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AdamBudzinski
Beginner

dhcpv4 DHCPDISCOVER packet

Hello,

I'm reading about DHCPv4 on NetAcad and it's written "The DHCP client sends a directed IP broadcast with a DHCPDISCOVER packet" 

isn't this wrong ? while inspecting the DHCPDISCOVER packet the Destination Layer 3  address = 255.255.255.255     so, shouldn't they say it sends a limited broadcast

EDIT: I'm asking because the next line states "The server notes the GIADDR field is blank;therefore, the client is on the same segment" 

Looking for some clarification! 

Thank you in advance and have a great day ! 

Adam 

1 ACCEPTED SOLUTION

Accepted Solutions
acampbell
Advocate

Hi Adam,

You are correct 255.255.255.255 is a "limited IP broadcast address".
This is relevant to devices in the same broadcast domain (VLAN)

A router will not forward a packet with a destination of 255.255.255.255.

However if a router has its interfaces configured for DHCP Relay it
will tranform the destination of the packet and forward it to the DHCP servers.
It will also insert its interface ip address as the "giaddr" field in the dhcp
packet.

!
interface fast 0/0
description *** MY LOCAL VLAN 100 ***
ip address 10.1.100.1 255.255.255.0
ip helper-address 10.1.254.21
ip helper-address 10.1.255.21
!

So this wil DHCP relay to my 2 DHCP servers 10.1.254.21 & 10.1.255.21
And insert 10.1.100.1 as the "giaddr"

A directed broadcast
http://www.cisco.com/web/techdoc/dc/reference/cli/nxos/commands/l3/ip_directed-broadcast.html

An IP directed broadcast is an IP packet whose destination address is a valid broadcast address
for some IP subnet but which originates from a node that is not itself part of that destination subnet.


I hope this helps

Regards
Alex

Regards, Alex. Please rate useful posts.

View solution in original post

1 REPLY 1
acampbell
Advocate

Hi Adam,

You are correct 255.255.255.255 is a "limited IP broadcast address".
This is relevant to devices in the same broadcast domain (VLAN)

A router will not forward a packet with a destination of 255.255.255.255.

However if a router has its interfaces configured for DHCP Relay it
will tranform the destination of the packet and forward it to the DHCP servers.
It will also insert its interface ip address as the "giaddr" field in the dhcp
packet.

!
interface fast 0/0
description *** MY LOCAL VLAN 100 ***
ip address 10.1.100.1 255.255.255.0
ip helper-address 10.1.254.21
ip helper-address 10.1.255.21
!

So this wil DHCP relay to my 2 DHCP servers 10.1.254.21 & 10.1.255.21
And insert 10.1.100.1 as the "giaddr"

A directed broadcast
http://www.cisco.com/web/techdoc/dc/reference/cli/nxos/commands/l3/ip_directed-broadcast.html

An IP directed broadcast is an IP packet whose destination address is a valid broadcast address
for some IP subnet but which originates from a node that is not itself part of that destination subnet.


I hope this helps

Regards
Alex

Regards, Alex. Please rate useful posts.