Question

DHCP server returning address from wrong scope.

Asked by: dpweiss

I am adding a new switch for a new office.  The existing network works like this:

The DHCP server is: 192.168.1.4.  It has three scopes defined.  192.168.1.x, 192.168.3.x and 192.168.4.x.  3.x and 4.x are identically configured.

192.168.1.0/24: the primary network served by a Procurve 3400
192.168.3.0/24: a remote office service by a Procurve 2626 (this network works perfectly)
192.168.4.0/24: remote office served by a Procurve 2610.  Routing is fine.  Only DHCP doesn't work right.  DHCP addresses returned by PCs on this switch are 192.168.1.x (but should be 192.168.4.x).  This switch is configured identically to the 2626.

What am I missing?

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Asked On
2009-09-11 at 07:10:15ID24724580
Tags

DHCP

,

HP Procurve

,

Networking

Topics

Dynamic Host Configuration Protocol (DHCP)

,

Network Switches & Hubs

Participating Experts
3
Points
500
Comments
13

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Answers

 

by: DGSGuyPosted on 2009-09-11 at 09:00:35ID: 25310834

Did you check that workstations in the 192.168.4.x network are truly getting their DHCP from your 192.168.1.4 server?  Sounds like you might have something else running a default DHCP server (other small switch somewhere?).  Please post the results of an "ipconfig /all" from one of the workstations.

 

by: dpweissPosted on 2009-09-11 at 09:22:09ID: 25311028

Yes.  The reservations are in the DHCP server Address Leases for the incorrect scope and there are entries for the address in the DHCP log on server 192.168.1.4.  I am 100% confident that that a rogue DHCP device is not the cause.  192.168.1.4 is service the request but returning an address from an incorrect scope.

For some reason DHCP recognizes the gateway from the PC 2626 as 192.168.3.1 and issues leases appropriately to that VLAN.  HOwever, the VLAN gateway address fro the new 2610 of 192.168.4.1 is not being recognized???

This is a strange one.  Might have more to do with the Procurve than DHCP, but not sure.

 

by: DGSGuyPosted on 2009-09-11 at 09:37:16ID: 25311193

I'm not sure about that model of Procurve, but you might want to check and make sure routing is disabled - I've had some strange results due to that.

 

by: jonathanalesPosted on 2009-09-11 at 10:02:52ID: 25311426

I don't know much about the Procurve model, but the DHCP Server will give you and address depending on the interface it receive the request, if you are using a DHCP Relay Agent (at the remote offices) consider to check to what interface or subinterface (vlan) of the Procurve 3400 you are pointing, it should be one different to the main network address range 192.168.1.x.

Please post more about the output of the PC IP interface configuration.

You can also check the binding of the DHCP server to see the physical address of the interface that is requesting an IP address.

 

by: thehagmanPosted on 2009-09-13 at 08:31:42ID: 25320569

Do the switches share a VLAN / broadcast domain? This might cause the clients to see the DHCP server directly instead of the local relay agent ...

 

by: thehagmanPosted on 2009-09-13 at 08:35:13ID: 25320579

And "identically" configured of course cannot be right.
I assume that you have mutatis mutandis something like

ip address 192.168.4.1 255.255.255.0
ip helper-address 192.168.1.1

 

by: dpweissPosted on 2009-09-13 at 16:05:55ID: 25322227

Yes on both.  Of course, why the 2626 works and then 2610 does not is inexplicable to me unless the 2626 (older) has a flaw that was fixed in the new 2610.  Should I try renaming the VLAN on the 2610?  Would this force routing to be recognized?

 

by: dpweissPosted on 2009-09-13 at 17:41:04ID: 25322435

I have attached a file with all three router configs.  Suggestions?

 

by: thehagmanPosted on 2009-09-13 at 23:38:44ID: 25323210

It appears that you included the 192.168.3.1 config twice.
Anyway, the 192.168.4.1 config looks okay so far.
You mentioned only one DHCP server before, but there are 3 helper addresses?

