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  1. #1
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    ALC Bacnet and subnets?

    I have a question for ALC and how they manage going across subnets. We currently have some schools that will have the same bacnet Device ID. They are on different subnets so this shouldn't matter. I have the ALC guy telling me though he has to have unique Device IDs to talk to my equipment.

    I have never heard of this nor have i had this problem else where. Once you cross subnets your panels should be looking at the IP of the device not just the Device ID. I have done other schools with ALC/JCI, etc.. before and we always do this scenario without a problem. Not sure why this issue is coming up now.

    I believe he is using a LGR. I've always been taught that device id cannot be the same within a subnet, but once you are on a different one, you are on a different network so at that point you can duplicate again. Then the BBMDs will route message via IP between the subnets to transfer the info.

  2. #2
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    I don't know about the LGR's routing but I'm confused as to why you would purposefully set up duplicate device ID's on the same network (subnets aside).

    There are over 4 million device ID's for you to choose from. It seems unlikely that you've run out at that particular site...

  3. #3
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    Some systems index the device's information using the device instance number. He may be running into an issue like that. I also have to agree with BACnet that unique instance numbers should be used for the devices regardless of the subnet on which they reside.

  4. #4
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    Wasn't purposely setup that way. Owner has some BACnet devices out there that we did not know about and they ended up on the same device ID. But they are in a separate schools on separate subnets so this has never been a issue for our company.

    I look at it like having two mstp links, if you have 2 full links on a system. They are separated by a subnet. You have same device id numbers on both links. So when you access device 1 its knows based on how you set it up to either hit link 1 or link 2 based on your subnet address. So in my mind i dont see why IP would be any different.

  5. #5
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    Quote Originally Posted by scrooloose View Post
    I have a question for ALC and how they manage going across subnets. We currently have some schools that will have the same bacnet Device ID.

    I have never heard of this nor have i had this problem else where. Once you cross subnets your panels should be looking at the IP of the device not just the Device ID. I have done other schools with ALC/JCI, etc.. before and we always do this scenario without a problem. Not sure why this issue is coming up now.
    It becomes a problem when you start adding BBMD routers on those two IP subnets.

    The BBMD router hears the "who is" xxx (your duplicate address) and shares it with all other BBMD routers on other IP subnets and now you are open to multiple devices responding to that "who is" statement.

    IMO best practice is to prefix all your device instances and network numbers with your ashrae vendor ID. So then if you get two guys that like to start with 1,2,3 etc they end up with 5601, 5602, 5603 and the other guy is 2401, 2402, 2403.
    http://www.bacnet.org/VendorID/BACne...ndor%20IDs.htm
    Scott Jalbert, WebCTRL ninja, Naiagara AX and Smartstruxure newb, SiteScan Retired

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  6. #6
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    Thread Starter
    Quote Originally Posted by MaxBurn View Post
    It becomes a problem when you start adding BBMD routers on those two IP subnets.

    The BBMD router hears the "who is" xxx (your duplicate address) and shares it with all other BBMD routers on other IP subnets and now you are open to multiple devices responding to that "who is" statement.

    IMO best practice is to prefix all your device instances and network numbers with your ashrae vendor ID. So then if you get two guys that like to start with 1,2,3 etc they end up with 5601, 5602, 5603 and the other guy is 2401, 2402, 2403.
    http://www.bacnet.org/VendorID/BACne...ndor%20IDs.htm

    Makes sense, guess you could also change UDP ports and that would isolate it. Assuming third-party vendors can also specify different UDPs for each connection.

  7. #7
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    Yes we have been asked to change port numbers sometimes, that isolates things completely as well.
    Scott Jalbert, WebCTRL ninja, Naiagara AX and Smartstruxure newb, SiteScan Retired

    The master has failed more times than the beginner has even tried

  8. #8
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    Quote Originally Posted by MaxBurn View Post
    IMO best practice is to prefix all your device instances and network numbers with your ashrae vendor ID. So then if you get two guys that like to start with 1,2,3 etc they end up with 5601, 5602, 5603 and the other guy is 2401, 2402, 2403.
    http://www.bacnet.org/VendorID/BACne...ndor%20IDs.htm
    That there is a great idea if everyone would do it.
    Beware of the prophet trying to make a profit.

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  9. #9
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    Last edited by bobby m; 07-15-2010 at 08:34 PM. Reason: azghy

  10. #10
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    I go to great lengths to keep instace numbers unique. The scheme I use is this: in the instance number 240335 The 24 is the alc bacnet network id, the 03 would be the LGR#, and the 35 would be the module address. This can be a pain sometimes but it realy helps keep things unique.
    Also in the school districts I have, all of their buildings are on the same subnet mask , different subs seems wrong somehow...........

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