•Networkingat Layer 2: Benefits of Ethernet Frames on page9
•Networkingat Layer 2: Forwarding VLAN TaggedFrames on page 11
•Networkingat Layer 2: Forwarding Dual-TaggedFrames on page 13
•Networkingat Layer 2: Logical InterfaceTypes on page 14
•AMetro Ethernet Network with MX Series Routers on page15
•Layer2 Networking Standards on page 17
Networkingat Layer 2: Forwarding VLAN TaggedFrames
VLANtags were not developed as a wayto limit network node table entries. They were
originallyinvented to allowLAN switches to distinguish between physical groups of LAN
portsand logical groups of LAN ports. In other words, there was a needto configure a
LANswitch (or group of local LAN switches) toknow that “these ports belong to VLAN
A”and “these ports belong to VLAN B.”
Thiswas important because of how all LANs, not just Ethernet, work atthe frame level.
Lotsof frames on a LAN are broadcast to all stations(hosts and network nodes) on the
LANsegment. Also, multicasting works byflooding traffic within the VLAN. The stations
thatreceived broadcast framesform the broadcast domain of the LAN. Only Ethernet
framesbelonging to same broadcastdomain are forwarded out certain ports on the LAN
switch.This preventsbroadcast storms and isolates routinecontrol frames onto the LAN
segmentwhere they make the most sense.
TheVLAN tag was invented todistinguish among different VLAN broadcast domains on
agroup of LAN switches. The VLAN tag is a two-byte fieldinserted between the source
MACaddress and the Ethertype (or length) fieldin an Ethernet frame. Another two-byte
field,the TagProtocol Identifier (TPI or TPID), precedes the VLAN tag field.
Twofields were necessaryto hold one piece of information, the VLAN tag, to enable
receiverstodistinguish between untagged or plain Ethernet framesand those containing
VLANtags. A mechanism wasrequired to differentiatebetween the Ethertype and length
fieldfor the untagged case and to distinguishamong VLAN tag, Ethertype, and length
fieldfor the tagged case. Theanswer was to constrain the TPID field to valuesthat were
notvalid Ethernet frame lengthsor defined as valid Ethertypes. The first VLAN tag added
toan Ethernet frame is alwaysindicated by a TPID value of 0x8100. This is not the VLAN
identifier,which appears in the next two bytes.
InFigure 1 on page 12, a native or normal Ethernet frame is comparedto a VLAN-tagged
Ethernetframe. The lengths of each field, in bytes,is shown next to the field name.
11Copyright© 2010, Juniper Networks,Inc.
Chapter1: Overview of Ethernet Solutions