ip helper-address behavior

Discussion in 'Cisco' started by Terry D, Feb 12, 2007.

  1. Terry  D

    Terry D Guest

    If I have multiple DHCP servers, and in turn, multiple ip helper-
    address lines in my router, how does the router handle this? Does it
    round robin the requests between the two entries?
     
    Terry D, Feb 12, 2007
    #1
    1. Advertising

  2. It sends it to all servers. When we studied our network, we noticed that
    routers convert DHCP request to unicast, and sends to all configured DHCP
    servers. So, you cannot control which server will accept and give IP
    address.

    Good luck,

    Mike
    ------
    Cisco IP Phone PC Headset Adapters
    www.ciscoheadsetadapter.com


    "Terry D" <> wrote in message
    news:...
    > If I have multiple DHCP servers, and in turn, multiple ip helper-
    > address lines in my router, how does the router handle this? Does it
    > round robin the requests between the two entries?
    >
     
    headsetadapter.com, Feb 12, 2007
    #2
    1. Advertising

  3. Terry  D

    Terry D Guest

    On Feb 12, 2:11 pm, "headsetadapter.com" <> wrote:
    > It sends it to all servers. When we studied our network, we noticed that
    > routers convert DHCP request to unicast, and sends to all configured DHCP
    > servers. So, you cannot control which server will accept and give IP
    > address.
    >
    > Good luck,
    >
    > Mike
    > ------
    > Cisco IP Phone PC Headset Adapterswww.ciscoheadsetadapter.com
    >
    > "Terry D" <> wrote in message
    >
    > news:...
    >
    > > If I have multiple DHCP servers, and in turn, multiple ip helper-
    > > address lines in my router, how does the router handle this? Does it
    > > round robin the requests between the two entries?


    Interesting. What's best practice? So, both servers could respond
    with an ip address but the client only accepts one? Isn't this
    wasteful?
     
    Terry D, Feb 12, 2007
    #3
  4. Hi,

    the IP "refused" by the client is not wasted; actually only one IP in the
    pool is used.

    For High-Availability of DHCP serve I suggest you to use a cluster or the
    classical 80/20 config.

    Regards,
    Gabriele

    "Terry D" <> ha scritto nel messaggio
    news:...
    > On Feb 12, 2:11 pm, "headsetadapter.com" <> wrote:
    > Interesting. What's best practice? So, both servers could respond
    > with an ip address but the client only accepts one? Isn't this
    > wasteful?
    >
     
    Gabriele Beltrame, Feb 13, 2007
    #4
  5. Terry  D

    Sam Wilson Guest

    In article <>,
    "Terry D" <> wrote:

    > On Feb 12, 2:11 pm, "headsetadapter.com" <> wrote:
    >
    > > "Terry D" <> wrote in message
    > >
    > > news:...
    > >
    > > > If I have multiple DHCP servers, and in turn, multiple ip helper-
    > > > address lines in my router, how does the router handle this? Does it
    > > > round robin the requests between the two entries?

    > >
    > > It sends it to all servers. When we studied our network, we noticed that
    > > routers convert DHCP request to unicast, and sends to all configured DHCP
    > > servers. So, you cannot control which server will accept and give IP
    > > address.

    >
    > Interesting. What's best practice? So, both servers could respond
    > with an ip address but the client only accepts one? Isn't this
    > wasteful?


    It's handled by the protocol: client broadcasts request, servers make
    offers, client broadcasts acceptance of one of the offers (it's
    broadcast so other servers know they've been declined), accepted server
    ACKs (or NAKs and the whole thing starts again). So only one address is
    used per client.

    If you're using dynamic pools then you have to make sure your servers'
    pools don't overlap or that they have a way of synchronising their pools
    (rare). With statically assigned addresses it doesn't matter - multiple
    servers are just for redundancy.

    Sam
     
    Sam Wilson, Feb 13, 2007
    #5
  6. Terry  D

    Thrill5 Guest

    "Sam Wilson" <> wrote in message
    news:...
    > In article <>,
    > "Terry D" <> wrote:
    >
    >> On Feb 12, 2:11 pm, "headsetadapter.com" <> wrote:
    >>
    >> > "Terry D" <> wrote in message
    >> >
    >> > news:...
    >> >
    >> > > If I have multiple DHCP servers, and in turn, multiple ip helper-
    >> > > address lines in my router, how does the router handle this? Does it
    >> > > round robin the requests between the two entries?
    >> >
    >> > It sends it to all servers. When we studied our network, we noticed
    >> > that
    >> > routers convert DHCP request to unicast, and sends to all configured
    >> > DHCP
    >> > servers. So, you cannot control which server will accept and give IP
    >> > address.

    >>
    >> Interesting. What's best practice? So, both servers could respond
    >> with an ip address but the client only accepts one? Isn't this
    >> wasteful?

    >
    > It's handled by the protocol: client broadcasts request, servers make
    > offers, client broadcasts acceptance of one of the offers (it's
    > broadcast so other servers know they've been declined), accepted server
    > ACKs (or NAKs and the whole thing starts again). So only one address is
    > used per client.
    >
    > If you're using dynamic pools then you have to make sure your servers'
    > pools don't overlap or that they have a way of synchronising their pools
    > (rare). With statically assigned addresses it doesn't matter - multiple
    > servers are just for redundancy.
    >
    > Sam


    You can also purchase a DHCP system that offers redundancy without having to
    use multiple pools. Many of the IP address management systems offer this
    capability.

    Scott
     
    Thrill5, Feb 14, 2007
    #6
    1. Advertising

Want to reply to this thread or ask your own question?

It takes just 2 minutes to sign up (and it's free!). Just click the sign up button to choose a username and then you can ask your own questions on the forum.
Similar Threads
  1. Northern Lights

    ip helper-address confusion

    Northern Lights, Jul 24, 2003, in forum: Cisco
    Replies:
    1
    Views:
    2,535
    Martin Gallagher
    Jul 25, 2003
  2. º°·.·°º   M¤§fët   º°·.·°º

    ip helper-address stats

    º°·.·°º M¤§fët º°·.·°º, Nov 1, 2003, in forum: Cisco
    Replies:
    0
    Views:
    982
    º°·.·°º M¤§fët º°·.·°º
    Nov 1, 2003
  3. tgot
    Replies:
    2
    Views:
    4,144
    Martin Gallagher
    Feb 13, 2004
  4. Wulf Losee
    Replies:
    6
    Views:
    18,880
  5. Gary Williams

    IP Helper-Address on Cisco 2900XL

    Gary Williams, Oct 31, 2004, in forum: Cisco
    Replies:
    4
    Views:
    10,532
    Robert B. Phillips, II
    Oct 31, 2004
Loading...

Share This Page