dhcp broadcasts

Unanswered Question
Jun 21st, 2007
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Hi all, what address does a dhcp brodcast use, as when we issue the ip helper addresses on switches I would expect it to allow all broadcasts across, or are dhcp broadcasts different ?

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Overall Rating: 3 (1 ratings)
guruprasadr Thu, 06/21/2007 - 03:03
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HI Carl, [PLS Rate if HELPS]

DHCP Discover message uses below format:


Destination = BROADCAST FFFFFFFFFFFF, Broadcast

Source = Station 0005DCC9C640

Refer the below RFC's:

RFC 2131 - DHCP

RFC 2132 - DHCP Options and BootP Vendor Extensions

RFC 1534 - Interoperation between DHCP and BootP

RFC 1542 - Clarifications and Extensions for the BootP

RFC 2241 - DHCP Options for Novell Directory Services

RFC 2242 - Netware/IP Domain Name and Information


Best Regards,

Guru Prasad R

mohammedmahmoud Thu, 06/21/2007 - 03:24
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Actually ip-helper command can use a destination broadcast or host address to be used when forwarding UDP broadcasts, thus if you configured it with the IP address of the remote DHCP server, it'll complete the DHCP request as unicast rather than brodacst.


Mohammed Mahmoud.

vinayrajkp Thu, 06/21/2007 - 05:01
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Agree with Mohammed that the request will go as a DHCP request as unicast.But consider below statement from one of the cisco docs -

By default, the IP helper-address will forward the following UDP broadcasts:

Trivial File Transfer Protocol (TFTP) (port 69)

DNS (port 53), time service (port 37)

NetBIOS name server (port 137)

NetBIOS datagram server (port 138)

Boot Protocol (DHCP/BootP) client and server datagrams (ports 67 and 68)

Terminal Access Control Access Control System (TACACS) service (port 49)

IEN-116 name service (port 42)

Hope this helps


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