Re: DHCP + TFTP helper

aruzsinszky · ‎12-01-2010

Hi,

My problem is the next:

I wanted to change our DHCP relay device (Linksys WRT54GL with OpenWRT) to Cisco 1751 router. There are in the same LAN.

Far from that LAN there is a remote LAN where the DHCP+TFTP sever resides.DHCP relay worked with Linksys so routing is working between the clients and server.

When I configured c1751 the DHCP packets not reach the DHCP server!

The relay router not on the path to the remote LAN (serial interfaces are connected to other WAN directions), but the target LAN reachable.

Of course I disabled DHCP relay function on Linksys. ;-)

c1751's IOS: 12.4

The config: (relevant command only)

ip dhcp relay information option
ip dhcp relay information trust-all

...

interface FastEthernet0/0
ip address 192.168.1.240 255.255.255.0
ip helper-address 192.168.52.30
ip directed-broadcast

...

no ip forward-protocol nd
no ip forward-protocol udp domain
no ip forward-protocol udp time
no ip forward-protocol udp netbios-ns
no ip forward-protocol udp netbios-dgm
no ip forward-protocol udp tacacs
ip forward-protocol udp bootpc

...

access-list 150 permit udp host 192.168.1.240 host 192.168.52.30

...

K_dgr#sh deb
Generic IP:
IP packet debugging is on for access list 150
DHCP server packet debugging is on.
DHCPC:
DHCP client activity debugging is on

Dec 1 11:52:27: DHCPD: setting giaddr to 192.168.1.240.
Dec 1 11:52:27: DHCPD: adding relay information option.
Dec 1 11:52:27: IP: s=192.168.1.240 (local), d=192.168.52.30 (FastEthernet0/0), len 362, sending
Dec 1 11:52:27: IP: s=192.168.1.240 (local), d=192.168.52.30 (FastEthernet0/0), len 362, encapsulation failed
Dec 1 11:52:27: DHCPD: BOOTREQUEST from 0100.0ea6.1e62.32 forwarded to 192.168.52.30.
Dec 1 11:52:31: DHCPD: setting giaddr to 192.168.1.240.
Dec 1 11:52:31: DHCPD: adding relay information option.
Dec 1 11:52:31: IP: s=192.168.1.240 (local), d=192.168.52.30 (FastEthernet0/0), len 362, sending
Dec 1 11:52:31: IP: s=192.168.1.240 (local), d=192.168.52.30 (FastEthernet0/0), len 362, encapsulation failed
Dec 1 11:52:31: DHCPD: BOOTREQUEST from 0100.0ea6.1e62.32 forwarded to 192.168.52.30.

I've got no ideas what could be the problem!

What is 'encap. failed'? And the sending? If it was sent why can't my DHCP see tha packets?

Between the relay agent (c1751) and DHCP+TFTP server thera are some non Cisco routers (3Com+Proteon) without any DHCP or TFTP config.

TFTP doesn't work, either.

It is reach the server, server send the answer back and thats all. Timeout ...

Please, help me! I dislike these kind of non routeable protocols ...

TIA

cadet alain · ‎12-04-2010

Hi,

encapsulation failed means it can't send the frame because it hasn't got the mac address of destination.

if you got another router in between then maybe you put a route to destination pointing on interface instead of next hop on your 1721 ans so as the other router is not doing proxy arp your arp requests from 1751 are never replied.

Can you undebug all then debug arp and try again and post result along with show ip route static output.

Regards.

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aruzsinszky · ‎12-04-2010

Hi,

Is that enough if I can ping the DHCP server from the router?

There is route in its routing rable:

O 192.168.52.0/24 [110/59] via 192.168.1.254, 01:54:35, FastEthernet0/0

192.168.1.254 is a 3Com NetBuilderII.

TIA,

cadet alain · ‎12-04-2010

Hi,

your route is an ospf route which is pointing to your 3com but if you were having encapsulation failed no communication should work because there is a Layer 2 problem.

