Analysing The IEEE802.1q Link Protocol
Introduction
Our VLAN Tagging page briefly covered the IEEE802.1q protocol and we are about to continue its analysis here.As m entioned previously,the IEEE802.1q tagging method is the most popular as it allows the em less integration of VLAN capable devices from all vendors who support the protocol.
So,without any m ore delay,let's get right into the protocol.
IEEE802.1q Analysis
soheThe IEEE802.1q tagging mechanism ems quite simple and efficient thanks to its 4-byte overhead squeezed between the Source Address and Type/Length field of our Ethernet II fram e:
The process of inrting the802.1q tag into an Ethernet II frame results in the original Fram e Check Sequence(FCS)field to become invalid since we are altering the frame, hence it is esntial that a new FCS is recalculated,bad on the new fram e now containing the IEEE802.1q field.This process is autom atically performed by the switch, right before it nds the fram e down a trunk link.Our focus here will be the pink3D block, labeled as the IEEE802.1q header.
The IEEE802.1q Header
As noted,the802.1q header is only4bytes or32bits in length while within this space there is all the necessary information required to successfully identify the fram e's VLAN
and ensure it arrived to the correct destination.The diagram below analys all fields contained in a802.1q header:
情商是什么意思
The structure is quite simple as there are only4fields when com pared with the11ISL has.We will continue by analysing each of the fields in order to discover what the protocol is all about.
TPID-Tag Protocol IDentifier
女娲补天翻译The TPID field is16bit long with a value of0x8100.It is ud to identify the fram e as an IEEE802.1q tagged fram e.
Note:The next three fields,Priority,CFI and VLAN ID are also known as the TCI(Tag Control Inform ation)field and are often reprented as one single field(TCI Field).
Priority
The Priority field is only3bits long but ud for prioritisation of the data this fram e is carrying.
ffcData Prioritisation is a whole study in itlf but we won't be analysing it here since it's well beyond the scope of our topic.However,for tho interested,data prioritisation allows us to give special priority to time-latency nsitive rvices,such as Voice Over IP (VoIP),over normal data.This m eans that the specified bandwidth is allocated for the critical rvices to pass them through the link without any delay.
晚上好的英语
The IEEE802.1p priority protocol was developed to provide such rvices and is utilid by the IEEE802.1q tagging protocol.
The Priority field is approximately3bits long,allowing a total of2^3=8different priorities for each frame,that is,level zero(0)to ven(7)inclusive.
CFI-Canonical Format Indicator
专科考研究生The CFI field is only1bit long.If t to'1',then it means the MAC Address is in
non-canonical form at,otherwi'0'means it is canonical form at.For Ethernet switches, this field is always t to zero(0).The CFI field is m ainly ud for com patibility reasons between Ethernet and Token Ring networks.
In the ca where a fram e arrives to an Ethernet port and the CFI flag is t to one(1), then that fram e should not be forwarded as it was received to any untagged port(Access Link port).
VLAN ID-Virtual Local Area Network Identifier
km是什么单位的The VLAN ID field is perhaps the most important field out of all becau we are able to identify which VLAN the fram e belongs to,allowing the receiving switch to decide which ports the fram e is allowed to exit depending on the switch configuration.
For tho who recall our VLAN Tagging page,we mentioned that the IEEE802.1q tagging method supports up to4096different VLANs.This number derives from the12bit VLAN ID field we are analysing right now and here are the calculations to prove this:
网上学韩语2^12=4096,which translates from VLAN0to VLAN4095inclusive.
Summary
国有化That com pletes our analysis on the IEEE802.1q protocol.As a last note,you should remember that this protocol is the most wide spread tagging method ud around the world that supports up to4096VLANs!
吉隆坡英文
Next up is the popular InterVLAN Routing topic,which is often a misunderstood and confusing subject,but we have managed to make it simple and clear.It's now time for a break-go get som e fresh air and we'll e you back in a few moments for the rest of our cool VLAN topic!
