Networkingat Layer 2: Forwarding Dual-TaggedFrames
Theuse of VLAN tagging to group (or bundle) sets of MACaddresses is a start toward
amethod of forwarding LAN traffic based on informationfound in the frame, not on IP
addressin the packet. However,there is a major limitation in trying to build forwarding
tablesbased on VLAN tags. Simply put, there arenot enough VLAN tags.
Twelvebitsonly supply enough space for 4096 unique VLAN tags. This is hardlyenough
forall the LANs on a largecorporate campus, letalone the whole world. A 12-bit tag might
sufficefor the local campus arena, but for the metropolitanarea, comprising a whole
city,more bits are needed.
Thenumber of bits in the VLAN tag, two bytes for the TPID and twobytes for the TCI
field,are fixed and cannot be extended.However, another VLAN tagcan be added to the
frame,forming an inner and outer VLAN tag arrangement. This arrangementis defined
inthe IEEE 802.1ad specification and applies to devices thatfunction on the provider
bridgelevel. This means that Ethernet framestagged at the local (or customer) VLAN
levelcan receive another outer VLAN tagwhen they are sent to the provider's LAN
switches.As a result, Ethernet frames can be switchedacross a metropolitan area, not
justamong the local organizations devices atthe campus level.
Theouter tagdefined in IEEE 802.1ad is often called the VirtualMetropolitan Area Network
(VMAN)tag, a good way to recall the intended scopeof the specification. The outer tag
isplaced after the MAC source address,moving the inner tag backwards in the frame.
Bothtags can be added at the same time by the same device (calleda push/push
operation),changed by a device (a swap operation),or removed by a device one at a
time(pop) or together (pop/pop). Devicescan perform elaborate variations on these
operations(such as pop/swap/push)to accomplish the necessary networking tasks
withthe frames they process.
TheIEEE specification indicates that the outertag of a doubly-tagged Ethernet frame
shouldhave a TPID value of 0x88a8. Any network devicecan easily tell if it has received
aframe with one tag(0x8100) or two tags (0x88a8). However,because the value 0x8100
alwaysmeans that a VLAN tag is present, mostvendors and networks use the same
TPIDvalue (0x8100) for the inner and outer tags. As long asthe configuration and
processingare consistent, there is no confusion, and the TPID valuecan usually be
changedif necessary.
Howdo nested VLAN tags solve the VLAN numbering limitation?Taken together,the
twoVLAN tags can be thought of as providing 24 bits fortagging space: 12 bits at the
outerlevel and 12 bits at the inner level. However,it is important to realize thatthe bits
arenot acted on as if they were all one tag. Evenwhen the tags are nested, bridges on a
providerbackbone will normally only switch on the outer VLAN tag.All in all, the inner
12-bittagging space is more than adequatefor a Metro Ethernet network. Any limitations
inthe VLAN tag space can be addressedby adding more VLAN tags to the basic Ethernet
frame.
Related
Documentation
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13Copyright© 2010, Juniper Networks,Inc.
Chapter1: Overview of Ethernet Solutions