Do your ping along with debug arp

Let us know.

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aruzsinszky · ‎12-04-2010

Hi,

I read somewhere in a Cisco doc DHCP relay is working between different interfaces.

In my situation the DHCP DHCPDISCOVER packet arrives in fa0/0 and 3Com's !1 (ethernet)

interface is connected to fa0/0, too.

Is that the problem?

Now I did a workaround. I use a Linux PC as a DHCP relay from the same network as the

1751's fa0/0 is.

TIA,

cadet alain · ‎12-04-2010

Hi,

read my previous post. change your ip-helper address position: put it on the interface receiving dhcp requests from clients.

Regards.

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aruzsinszky · ‎12-04-2010

Hi,

Sorry, I don't understand what is the problem with my previous post.

If you check again my 1st post you can see the ip helper-address is on fa0/0.

DHCP client + 1750's fa0/0 + 3Com eth + Linux DHCP relay eth0 are in the same LAN segment.

TIA,

cadet alain · ‎12-04-2010

Hi,

You must use a dhcp relay agent if your dhcp server is on a different subnet that the dhcp clients.

This way dhcp requests broadcasts will be changed to unicast by the relay agent that will tell the dhcp server which pool to use by telling him on which subnet he received the original request.

So if you want dhcp client on one side of router to get ip addresses from dhcp server on another side( subnet) then you must put the ip-helper address

on the side where the client is and give the address of the server.

Here you were putting this command on the server side so it can't work.

Regards.

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aruzsinszky · ‎12-04-2010

Hi,

Can I use my 1751 as a DHCP relay, or not in my situation?

I've got a 1721 with ip helper-address but there the client is on fa0 and server is the same as now and reachable over serial interfaces.

TIA,

cadet alain · ‎12-04-2010

Hi,

if the server is on another interface yes you can but your sh ip route output was telling that to get to your server you had to go out f0/0 to your 3com.

So for your router the server is on same interface as ip-helper address and that's not good.

Can you post a topology with role of each device.

Regards.

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aruzsinszky · ‎12-04-2010

if the server is on another interface yes you can but your sh ip route output was telling that to get to your server you had to go out f0/0 to your 3com.

Yes, it is true.

DHCP client-----------|

Linux DHCP relay---|

1751 fa0/0--------------|

DHCP server--Router----serial 3Com eth---------------|

Is that enough for topology?

TIA,

cadet alain · ‎12-04-2010

Hi,

your dhcp server is on a serial link on 3com and 3com is linked to 1751 with ethernet.ok?

your client is connected to linux relay which is connected to 1751 ok? but on different interface than 3com?

in this case why put a relay on linux box? and if you want ip-helper on 1751 then no need for this linux relay and the command must be on the interface where this linux box is now.

But if you could post a diagram with device names, interfaces and ip address it woul be better to understand your topology.

Regards.

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aruzsinszky · ‎12-04-2010

your dhcp server is on a serial link on 3com and 3com is linked to 1751 with ethernet.ok?

Yes.

your client is connected to linux relay which is connected to 1751 ok?

Yes.

but on different interface than 3com?

No.

DHCP client: 192.168.1.47

Linux eth0: 192.168.1.99

3Com eth: 192.168.1.254

1751 fa0/0: 192.168.1.240

DHCP server: 192.168.52.30.

in this case why put a relay on linux box?

Because with 1751 ip helper doesn't work.

and if you want ip-helper on 1751 then no need for this linux relay and the command must be on the interface where this linux box is now.

I know not neded both Linux DHCP relay and ip helper on 1751. 1751 with IP helyper it doesn't work.

But if you could post a diagram with device names, interfaces and ip address it woul be better to understand your topology.

OK. I started to drawing ...

TIA,

aruzsinszky · ‎12-04-2010

Here is my drawing ...

TIA,

aruzsinszky · ‎12-04-2010

Hi,

Yes, 192.168.1.254 is a 3Com router toward my DHCP server's route.

TIA,