IP address schemes and VLANs

[snared04drummer]

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?

[Jon Marshall · Accepted solution]

The only time you would really need to be concerned with using different vlan IDs per site is if you were connecting the sites with L2 links. If you are using L3 routed links between sites which is probably more common then it really makes no difference and in fact may make it easier to troubleshoot if you do use the same IDs for the same purpose in each site.

It's really a matter of preference.

IP addressing, key thing to ensure is that you can summarise from each site which if you are allocating a class B to each site sounds like you are.

Ideally you would want to allocate the same sized subnet per vlan so it really shouldn't make much difference which end you start although I would generally start from the bottom and work upwards.

For data/VOIP vlans a common size would be a class C or even half a class C which I have used before.

You will also need a class C (or more) for point to point links and you can if you want start at the top end for this.

Again, a matter of preference really.

Like I say summarisation is the most important thing.

Edit - do you need different sized subnets ?

Jon

[snared04drummer]

My gut reaction is to agree with you about VLAN standardization since they are in fact connecting their sites with L3 links.  However, their Cisco representative had previously insisted that unique VLAN id's would help for bringing the network under central management using WiSm.  I'm not familiar with this myself, but this was the conversation under which this idea was floated.

Yes, the subnets will need to be different sizes.  I'm assigning a /16 block of addresses for each site, and then trying to match their individual subnet needs as far as subnetting that address space.

[Jon Marshall]

Can't really comment on the wireless aspect.

You may want to post the same question in the Wireless forums or someone on this forum my be able to comment.

IP addressing, probably easiest to just start from bottom and work you way up.

Jon

[devils_advocate]

I am not sure on the wider aspect of using unique VLAN ID's at each site in regards to WiSm but my previous company had over 400 sites and we used the same VLAN ID's at each without issue.

Personally I prefer the consistency of using the same ID's at each site, especially if the format of the hosts is going to be the same, i.e VLAN 10 for Data, 20 for VOIP, 30 for Wireless etc etc.

VLAN ID's are usually down to personal preference, likewise with Subnets.

Using a /16 as a 'supernet' is good practice in regards to consistency as far as I am concerned and its something I would do myself to make management easier. Using a /24 for each subnet is also something I try and do, unless there is a reason for a larger subnet. You usually have to look at the bigger picture when planning for IP addressing as the objective is to make routing tables as small as possible by using summary routes where possible :)

[Joseph W. Doherty]

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Posting

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.