Figure1: Native (Normal) and VLAN-TaggedEthernet Fames
TheVLAN tag subtracts four bytesfrom the total MTU length of the Ethernet frame, but
thisis seldom a problem if kept in mind. When this tag is used in an Ethernetframe, the
framecomplies with the IEEE 802.1Q (formerly IEEE 802.1q) specification.
Together,thefour added bytesform the VLAN tag, but the individual fields that comprise
itare more important. The 2–byte TPID field is just a number and hasno structure, only
havingallowed and disallowedvalues. However, the 2-byte TagControl Information
(TCI)field has a defined structure:
Thethree bits of the User Priority field are defined by the IEEE 802.1pspecification.
Thesecan mimic class-of-service (CoS)parameters established at other layersof the
network(IP precedence bits, or MPLS EXP bits, and so on).
TheCanonical Format Indicator (CFI) bit indicateswhether the following 12 bits of
VLANidentifier conform to Ethernet or not. For Ethernet frames, this bit is alwaysset
to0. (The other possible value, CFI=1, is usedfor Token Ring LANs, and taggedframes
shouldnever be bridged between an Ethernet and TokenRing LAN regardless of the
VLANtag or MAC address.)
The12-bit VLAN ID allows for 4096 possible VLANs, but not all valuesare used in all
cases.
Related
Documentation
MXSeries Ethernet Services Routers SolutionsPage
EthernetTerms and Acronymson page 3
Networkingand Internetworking with Bridges and Routerson page 6
NetworkAddressing at Layer 2 and Layer3 on page 7
Networkingat Layer 2: Benefits of Ethernet Frames on page9
Networkingat Layer 2: Challenges of EthernetMAC Addresses on page 10
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
Copyright© 2010, Juniper Networks,Inc.12
Junos10.4 MX Series Ethernet ServicesRouters Solutions Guide