IP address schemes and VLANs

Question

I'm in the middle of working on Re-IPing a network for a client and wanted to clarify a couple of concepts before I got started:
First of all, the current setup is that all 50 sites use identical VLAN ID's at each site to relate to the same services (I.e. VLAN 10 - Data, VLAN 20 - VOIP, etc.).
However, I have had read some discussions that seem to suggest that using completely unique VLAN's at each site would be better practice.
Can someone confirm or deny this, and elaborate as to why this is/isn't best practice?
Secondly, if I'm using a /16 format as a template for each site, and referencing a pre-assigned device ID as the second octet (10.[site ID].x.x/16), would it be best to assign the subnets contiguously from bottom to top, starting with the biggest subnet, or is there a better approach?

Answer 1

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If you're sites are tied together with L3, I would suggest reusing the same VLAN numbers for the same VLAN purposes.
Regarding your /16, that's a rather large (IPv4) allocation.  Your sites are very large?  If not, you might want to use a smaller reservation per site, or even different reservations for different site tier sizes.
When it comes to allocating IP space, I would recommend you try to preserve large blocks for future allocations.  This can be accomplished by keeping the binary nature of address space allocations in mind.
This can be accomplished by keeping track of the binary tree "above" the allocated network block.  Only allocate similar or related network blocks from within the same "parent" tree.
For example, if your first allocation is a /30, anywhere within your /16, you now still have an available /15 and /14.  However, where you allocate your second allocation, another /30, could lose your /15 or the /14.  Consider if the two /30s were sequentially allocated, the one at the last /30 of the top /15 and the other at the first /30 at the top of the bottom /15.