13.4 DHCP Relay

Hi all,

In order to avoid conflict with other ips in the vlan you could also use:

ip dhcp ping packets 1

This would get the server to ping the ip address before assigning it to make sure nobody else is using it.



  • Eric, this is correct.  However, by default, the DHCP server pings a pool address twice before assigning it to try and make sure it isn't in use.  If it doesn't get a response it assumes its available and assigns the address so I really don't think adding this command is necessary unless you wanted to reduce the number of pings from 2 (the default) to 1.  I found this information on the following link in case anyone wants to verify: http://www.ccda.biz/en/US/docs/ios/12_2/ip/configuration/guide/1cfdhcp.html#wp1001176 



  • In Solution guide for this secrtion interfaces used on R5 should be "FastEthernet 0/0".



    Inder Vaid

  • In Solution guide for this secrtion interfaces used on R5 should be "FastEthernet 0/0".

    This seems to have been fixed now as the Solution reads:

    interface FastEthernet 0/0
     ip helper-address

  • Having had another look on this task, hasn't the previous tasks asked
    to have R5 Fa0/0 on VLAN 146? I think in order for this task to work we
    need the following additional configuration.


    ! SW1 


    conf t


    int fa0/5

     switchport access vlan 58




    ! R5

    conf t


    int fa0/0

     ip add


    Otherwise, R5 will never relay the request to R6 because it will never receive it since its fa0/0 is configured to be part of VLAN 146. What do you think?

  • Hello,


    Just wanted to add something:

    Exactly, DHCP has already embedded a way to determine whether an IP is being user or not via ICMP messages (As Wil said) but what happens if the end device that previously obtained an IP address has a firewall rule set that denies inbound ICMP messages, then it will not work.

    So to make it happen at all times we must use the " ip dhcp excluded-address"



    Julio Carvajal 

  • hfjardim, the last bullet point in 13.1 says to revert R5 and SW2 to their original configuration. Almost missed that myself.

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