the DOC CD, which states "The IP time-to-live (TTL) value must
be at least 2" for ip helper-address.
We know that a Windows box requesting a DHCP address has a TTL greater then
one, does a Cisco router? I would guess it would, but I can't test since I
don't have access to my equipment until next week.
Also of interest in the DOC CD The MAC address of the received frame must
be all-ones broadcast address (ffff.ffff.ffff). Obviously an ISDN frame
does not have a MAC address, but it states all conditions must be met.
J3
>From: "ccie2be" <ccie2be@nyc.rr.com>
>To: "Joe Smith" <j333smith@hotmail.com>, <ccielab@groupstudy.com>
>Subject: Re: I E lab 13 - dhcp-server vs ip helper
>Date: Wed, 29 Dec 2004 19:41:23 -0500
>
>Joe,
>
>That's a good point to keep in mind.
>
>In the scenario I was working with, that couldn't be an issue - the dhcp
>server was one hop
>
>from the dhcp relay.
>
>R4 --- isdn --- R5 ---- R1 (dhcp server)
>
>thanks for pointing that out.
>----- Original Message -----
>From: "Joe Smith" <j333smith@hotmail.com>
>To: <ccielab@groupstudy.com>
>Sent: Wednesday, December 29, 2004 6:49 PM
>Subject: Re: I E lab 13 - dhcp-server vs ip helper
>
>
> > Tim,
> >
> > Do you know the TTL of the DHCP request? If its one, then ip helper
> > wouldn't work per the Doc CD, however the Doc CD has no TTL requirement
>for
> > ip dhcp-server.
> >
> > Just a thought.
> >
> > J3
> >
> >
> > >From: "ccie2be" <ccie2be@nyc.rr.com>
> > >Reply-To: "ccie2be" <ccie2be@nyc.rr.com>
> > >To: "Joe Smith" <j333smith@hotmail.com>, <ccielab@groupstudy.com>
> > >Subject: Re: I E lab 13 - dhcp-server vs ip helper
> > >Date: Wed, 29 Dec 2004 16:17:22 -0500
> > >
> > >Joe,
> > >
> > >On the chance that you were right, I manually added the global, ip
> > >forward-protocol, command
> > >
> > >to see if that would make a difference.
> > >
> > >Unfortunately, it didn't.
> > >
> > >I don't understand this.
> > >
> > >Using the dhcp-server command works. Using the ip helper-address
>doesn't.
> > >
> > >I even rebooted the router after making the change just to make sure.
> > >
> > >Could it possibly be that I need to change something else in addition
>to
> > >the
> > >above?
> > >
> > >(To test the above, I just removed the dhcp-server and added the ip
>helper
> > >command.)
> > >
> > >Tim
> > >----- Original Message -----
> > >From: "Joe Smith" <j333smith@hotmail.com>
> > >To: <ccielab@groupstudy.com>
> > >Sent: Wednesday, December 29, 2004 1:38 PM
> > >Subject: Re: I E lab 13 - dhcp-server vs ip helper
> > >
> > >
> > > > Then I'm mistaken! Well sort of since you still need it.
> > > >
> > > > J3
> > > >
> > > > >From: "ccie2be" <ccie2be@nyc.rr.com>
> > > > >To: "Joe Smith" <j333smith@hotmail.com>, <ccielab@groupstudy.com>
> > > > >Subject: Re: I E lab 13 - dhcp-server vs ip helper
> > > > >Date: Wed, 29 Dec 2004 13:31:22 -0500
> > > > >
> > > > >Actually, according to the Doc-Cd, ip forword-protocol, is enabled
>by
> > > > >default.
> > > > >
> > > > >When the ip helper command is used only to forward dhcp/bootp
>messages,
> > >you
> > > > >may want to disabled the
> > > > >
> > > > >forwarding of other protocols which are also forwarded by default
>by
> > >using
> > > > >no ip forword prot xxx.
> > > > >
> > > > >
> > > > >----- Original Message -----
> > > > >From: "Joe Smith" <j333smith@hotmail.com>
> > > > >To: <ccielab@groupstudy.com>
> > > > >Sent: Wednesday, December 29, 2004 1:17 PM
> > > > >Subject: RE: I E lab 13 - dhcp-server vs ip helper
> > > > >
> > > > >
> > > > > > If I'm not mistaken don't you also need the "ip
>forward-protocol"
> > > > >command
> > > > >in
> > > > > > combination with the ip helper?
> > > > > >
> > > > > > J3
> > > > > >
> > > > > > >From: "ccie2be" <ccie2be@nyc.rr.com>
> > > > > > >Reply-To: "ccie2be" <ccie2be@nyc.rr.com>
> > > > > > >CC: "Group Study" <ccielab@groupstudy.com>
> > > > > > >Subject: I E lab 13 - dhcp-server vs ip helper
> > > > > > >Date: Wed, 29 Dec 2004 12:59:39 -0500
> > > > > > >
> > > > > > >Hi guys,
> > > > > > >
> > > > > > >R4's bri interface is configured to have it's ip address
> > >negotiated.
> > > > > > >
> > > > > > >R5 is configured as a dhcp relay & is suppose to get an ip
>address
> > > > > > >
> > > > > > >for R4 from a dhcp server configured on R1.
> > > > > > >
> > > > > > >R4 --isdn --- R5 ----- R1 (dhcp server).
> > > > > > >
> > > > > > >This works as it's suppose to when R5 has the global command,
> > > > > > >
> > > > > > >ip dhcp-server 139.1.15.1, configured.
> > > > > > >
> > > > > > >But, if the interface command, ip helper 139.1.15.1, is
>configured
> > >on
> > > > > > >
> > > > > > >R5, then R4 doesn't get an ip address.
> > > > > > >
> > > > > > >Can anyone explain why that is?
> > > > > > >
> > > > > > >I thought, the ip dhcp-server command does the same thing as
>the
>ip
> > > > >helper
> > > > > > >command except
> > > > > > >
> > > > > > >that it's more specific.
> > > > > > >
> > > > > > >TIA, Tim
> > > > > > >
> > > > > >
> > >
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