1Q Introduction VLAN
VLAN is logical network distributed across multiple physical LANs. Since its a logical network, its scalable and flexible in terms of resource [bandwidth] management.
Tagging
Tagging assists logical seperation of networks. There are numerous advantages in doing this:
- Broadcast/Multicast messages no longer hogs network bandwidth.
- Security between virtual networks.
Trunk
Trunk is a physical link between two network devices, typically, switches that carries multiple virtual lans. There are some advantages in doing this:
- Scalability - Can't have a physical link for every vlan.
- Usage - Every vlan may not need entire bandwidth.
A tag is used on each frame to identifing its vlan. There are two common tagging mechanism:
- 802.1Q [IEEE standard]
- Inter-Switch Link [ISL, Cisco's proprietary standard]
Ethernet Frames
There are two types of ethernet standards.
- Original Ethernet standard published by DEC and Xerox in '82. RFC 894 defines this standard.
- Revised version by IEEE 802 committee. RFC 1042 captures this version and is commonly referred to as 802.3 Ethernet frame.
Below are the diagrams for these two ethernet frames.
Vlan tags apply to both types of ethernet frames.
802.1Q
Tags in 802.1Q mechanism are inserted inside the frame. A new 'Tag' field of 4 bytes width is inserted before the length field as shown below
- TPID carries a value of 0x8100 for IEEE 802.1Q [+ IEEE 802.1p]. It can also have 0x9100 and 0x9200. Double-tagged traffic uses these values.
- TCI comprises of User Priority, Canonical Format Indicator and VLAN ID values.
- User Priority - Priority levels [up to 8] can be set using these bits. IEEE 802.1p discusses these priority levels.
- Canonical Format Indicator [CFI] - This is used for backwards compatability between Token Ring networks and Ethernet switches. For Ethernet frames, CFI is always 0 and for frames in Token Ring network, CFI is 1. An ethernet switch drops frames with CFI 1.
- VLAN ID - VID with 0 and 4095 are reserved.
Introduction of this 4 byte tag has increased the maximum ethernet frame size from 1518 to 1522 bytes.
Native VLAN
Frames of native VLAN do not have tags when sent over a trunk. They are some times referred to as management VLANs. Any physical port configured to send/receive 802.1q traffic can have only one native VLAN.Labels: 802.1P, 802.1Q, Ethernet Frames, VLAN
