Cisco Support Community
cancel
Showing results for 
Search instead for 
Did you mean: 
Announcements
Webcast-Catalyst9k
New Member

Source of IP packet from lan via rtr

Hi

Can someone confirm that the source ip of an ip packet when communicating with a central dhcp server will be the source ip of workstation?  IP helper-addressing configured on lRouter Lan interface so incoming dhcp request will be forwarded on as a result of helper-address to remote Central DHCP Server but I just want to ensure that the router doesn't remark the IP SRC IP to its own WAN int? Or is the incoming ip packet source ip address amended to router ethernet ip where the helper-address is configured? What happens if there is secondary addressing configured on the same ethernet?

Rgards

Mary

2 ACCEPTED SOLUTIONS

Accepted Solutions
Hall of Fame Super Silver

Re: Source of IP packet from lan via rtr

Hello Mary,

there is a specific internal field called gatway in DHCP message that allows to identify the correct vlan on which original DHCP request was recevied so the real ip source of packet can be different without causing issues.

the field is filled with router lan interface ip address during ip packet manipulation triggered by ip helper address

http://www.tcpipguide.com/free/t_DHCPMessageFormat.htm

>> What happens if secondary address is configured?

to be investigated..

Hope to help

Giuseppe

Hall of Fame Super Blue

Re: Source of IP packet from lan via rtr

maryodriscoll wrote:

Hi

Can someone confirm that the source ip of an ip packet when communicating with a central dhcp server will be the source ip of workstation?  IP helper-addressing configured on lRouter Lan interface so incoming dhcp request will be forwarded on as a result of helper-address to remote Central DHCP Server but I just want to ensure that the router doesn't remark the IP SRC IP to its own WAN int? Or is the incoming ip packet source ip address amended to router ethernet ip where the helper-address is configured? What happens if there is secondary addressing configured on the same ethernet?

Rgards

Mary

Mary

Until the client has been assigned an IP it can't be the client IP because it doesn't have one. The IP address will be the LAN interface IP address of the router ie. the interface where the ip helper-address is configured.

The router will use the primary address because it has no way of knowing that a client without an IP wants one of the secondary addresses.

Jon


Cisco are currently donating money to the Haiti earthquake appeal for every rating so please consider rating all helpful posts.

3 REPLIES
Hall of Fame Super Silver

Re: Source of IP packet from lan via rtr

Hello Mary,

there is a specific internal field called gatway in DHCP message that allows to identify the correct vlan on which original DHCP request was recevied so the real ip source of packet can be different without causing issues.

the field is filled with router lan interface ip address during ip packet manipulation triggered by ip helper address

http://www.tcpipguide.com/free/t_DHCPMessageFormat.htm

>> What happens if secondary address is configured?

to be investigated..

Hope to help

Giuseppe

New Member

Re: Source of IP packet from lan via rtr

Thanks Giuseppe, the link helped

Hall of Fame Super Blue

Re: Source of IP packet from lan via rtr

maryodriscoll wrote:

Hi

Can someone confirm that the source ip of an ip packet when communicating with a central dhcp server will be the source ip of workstation?  IP helper-addressing configured on lRouter Lan interface so incoming dhcp request will be forwarded on as a result of helper-address to remote Central DHCP Server but I just want to ensure that the router doesn't remark the IP SRC IP to its own WAN int? Or is the incoming ip packet source ip address amended to router ethernet ip where the helper-address is configured? What happens if there is secondary addressing configured on the same ethernet?

Rgards

Mary

Mary

Until the client has been assigned an IP it can't be the client IP because it doesn't have one. The IP address will be the LAN interface IP address of the router ie. the interface where the ip helper-address is configured.

The router will use the primary address because it has no way of knowing that a client without an IP wants one of the secondary addresses.

Jon


Cisco are currently donating money to the Haiti earthquake appeal for every rating so please consider rating all helpful posts.

189
Views
0
Helpful
3
Replies
CreatePlease to create content