 

by: dpweissPosted on 2009-09-14 at 05:22:03ID: 25324666

Yes.  The third config was a copy/paste error.  The correct one follows.  The other DHCP helper addresses are obsolete.  Only 192.168.1.4 is a DHCP server.

; J4905A Configuration Editor; Created on release #M.10.06

hostname "ProCurve Switch 3400cl-24G"
interface 22
   speed-duplex 100-full
exit
ip default-gateway 192.168.1.2
ip routing
snmp-server community "public" Unrestricted
vlan 1
   name "LAN"
   untagged 1-21,23-24
   ip address 192.168.1.3 255.255.255.0
   ip helper-address 192.168.1.4
   ip helper-address 192.168.1.2
   tagged 22
   exit
vlan 2
   name "WAN"
   untagged 22
   ip address 10.0.0.1 255.255.255.0
   exit
dhcp-relay option 82 keep
ip route 192.168.3.0 255.255.255.0 10.0.0.3
ip route 0.0.0.0 0.0.0.0 192.168.1.2
ip route 192.168.4.0 255.255.255.0 10.0.0.4
primary-vlan 2

 

 

by: thehagmanPosted on 2009-09-14 at 08:18:30ID: 25326338

OK, so it appears there are no additional routers between the switches.
I guess that the link between thw switches is via ports 22 on switch 1, port 26 on siwtch 3 and port 26 on switch 4.
(BTW: Why do you have "vlan 2 tagged 25" in switches 3 and 4? Is there a backup link between them via those ports?)

Anyway, I think you should remove the lines "vlan 1 tagged 22" on Switch 1 and "vlan 1 tagged 26" on Switch 3 and 4.
These statements join the  various vlan 1's into just one big vlan 1.
While the "v" for "virtual" may make this setup practical if you don't want to sesort to routing, it contradicts the "l" for "local".
In fact this setup makes the vlan 1's a single broadcast domain and hence
- broadcast noise (esp. by Windows hosts) fills up the WAN link
- remote hosts can obtain a DHCP lease directly from the DHCP server (and hence from its directly connected address range 192.168.1.0/24).

 

by: dpweissPosted on 2009-09-14 at 08:39:01ID: 25326560

Now we're getting somewhere!  You are correct, Port 22 on the 3400, and port 26 on the other two all plug into a Metro Ethernet WAN link (switched but not routed).  One port per office.  This is the "WAN" VLAN.  The LAN VLANs are such that there is one per office.

Let's make sure I understand, since I only just inherited this network.  I'm a windows guy, not a routing guy.  What we want here is three networks that are NOT on the same broadcast domain for sure.  I want them routed (which is what I thought was in place).  Sounds to me like it is not.

The only switch I can mess with during business hours is the 2610, so we will start with that one.  If I understand what I need to do, the config for the Procurve 2610 should look like this (do you agree?):


; J9085A Configuration Editor; Created on release #R.11.25
hostname "ProCurve Switch 2610-24"
interface 26
   speed-duplex 10-full
exit
ip default-gateway 10.0.0.1
ip routing
vlan 1
   name "LAN"
   untagged 1-25,27-28
   ip address 192.168.4.1 255.255.255.0
   ip helper-address 192.168.1.4
   exit
vlan 2
   name "WAN"
   untagged 26
   ip address 10.0.0.4 255.255.255.0
   exit
dhcp-relay option 82 keep
ip route 0.0.0.0 0.0.0.0 10.0.0.1
ip route 192.168.1.0 255.255.255.0 10.0.0.1
ip route 192.168.3.0 255.255.255.0 10.0.0.3
primary-vlan 2  

 

by: dpweissPosted on 2009-09-29 at 13:11:36ID: 31627576

This was this issue in the end.  Because there was a single broadcast domain, broadcast DHCP requests got to the DHCP server first and returned an incorrect IP, while routed DHCP requests got there after and were NACKed.  By separating all 3 nodes into separate broadcast domains, the problem was resolved.  Thank you.

20120131-EE-VQP-002

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