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Way to forward DHCP requests w/o an 'ip helper-address'?

Hello,

About 18 months ago, we stopped using VTP and switched to assigning 'allowed VLAN' to individual interfaces.  Everything has been working well.  At that time, the contractor had me turn off VTP on the core switch (default gateway) and leave VTP in transparent mode on the access and distribution switches.  He said going forward, we could stop creating VLAN interfaces and simply just create named VLANs.  More recently, perhaps when a client lease expires, a DHCP client will not obtain an ip address in a timely manner, if at all.  I suspect this might be due to the lack of an 'ip helper-address' on a VLAN interface.  Is that likely?  Is the configuration okay as described?  And how can I help clients reach the DHCP server?  

Perhaps this clarifies my question:  We were trying to reduce traffic in an OT environment but appear to lose DHCP acknowledgements periodically.  Can create a VLAN interface and still isolate traffic using 'allowed vlan'?

P.S.  The DHCP server is connected to the core stack of switches.

20 Replies 20

No, it was suggested we use 'allowed vlan' in place of VTP Pruning.  I meant to indicate the contractor had us turn VTP off on the core, and then left downstream switches in transparent mode.  However, I still see your point.


haggittmark@tm-america.com wrote:

No, it was suggested we use 'allowed vlan' in place of VTP Pruning.  I meant to indicate the contractor had us turn VTP off on the core, and then left downstream switches in transparent mode.  However, I still see your point.


If you have an all Cisco VTP network, that is exactly the scenario that VTP pruning was built to serve. It does that very well, and you don't have to remember to change allowed VLAN statements on a bunch of trunks if you need a VLAN in a switch where it wasn't configured before. IMHO that is creating a lot of work for no reason that I can determine.

Friend make it simple to you, 
VLAN in below meaning the VLAN of client not VLAN of DHCP Server 
***the VLAN is allow and add in all SW 
here 
**either you turn OFF VTP and config it manually (ADD VLAN in EACH SW) and allow VLAN in trunk 
**or run VTP and make sure the VLAN is allow in trunk between the Access to Core SW


***the VLAN in Access SW only 
here you need to config ip helper address in access SW and no need to allow VLAN 
only e sure that the DHPC server is ping from VLAN SVI.

Starting a new post for related issue.  When I create a VLAN interface on an access switch and add an ip helper-address, the network slows down noticeably.

I have started a related post for a problem after adding a VLAN interface:  

https://community.cisco.com/t5/switching/creating-a-vlan-interface-impairs-connectivity-on-named-vlan/m-p/4669231#M529161 

Thank you Elliot, Reza, and MHM for providing helpful information.  

My boss would like me to move on from this post now.

Thank you all community!

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