Solved: "DHCP Snooping" question

bigbrother74 · ‎09-14-2007

Hi

If installed dhcp snooping for testing perpose on an access switch.

We have a big network but we don't need a distribution layer in our hierarchical network design model.

We have PE switches 65xx in the core and 35xx in the access layer.

I activated "dhcp snooping" from the folling link:

http://www.cisco.com/en/US/docs/switches/lan/catalyst3560/software/release/12.2_25_se/configuration/guide/swdhcp82.html

I'm studying to become a ccnp and also the curriculum describes the following task to activate snooping for dhcp packets:

[global]

ip dhcp snooping

ip dhcp snooping vlan vlan-range

[interface]

ip dhcp snooping trust

Unfortunately it does not as long as I disable "ip dhcp snooping information option"

The workable config look's like:

[global]

ip dhcp snooping

ip dhcp snooping vlan vlan-range

no ip dhcp snooping information option

[interface]

ip dhcp snooping trust

What does the command "no ip dhcp snooping information option" realy do???

Thanx for help

:-)

Jozef Janitor · ‎09-14-2007

It prevents the switch from inserting Option 82 header into the DHCP packet while forwarding.

I am not sure but afaik if this option is turned off then the upper layer switches can build the DHCP Snooping table, otherwise if the dhcp packet already contains the Option 82 header they will not build it.

View solution in original post

Jozef Janitor · ‎09-14-2007

It prevents the switch from inserting Option 82 header into the DHCP packet while forwarding.

I am not sure but afaik if this option is turned off then the upper layer switches can build the DHCP Snooping table, otherwise if the dhcp packet already contains the Option 82 header they will not build it.

wochanda · ‎09-14-2007

The 'information option' adds an Option-82 header which contains additional information about the host connecting to the network. This extra information consists of the Switch ID and Port ID that the user connects to.

This is usually used in service provider networks where the DHCP server desires extra information in the DHCP discovers. Unfortunately, many DHCP servers do not support the option-82 header.

Chances are your DHCP server doesn't support these headers, and this is why your hosts cannot get DHCP addresses when insertion is configured.

Will

bigbrother74 · ‎09-15-2007

@ wochanda

This is very helpful answer. It explains clearly what I want to know...

Thanx for help