VLANS

VLAN's

What are VLAN's

Vlan's are virtual Local Area Networks. The idea here is to break up BCST domains into smaller chunks. Host in the same vlan can communicate with each other, but will have to be routed to get outside the vlan due to the ARP protocol being a BCST which is stopped at the layer 3 boundary.


Vlans should be defined by physical location. Like a switch closet should contain different vlans from other switching closets.

Vlans are a number that the switch uses with a port. 12 Bits 0-4095

Normal VLAN's 1-1005

1 Default Vlan Ports default here. Should move them to another non routed vlan for security reasons.

1002/4 Default FDDI vlans
1003/5 Default Token Rings

Extended Vlans 1006-4094


Once a vlan is created the switch or device will make a Cam table for that individual vlan. Also a spanning tree instance is created. Ports can be one of 3 things when working at the layer 2 level.

- Access
- Trunk
- Dynamic

Access - Where PC, Phones and printers plug in.

Trunks - Where Multiple Vlans can come through these ports usually used when leaving the device and going to the next devices

Dynamic - Automatically chooses port type


More on trunks:

Trunk Links are used when we want to have our vlans go between switches or routers. To do this we have to TAG the packets when they leave the device. We will add ISL(cisco prop) or 802.1Q(IEEE).


802.1Q has a 4 byte tag between the source and destination MAC and the Lenth/ethertype fields(check the pic below). The frame is modified for .1Q except for the native vlan. we don't usually see to much with ISL. Both sides of the link have to be the same, but they are local to the device.

FCS is also modified since we had changed the frame.


Native Vlan

Normal traffic will be sent untagged across the native Vlan. This is usually Vlan 1 in most devices.

QinQ support

we can have vlans stacked on top of each other on each frame. Sometimes known as Metro Tagging when used with Metro E environments.



Stay tuned for more later on...