From: Joe Smith (j333smith@hotmail.com)
Date: Thu Dec 30 2004 - 12:34:45 GMT-3
Tim,
I think you have answered your question. R4 is not sending out a DHCP
request, so even if ip helper forwards the broadcast, it's not a dhcp
request, so the DHCP server would not respond.
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: Thu, 30 Dec 2004 07:30:40 -0500
>
>Joe,
>
>If you lab this up and run debug dhcp packets on R4, it's interesting to
>see
>
>that R4 never sends out any dhcp packets (even though it does get it
>
>address originally from the dhcp server.).
>
>In fact, with the current release of IOS, the dhcp client function is NOT
>
>supported on isdn interfaces.
>
>What happens is this:
>
>When the isdn circuit comes up because ppp is configured on the isdn
>
>link, ppp, not dhcp, is responsible for negotiating an ip address for R4's
>
>bri interface.
>
>However, R5 acts as a dhcp client proxy for R4 and issues a dhcp discover
>
>packet to R1 to get an ip address for R4.
>
>I haven't been able to piece together the exact sequence of events from the
>debugs
>
>but, I'm pretty confident that at a high level that's what's happening - at
>least when
>
>R4 is configured with ip address nego and R5 is configured with dhcp-server
>x.x.x.x
>
>Pretty interesting, right?
>
>Tim
>
>
>
>
>----- Original Message -----
>From: "Joe Smith" <j333smith@hotmail.com>
>To: <ccielab@groupstudy.com>
>Sent: Wednesday, December 29, 2004 10:56 PM
>Subject: Re: I E lab 13 - dhcp-server vs ip helper
>
>
> > 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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