DHCP Odom's latest ICND1 vs Cisco

Hi there,

From reading old forum posts I can see this topic has come up before. Odom's text seems to have changed but I'm still left wondering what the correct position is for the exam?

On page 125 of Odom's ICND1 book the graphic states that:

1. DHCP Discover Message (LAN Broadcast)
2. DHCP offer message (LAN Broadcast)
3. DHCP Request Message Directed to Server
4. DHCP Acknowledgement Directed to Client

This Cisco doc states that the offer message is unicast (obviously at layer 2) - which makes more sense to me: Cisco IOS DHCP Server - Cisco Systems

My thoughts are, follow Cisco. But why on earth did Odom change the latest edition of the book to state the offer message is a LAN Broadcast when previously it read "direct to client"?

Grateful for your thoughts!

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Comments

SteveO86

I've attached a screen shot of a packet capture of the DHCP process.. (Part of an upcoming blog post I've been working on)

I've highlighted the DHCP Offer packet and it is indeed a broadcast. Note the destination MAC address FF:FF:FF:FF:FF:FF and the IP Destination 255.255.255.255 (Hope you can make it out)

Not sure If I'm going too deep for CCNA level knowledge.. but packet is right there.

Hope this helps.

My packet capture came from a small 48 port Cisco Catalyst 2960.

DHCP Offer Packet.jpg

wave

Thanks Steve,

Wow o.O They are *ALL* Broadcasts! Discover, Offer, Request and ACK.

That contradicts everything I've read (Lammle & Odom), but I the proof is in the pudding.

Cheers,

luisYme

from my understanding, one reason the dhcp offer has to be a broadcast because the client doesn't have an ip address yet, so a unicast packet just isn't appropriate. in order for the client to receive any communication from the dhcp server the server must use broadcast packets (again, because the client doesn't have an ip address yet).

when in doubt, i'd believe the actual packet captures than a book any day