DHCP on Cisco 3750

Hi all,

I am in need to configure two DHCP pools on my Cisco 3750 switch (I have

2 other VLANs for which I do not want to enable the DHCP service). This seems to be quite straightforward, but I need clarification on something. If I define pools like this:

ip dhcp pool VLAN40 network 192.168.15.0 255.255.255.0 dns-server 10.99.212.1 default-router 192.168.15.1 ip dhcp excluded-address 192.168.15.1

ip dhcp pool VLAN60 network 192.168.1.0 255.255.255.0 dns-server 10.99.212.1 default-router 192.168.1.1 ip dhcp excluded-address 192.168.1.1

I am a bit lost where does the actual binding takes place. How do I define that pool VLAN40 is supposed to be set on my VLAN40 and pool VLAN60 on my VLAN60 subnets and not other subnet (VLAN) configured on the switch? Is there a way to disable temporarily one of the DHCP pools? Or do I need to disable it globally on the switch impacting other DHCP pools as well?

Thanks, AP

Reply to
Adam Przestroga
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The DHCP server looks for which IP range the directed broadcast (that is a DHCPDISCOVERY) comes from, and replies appropriately for that range.

You don't have to bind each scope to a VLAN, it'll just work like it should.

The only way to disable a scope is to remove its config AFAIK. That wouldn't affect other scopes configured though.

Reply to
Doug McIntyre

Doug, I am sorry but I do not get it. If you have a host on a VLAN, the host will broadcast DHCP request. How does the switch know to respond with IP address from pool A and not B? What makes this request so special?

Reply to
Adam Przestroga

Thanks for the clarification, but this implies two things:

1) you cannot simply create two VLANs and not to assign an IP address to the VLAN interfaces - right? 2) The IP address of the VLAN interface is the one which "translates" DHCP requests from hosts on the VLAN to its own IP address (indicating network address) and because of that DHCP server running on the switch knows which pool to use?

Sorry, if I am oversimplifying this.

Reply to
Adam Przestroga

Err, no. You only assign an IP to a VLAN interface if you want to.

When a DHCP request arrives on an interface (well, one with an IP address) the router "knows" which interface it arrived on. It looks in the DHCP configuration to see if there is a pool that matches the IP address range of the interface. If so it attempts to allocate an address from the pool. If no pool matches the address range of the interface it ignores the request.

Err. NO. You are overcomplicating it - somehow or other. Apology accepted:)

Reply to
bod43

Thanks for the clarification.

Reply to
Adam Przestroga

Err, no. You only assign an IP to a VLAN interface if you want to.

When a DHCP request arrives on an interface (well, one with an IP address) the router "knows" which interface it arrived on. It looks in the DHCP configuration to see if there is a pool that matches the IP address range of the interface. If so it attempts to allocate an address from the pool. If no pool matches the address range of the interface it ignores the request.

Err. NO. You are overcomplicating it - somehow or other. Apology accepted:)

Reply to
Thrill5

I could think of no reason at the time to configure an SVI (to use the formal term) and then not configure an IP address but I thought there must be one, somewhere.

Let's see - IPV6 on SVI and no need for IPV4 might be one case. In fact on an 877W for example, for home adsl with wireless it is normal not to have a ip on a VLAN interface (SVI) and to just have a bridge group on it. Of course that is not a 3750.

Reply to
bod43

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