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CCNP SWITCH Exam Tutorial:

The Difference Between Static And Dynamic VLANs

By Chris Bryant, CCIE #12933

Yeah, I know. You look at that title and say "I'll tell you the difference - one is static and the other is dynamic!"

Well, I can't argue with that answer. But for your CCNP SWITCH 642-813 exam, you might just need to give a bit more detail.

(Also, that's not the best answer to give in a job interview.)

During your CCNA studies, you learned that all Cisco switch ports are in VLAN 1 by default.. and you also learned that it's usually a good idea to create multiple VLANs and place the switch ports into the appropriate VLANs.

Why? When we have all of our hosts in one VLAN, that means they're all in one big broadcast domain. In turn, that means that when one host sends a broadcast, every other host in that VLAN will get a copy of that broadcast.

If that sounds like a lot of broadcasts, you're right. One reason we use multiple VLANs is to limit the scope of broadcasts - because when one host in a VLAN sends a broadcast, only members of that same VLAN will receive it.

Static VLANs aren't perfect, though.. If you have a host connected to port 0/2 and assigned it to VLAN 12, the config would look like this:

SW1(config-if)#int fast 0/2

SW1(config-if)#switchport mode access

SW1(config-if)#switchport access vlan 12

All well and good. But what happens if that port goes down? With Static VLANs, you'd have to plug that host into another switch port and statically assign the VLAN membership on that new port. Assuming we're connecting this host to fast0/3, the config is:

SW1(config-if)#int fast 0/3

SW1(config-if)#switchport mode access

SW1(config-if)#switchport access vlan 12

That doesn't seem like a big deal, but wouldn't it be nice to just be able to plug the cable into port 0/3 and have the VLAN membership changed dynamically?

We can have that happen by using Dynamic VLANs, where the port VLAN membership is changed...

wait for it...

... dynamically.

Dynamic VLANs are able to do this thanks to VMPS - a VLAN Membership Policy Server. VMPS is able to react to such a situation dynamically since the port's VLAN membership is actually decided by the source MAC address of the device connected to that port.

Yet another reason the first value a Cisco switch examines on an incoming frame is the source MAC address!

To review, the VLAN membership of a host is decided by one of two factors. With static VLANs, the host's VLAN membership is the VLAN to which its switch port has been assigned. With dynamic VLANs, it is dependent upon the host's MAC address.

For many more CCNP SWITCH exam tutorials and videos, just click that link... right after you take advantage of the following opportunity.

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