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Creating a VLAN interface impairs connectivity on named VLAN?

We name VLANs and then assign them to trunks with 'allowed VLAN'. This seems to work. On occasion, a client on a different VLAN than the DHCP server does not obtain an IP address. So, I tried to create a VLAN interface for a named VLAN and added an ip helper-address. However, the clients become nearly unresponsive. Communication seems to occur to some degree but seems to be sporadic and suffering from frequent time-outs.

5 Replies 5



 ip-helper command only provisions dhcp server for a particular vlan/subnet , check if the client can acquire an address, if then there are client connectivity problems, then these are not related to address provisioning (ip-helper).


-- Each morning when I wake up and look into the mirror I always say ' Why am I so brilliant ? '
    When the mirror will then always repond to me with ' The only thing that exceeds your brilliance is your beauty! '

My previous answer was two option 
allow VLAN in path between client and DHCP Server, 
this make client send broadcast and since the VLAN allow along path to DHCP Server then the DHCP receive the broadcast and assign IP to client.

using IP helper 
config the IP helper in SW the client direct connect. 

I think you merge both solution and you now totally lost 

I think I stumbled across a band-aid for the immediate issue.  The band-aid is creating a VLAN interface for the named VLAN without assigning a subnet address and then adding the ip helper-address.  
However, I am not 100% sure because I don't have enough evidence yet.  I placed a client on the subnet in question, and it obtained an IP address right away.  But I could have just gotten lucky.  

There is much that I do not understand about what you describe. One thing is clear: if you configure a vlan interface (a layer 3 interface for the layer 2 vlan), do not configure an IP address for it, and configure a helper-address for it then the helper address does not accomplish anything. If the client does get an IP address then either there is some layer 2 connection between the client and the server or somewhere there is a layer 3 interface in the vlan that does have an IP address and forwards DHCP requests from the client to the server.

If the client does receive an IP address and then becomes unresponsive it would be some kind of issue about the network (perhaps a duplex mismatch, perhaps an undetected loop, perhaps something else) but not just that the client learned an address using DHCP.



Thanks for the confirmation that the ip helper-address has no impact without a subnet defined on the VLAN interface.

When i do define a subnet to the VLAN interface, clients become less responsive.   

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