DHCP on 6509 with no SVI

Unanswered Question
Aug 16th, 2010

   Hello,

I have a setup where I have a 6509 with no SVIs. I need to create different dhcp pools that will be used for voice without creating SVIs, is this possible? how?

I have this problem too.
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Peter Paluch Mon, 08/16/2010 - 04:48

Hello,

What do you mean by saying that "your 6509 has no SVIs configured"? Does that mean there are absolutely no SVIs configured and only routed interfaces are used, or there are some SVIs but not in those networks for which you want to create the DHCP pools?

In any case, DHCP pools are basically independent of local interfaces. You can create them for any network, local or remote, without any difference. For a local network, an incoming DHCP request is associated with its appropriate pool depending on the incoming interface's address (by matching the configured network command). For a remote network, the DHCP request will be forwarded to you by means of a DHCP Relay service, and the original network where the request was originated will be identified in the DHCP request contents (somewhat simplified explanation). In any case, your DHCP server will be always aware which from DHCP pool should a particular request be served.

I am not sure if this is what you were asking about so please keep this discussion going.

Best regards,

Peter

k.abillama Mon, 08/16/2010 - 05:02

Hi Peter,

My 6509 switch is not the Default gateway for users, I have no SVIs at all on it. My concern is I have some IP phones which will be in two subnets, i need to create on the 6509 two dhcp pools and the gateway for the voice subnet will not be an SVI on the Core switch neither. so i don't want to create any SVI interface on the 6509, is this possible? how will an ip phone ( let's say in the voice vlan 10 take and ip from the 192.168.10.x and another one on the vlan 20 take from the 192.168.20.x range without having SVIs configured?)

Regards

Peter Paluch Mon, 08/16/2010 - 05:30

Hello,

As a rule, a device acting as DHCP server must have an IP address, and therefore it must have some IP interface, be it a routed interface or a SVI. If your 6509 has no IP interface at all, it cannot act as a DHCP server.

But I assume that your 6509 has at least one IP address in whatever VLAN and that this IP address is reachable from the default gateways of the voice VLANs. In such a case, you will configure DHCP pools in the 6509 as if the VLANs were directly connected:

ip dhcp pool VoiceVLAN10

network 192.168.1.0 /24

default-router 192.168.1.X

dns-server ...

...

...

ip dhcp pool VoiceVLAN20

network 192.168.2.0 /24

default-router 192.168.2.X

dns-server ...

...

...

Note that there need not be any SVIs for the voice VLANs present on the 6509. Any reachable IP address on the 6509 will do.

On both default gateways for voice VLAN, you will need to configure the DHCP Relay on the interface towards the respective voice VLAN:

interface Vlan 10

ip helper-address X.Y.Z.Q

where the X.Y.Z.Q is the reachable IP address of the 6509 switch, to make the gateway relay the DHCP message from the client to the 6509 switch. The 6509 switch will be able to associate this request with the proper pool because the IP address of the interface Vlan 10 in this example will be indicated in the relayed DHCP message, allowing the DHCP server to associate the request with the DHCP pool VoiceVLAN10.

Once again, if the 6509 does not have any reachable IP address then it cannot act as a DHCP server.

Best regards,

Peter

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