About Bootp (a entry level question)

Hi. I have used Bootp utility all the time to assign IP address to MLX1100 successfully, but I always had few questions. Just FYI, I am using Rockwell Bootp/DHCP Server Utility 2.3.

#1) Why doesn't my home router/modem send bootp requests? Or at least I don't see them in the request history panel of the Bootp utility?

#2) Can a PLC broadcast DHCP requests, rather than Bootp requests?

#3) Related to the previous question, what does the Enable DHCP button do? An industrial process network cannot have a PLC's IP address changing on its own, and I guess that is the reason for Bootp being used, not DHCP. But I guess in some applications (I don't know), dynamic addressing doesn't matter therefore DHCP can be used, hence the "Enable DHCP" function. Is my reasoning correct?

Easier for me to point you to Wikipedia articles on BOOTP and DHCP.

In a nutshell, BOOTP assigns an IP address to a device that has a specific and unique MAC address. Not very good for maintenance "swap-out".

DHCP can assign IP addresses to any device that identifies itself as belonging to the network. Mostly DHCP dishes out IP Addresses indiscriminately, giving the device a free IP address from the assigned "pool". DHCP can act like a BOOTP server, allowing for reserved addresses for specific MAC addresses. This is just BOOTP functionality, provided by the more capable DHCP server.

Used in an industrial environment, both BOOTP and DHCP will therefore require maintenance, to ensure that the actual hardware gets the correct addresses.

I don't like the use of either BOOTP or DHCP in industrial networks, as both require a machine running a BOOTP or DHCP server 24/7. I prefer to get the nodes assigned manually, using whatever method is available, and "seal-in" the IP address in the hardware.

daba said:

Used in an industrial environment, both BOOTP and DHCP will therefore require maintenance, to ensure that the actual hardware gets the correct addresses.

Click to expand...

In my limited experiences, I would have to think that bootp is a much safer way on industrial applications since it assigns a "permanent" IP address to a device. Where as with DHCP, I have to worry about the lease renewal or expiration. Am I right?

daba said:

I don't like the use of either BOOTP or DHCP in industrial networks, as both require a machine running a BOOTP or DHCP server 24/7. I prefer to get the nodes assigned manually, using whatever method is available, and "seal-in" the IP address in the hardware.

Click to expand...

Once you assign a permanent IP address to a device, why can't you disable bootp on that device? Then the device will no longer send bootp requests, and you don't need the bootp server running for that device anymore until you need to change the IP address for some reason later, right?

unsaint32 said:

In my limited experiences, I would have to think that bootp is a much safer way on industrial applications since it assigns a "permanent" IP address to a device. Where as with DHCP, I have to worry about the lease renewal or expiration. Am I right?

Click to expand...

DHCP can be set to "not expire" - just enter 0 for the lease duration. But neither BOOTP or DHCP will assign a "permanent" address to a device. Power it down, and it all has to be assigned again by BOOTP or DHCP. And that could have issues if the BOOTP or DHCP server machine doesn't boot itself properly. No BOOTP or DHCP server means that devices that rely on it won't work!

Far better to configure the device manually, once, and it will always power-up at the same IP adsress, IMHO.

unsaint32 said:

Once you assign a permanent IP address to a device, why can't you disable bootp on that device? Then the device will no longer send bootp requests, and you don't need the bootp server running for that device anymore until you need to change the IP address for some reason later, right?

Click to expand...

BOOTP does not have the capability to automatically send a "disable BOOTP" command once it has configured a device - yes you can manually do it with the BOOTP GUI, but that's about the same effort as using RSLinx to browse the device port parameters page, setting it as you want it, then click OK.

At least if you have manually configured the IP address of a node, and "locked it in", you know you will never need a machine running BOOTP on the network. Now some "networks" I have seen span different geographical locations, and you can have only one BOOTP or DHCP server, so imagine the problems a remote site might have if power is cycled, and there isn't a connection back to the BOOTP/DHCP server.

daba said:

Power it down, and it all has to be assigned again by BOOTP or DHCP.

Click to expand...

Really? Can you give me an example? All devices I used (Micrologix1100 and Panelview+) retain there previously assigned IP address through power cycles whether I used Bootp or not.

daba said:

Far better to configure the device manually, once, and it will always power-up at the same IP adsress, IMHO.

Click to expand...

When I used PanelView+, I could enter IP address manually directly using the PV's touchscreen and digital keypad. Is that one example of what you mean by "manually"? But some devices require Bootp utility and GUI and there is no other way to assign IP address, right? Such as Micrologix1100.

Thank you for your replies.

Thats because you clicked the "Disable BootP/DHCP" button on the GUI. Otherwise it will forget the IP you assigned.

daba said:

I don't like the use of either BOOTP or DHCP in industrial networks, as both require a machine running a BOOTP or DHCP server 24/7. I prefer to get the nodes assigned manually, using whatever method is available, and "seal-in" the IP address in the hardware.

Click to expand...

100% agree.

What I do is to put a special maintenance display that given the right code the operator can use to change the IP address of the device.

I'm using Windows CE PC104 computers as PLC's therefore I'm assigning the IP to the Windows OS.