A client: 3 sites with L2 links to our Data Centre. I create L3 links by providing IP's both sides. I want to create a GRE tunnel over each link with seperate IPs in order to route Data over these new links and splitting Voice this way using the overhead. The idea is to have a 30m line with a 27m GRE for the Data component and the Voice will be routed over the normal L3 link, therefore always able to have the additional 3m bandwidth.
Can this work?
Do not create GRE tunnels - this will appear to your L3 network as two distinct paths to the same site, meaning you'll end up with extra routing complexity you don't need.
GRE will also add overhead to your payload, and reduce your link MTU which you do not want.
On top of all this, GRE does nothing to solve the guaranteed bandwidth issue, for which you will need to use QoS anyway.
Keep it simple
First, if the three sites connect to the data center over WAN circuits with any appreciable latency, layer-2 connections are not a very good idea. Layer-2 must send all the broadcasts across the layer-2 links, and this can degrade performance and eat up expensive WAN bandwidth.
Sending the VoIP over the layer-2 link will force it to compete with the broadcast and link-local traffic inherent in a layer-2 connection, so you may not always have 3 Mb for VoIP as you envision.
The proper way to separate classes of traffic and guarantee bandwidth is with QoS.
You are going to need a router on each end of the link if you use GRE. In that case, you should use layer-3 links. Doing this will allow you to use QoS to mark priority traffic and guarantee bandwidth for VoIP. You could also have a lower-priority class of traffic for things like backups and replication which would normally hog bandwidth, starving more immediate, normal user traffic. When the higher-priority classes are not using their bandwidth, the lower-priority class(es) can use that bandwidth.
