DHCP relay on Layer 2 switches

DHCP relay on Layer 2 switches

Post by Rob Sco » Thu, 10 Jul 2003 21:46:49



We have some KTI 2240 switches that are Layer 2 Managed types.
Looking at the manuals, they are also strickingly similar to Linksys
PC22224 switches.

You can do nice things with them like set up VLANs and so forth.
It also looks like you can set it up so that the switch will relay
DHCP packets from one VLAN to another so you only need to have one
DHCP server for the whole system.

My problem is that I don't seem to be able to get this to work. The
manuals for these switches don't really help too much.

What I have is two VLANs: the default VLAN, tag = 1; and the test
VLAN, tag = 2.
I want to set up DHCP relay on VLAN 2 to relay to/back to a DHCP
server on VLAN 1.

Sooooo.... what I have done is to set a 'DHCP Gateway' on VLAN 2, with
'Server' specified as a server on VLAN 1, and the 'Outgoing port' is
the IP address of VLAN 1.

What I'm assuming I've done is that this relays DHCP requests on VLAN
2 to the specified server on VLAN 1, but it doesn't appear to work.
I've tried every other setup configuration that I can think of without
success.

It could be that I don't understand what I'm doing, that I've missed
some component that I need, or that this feature of the switch simply
doesn't work.
Extensive looking on Google and on Usenet didn't bring up any results.

Any help appreciated.

 
 
 

DHCP relay on Layer 2 switches

Post by ch.. » Sun, 13 Jul 2003 01:48:39




>We have some KTI 2240 switches that are Layer 2 Managed types.
>Looking at the manuals, they are also strickingly similar to Linksys
>PC22224 switches.

>You can do nice things with them like set up VLANs and so forth.
>It also looks like you can set it up so that the switch will relay
>DHCP packets from one VLAN to another so you only need to have one
>DHCP server for the whole system.

>My problem is that I don't seem to be able to get this to work. The
>manuals for these switches don't really help too much.

>What I have is two VLANs: the default VLAN, tag = 1; and the test
>VLAN, tag = 2.
>I want to set up DHCP relay on VLAN 2 to relay to/back to a DHCP
>server on VLAN 1.

>Sooooo.... what I have done is to set a 'DHCP Gateway' on VLAN 2, with
>'Server' specified as a server on VLAN 1, and the 'Outgoing port' is
>the IP address of VLAN 1.

>What I'm assuming I've done is that this relays DHCP requests on VLAN
>2 to the specified server on VLAN 1, but it doesn't appear to work.
>I've tried every other setup configuration that I can think of without
>success.

>It could be that I don't understand what I'm doing, that I've missed
>some component that I need, or that this feature of the switch simply
>doesn't work.
>Extensive looking on Google and on Usenet didn't bring up any results.

>Any help appreciated.

What happens when you try to get a dhcp lease?  Try running a packet
sniffer and look at what transpires.  www.ethereal.com is a good free
sniffer.

Does the switch have IPs assigned in both vlans?  If not, this
probably isn't going to work real well.

-Chris

 
 
 

DHCP relay on Layer 2 switches

Post by ch.. » Sun, 13 Jul 2003 01:49:43




>We have some KTI 2240 switches that are Layer 2 Managed types.
>Looking at the manuals, they are also strickingly similar to Linksys
>PC22224 switches.

>You can do nice things with them like set up VLANs and so forth.
>It also looks like you can set it up so that the switch will relay
>DHCP packets from one VLAN to another so you only need to have one
>DHCP server for the whole system.

>My problem is that I don't seem to be able to get this to work. The
>manuals for these switches don't really help too much.

>What I have is two VLANs: the default VLAN, tag = 1; and the test
>VLAN, tag = 2.
>I want to set up DHCP relay on VLAN 2 to relay to/back to a DHCP
>server on VLAN 1.

>Sooooo.... what I have done is to set a 'DHCP Gateway' on VLAN 2, with
>'Server' specified as a server on VLAN 1, and the 'Outgoing port' is
>the IP address of VLAN 1.

>What I'm assuming I've done is that this relays DHCP requests on VLAN
>2 to the specified server on VLAN 1, but it doesn't appear to work.
>I've tried every other setup configuration that I can think of without
>success.

>It could be that I don't understand what I'm doing, that I've missed
>some component that I need, or that this feature of the switch simply
>doesn't work.
>Extensive looking on Google and on Usenet didn't bring up any results.

>Any help appreciated.

I also forgot to ask if the DHCP server is properly configured, ie has
a range setup for the subnet on vlan2
 
 
 

DHCP relay on Layer 2 switches

Post by Charles R. Anderso » Mon, 14 Jul 2003 04:01:08



> We have some KTI 2240 switches that are Layer 2 Managed types.
> Looking at the manuals, they are also strickingly similar to Linksys
> PC22224 switches.

> You can do nice things with them like set up VLANs and so forth.
> It also looks like you can set it up so that the switch will relay
> DHCP packets from one VLAN to another so you only need to have one
> DHCP server for the whole system.

A DHCP Relay only relays broadcast packets.  The unicast part of the
DHCP protocol still needs a normal router between VLANs.  You still
need routed IP connectivity between the VLANs.  Since your switch is
only a Layer 2 device, do you have a router connecting VLAN 1 to VLAN
2?

Anyway, a Router/Layer 3 switch is usually where the DHCP Relay Agent
lives, rather than a Layer 2 switch.  If you have a unix box acting as
a router, you can use ISC dhcrelay to act as the relay agent.

 
 
 

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And a Layer 3 switch is really a router because it's routing/switching
based on the layer 3 IP address.  A Layer 3 switch has more ports than a
router and has more/all of the routing in hardware instead of software.

So the question I'm really wanting answered is:

Will a layer 3 Cisco 3750 switch do layer 2 switching out of the box?
I'd like to drop one into our network to replace a 3508G layer2 switch
without having to totally reconfigure the existing network.  If it'll do
layer2 I can configure the layer3 stuff latter.

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