I have several Layer 3 switch/routers which are all connected via an OSPF routed network. Also attached to each switch are two other networks. I have to assign a VLAN to each of these networks, I figure that I can just reuse the same two VLANS on each switch as all the traffic leaving the switch is untagged (as its routed, not on a VLAN trunk) - is there any reason I shouldn't do this and give each network a different VLAN? For example:
There is no significant reasons to use different vlan in your situation, but, some vendors(cisco at least) recommends to use this way:
for example, we have N branches, and we need four routed broadcast domains(LAN) on each, let`s calculate them:
branch1 = 1 branch2 = 2 ... branchN = N
- worstations - ip:10.N.0.0/24, vlan 100+N
- servers - 10.N.64.0/24, vlan 200+N
- voice - 10.N.128.0/24, vlan 300+N
- mgmt - 10.N.192.0/24, vlan 400+N
for tunnel interfaces networks, (we are) as usually, uses 172.16.0.0 network, with same rule: 172.16.N.0/30 - left side of the ring(or main uplink) 172.16.N.5 - right side of the ring(or reserve uplink)
if you have more than 255 branches, than you just need to use binary calculation, and you will get much more economical utilisation of address space(say if you need formula, i can provide it for you).
imho, this is good way, because you always know, in one look, what branch network, what branch vlan and what tunnel you are looking.
Here's another take in the opposite direction - a reason to reuse VLAN numbers since you can. I had a situation where I was working with some poor-quality IP phones that would download their configuration from a TFTP server, not from DHCP options. I wanted to be able to pick up a phone in any location and drop in any other without having to reconfigure, and the phones required the IP address and file name of the TFTP configuration to be statically configured on the phone. The config file had to specify the tagged VLAN number that the phones should jump to.
So, to make those horrible phones work the way I wanted, I had to make the voice VLAN the same VLAN number at every location. Sure I could have set up several different TFTP servers and/or config files and programmed each phone separately and reprogrammed them every time a phone moved. I just wanted to point out that there could be many other situations where re-using VLAN numbers would help you solve another problem.
There isn’t anything technologically wrong with it, but it's not ideal. From a design perspective it’s prone to problems down the road. Jens Link’s comment is very accurate; if you have an issue, this will compound the problem and complexity of trying to figure out what’s wrong. That would be even more exacerbated during intense, large-scale outages.
Although you haven’t mentioned network growth in your OP, it’s reasonable to assume that you’ll eventually deploy a switch downstream from one of your main routers. When you have matching VLAN-id’s in neighboring areas, this eliminates the possibility (well, makes it a PITA) of dual-homing connections to multiple upstream routers. You may not be forecasting this now, but it’s best to set things up for future success when it's an option, rather than a necessity.