Option 82 on Cisco switches.

[Evgeniy Ivanov]

Hi!
I have a question about how does option 82 work on cisco switches.
From one source, i read that Cisco switches always insert option 82 in every dhcp request by default.
From another source, i read that Cisco switches will insert option 82 information only if dhcp relay configured.

Which statement is correct?

Thanks!

[paul driver]

Hello

Please have a look at this link from a Peter Paluch it will explain much better than I ever could.

https://supportforums.cisco.com/discussion/11081266/dhcp-snooping#3262823

res

Paul

[Evgeniy Ivanov]

Paul, thanks for reply!

Also, nice explanation from Peter Paluch here:

https://supportforums.cisco.com/discussion/11703441/dhcp-snooping-not-working-dropping-packets

But i still have a problem with it.
I have tested insertion of option 82 in following configuration:
One 2960S switch, and two notebooks connected to it in vlan 1:
DHCP snooping and option 82 insertion are  enabled:

DHCP snooping is configured on following VLANs:
1
DHCP snooping is operational on following VLANs:
1

PC1 is configured for DHCP. PC2 is used for wireshark to sniff traffic.

As i understand, if dhcp snooping is not enabled, PC2 will receive dhcp request from PC1 without option 82.

If dhcp snooping is enabled , PC2 will receive  DHCP request from PC WITH option 82.

However, in second case i dont see option 82 in wireshark.

But why?
 

[Evgeniy Ivanov]

Any ideas?
I think that something wrong, but what?
For example, if i have small ISP, and want to use Cisco Catalyst as pure L2 access switch with DHCP snooping and option 82 insertion. Distribution switch will have L3 interfaces, with DHCP relay function.
In this scenario, access switch should insert option 82 information, and forward DHCP request inside same vlan to Distribution switch, and so on.

But as i said, wireshark shows that access switch doesnt insert option 82 information. Why?