帧中继基础.docx
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帧中继基础
帧中继基础
学习笔记
2009.6.13—6.15
障碍问题记录：
1.FECN/BECN由谁设置，是如何实现流量控制的；
2.LMI的数值记忆:
3.子接口
术语摘要:
VC/PVC/SVC/CIR/EIR/Bc/Be/Tc/DLCI/LMI/FECN/BECN/DE/DCE/DTE；
FRF,VC,PVC,SVC,DTE,DCE,LMI,accessrate,accesslink,FRF.9,FRF.5,
FRF.8,ServiceInterworking,FRF.12,FRF.11-c,VoFR,LAPF,NLPID,DE,FECN,
BECN,DualFIFO,LZS,DLCI
1.帧中继基本概念
源于80年代迅速发展于90年代，现在慢慢向基于IP的VPN和MPLS转移，在可见的未来，帧中继仍然是企业网络WAN连接的一个主要方式；
FR是第二层WAN连接技术，基于统计复用原理的分组交换（packet-switched）协议；
帧中继封装
标准Q.922-A封装：
（头部没有指示封装的上层协议，DTE设备无法识别封装的协议）
Cisco和RFC1490封装：
（cisco和IETF）
RFC2427封装：
问题1：
模拟实验中没有CISCO封装类型，是怎么回事？
如下所示：
R2（config-if）#encapsulationframe-relay?
（cisco3620）
ietfUseRFC1490/RFC2427encapsulation
+
R0（config）#ints1/0
R0（config-if）#encframe?
（cisco7200）
MFRMultilinkFrameRelaybundleinterface
ietfUseRFC1490/RFC2427encapsulation
问题2:
当将LMItype设置为cisco或ANSI，DLCI可用值都为16-1007，这是怎么回事？
（模拟实验中设置lmi-type为cisco时，lineprotocol是down的，是不是模拟器不支持lmi-type为CISCO，所以同时出现下面情况?
）如下所示：
R3（config-if）#encframe-relay
R3（config-if）#framelmi-typecisco
R3（config-if）#frameinterface-dlci?
<16-1007>DefineaswitchedorlocallyterminatedDLCI
R3（config-if）#framelmi-typeq933a
R3（config-if）#frameinter?
<16-1007>DefineaswitchedorlocallyterminatedDLCI
DLCI:
10个比特，因此数值范围为0-1023，一部分LMI使用,一部分用作组播，一部分用作单播；
显示拥塞通知：
FECN/BECN比特位；当帧中继网络发生拥塞的时候，交换机设置这些位来指示网络拥塞；接收者上层协议根据FECN降低速率，发送者根据BECN降低速率；？
？
DE（DiscardEligibility合适、适宜）比特：
也是用作拥塞管理，当速率超过了CIR时，交换机设置该位，在交换机因网络过于拥塞而不能处理时，交换机将丢弃标记了DE的帧；从头部可以看出，帧中继没有错误检测和恢复机制，它依靠虚电路端点设备的上层协议（如TCP,或应用）检测帧丢失和进行错误恢复；
帧中继标准化（standardization）
Gang-of-four,ITU-T
1990,developedanextensiontotheFrameRelayprotocolwithenhancementfeatures.Theseenhancements,knownastheLocalManagementInterface（LMI）,provideadditionalcapabilitiesforFrameRelayinacomplexinternetworkingenvironment.TheLMIenhancementfeaturesareasfollows:
∙Globaladdressing—ThisgivesFrameRelayDLCIglobalinsteadoflocalsignificance.
∙Multicasting—ThisallowsasingleframetobedeliveredtomultiplereceiversintheFrameRelaynetwork.
∙VirtualCircuitStatusMessages—TheseprovidekeepalivesandstatuscommunicationmechanismstoallowFrameRelaydevicestodetectnewPVCsortoperiodicallyreportthestatusofitsconnections.
LMI
TheLMIisastandardizedsuiteofprotocolsthatFrameRelayusestoexchangemessagesandinformationonthestateofaconnection.UsingtheLMIsignalingmechanism,bothendsofaFrameRelayconnectioncancommunicatewitheachothertodeterminethestatusofthePVC,theDLCIvaluedefinedfortheconnection,andwhethertheinterfaceisactive.Morerecently,functionalitiessuchascongestionmanagement,PVCauto-discovery,multicasting,andaddressresolutionhavebeenadded.
LMI帧格式：
∙Flag：
指示LMI帧的开始和结束；
∙LMIDLCI:
区分该LMI帧和普通数据帧，LMI类型为cisco时，该值为1023,LMI类型为ANSI和Q933A（ITU-T）时，该值为0；
∙Unnumberedinformationindicator:
Setsthepoll/finalbittozero
∙？
？
ProtocolDiscriminator（鉴别器）—ContainsavalueindicatingthatthisisanLMIframe
∙CallReference：
未使用，为0；
∙？
？
MessageType—Labelstheframeasfollows:
-Status-enquiry/inkwairi/询问message—Allowsauserdevicetoenquireaboutthestatusofnetwork
-Statusmessage—Respondstostatus-inquirymessages;includeskeepalivesandPVCstatusmessages
∙InformationElement（IE）—containsavariablenumberofindividualinformationelements;consistofthefollowing:
-IEIdentifier—UniquelyidentifiestheIE
-IELength—IndicatesthelengthoftheIE
-Data—Consistsofoneormorebytescontainingencapsulatedupper-layerdata
∙FrameCheckSequence（FCS）—Ensurestheintegrityofthetransmittedframe
R3#debugframelmi
FrameRelayLMIdebuggingison
DisplayingallFrameRelayLMIdata
R3#
00:
09:
20:
Serial0/0（out）:
StEnq,myseq18,yourseen17,DTEup
00:
09:
20:
datagramstart=0x39B0AF4,datagramsize=14
00:
09:
20:
FRencap=0x00010308
00:
09:
20:
00759501010103021211
00:
09:
20:
00:
09:
20:
Serial0/0（in）:
Status,myseq18,paksize14
00:
09:
20:
RTIE1,length1,type1
00:
09:
20:
KAIE3,length2,yourseq18,myseq18
00:
13:
30:
Serial0/0（out）:
StEnq,myseq43,yourseen42,DTEup
00:
13:
30:
datagramstart=0x39B0D74,datagramsize=14
00:
13:
30:
FRencap=0x00010308
00:
13:
30:
00759501010003022B2A
00:
13:
30:
Serial0/0（in）:
Status,myseq43,paksize19
00:
13:
30:
RTIE1,length1,type0
00:
13:
30:
KAIE3,length2,yourseq43,myseq43
00:
13:
30:
PVCIE0x7,length0x3,dlci301,status0x2
FrameRelayMulticasting
TheFrameRelayLMIextensionssupportthemulticastingfeature.InaFrameRelaynetwork,themulticastgroupsaredesignatedbyaseriesoffourreservedDLCIvalues（1019to1022）.WhenaFrameRelayDTEsendsdataononeofthesereservedDLCIs,theframesarereplicatedbytheFrameRelayswitchessupportingmulticastingandaresubsequentlysenttoallexitpointsinthedesignatedset.ThereceiversofthemulticastframeshavetobeconfiguredtolistenontheseDLCIs.
ThemulticastingLMIextensiondefinesadditionalLMImessagesfornotifyinguserdevicesoftheaddition,presence,anddeletionofneworexistingmulticastgroups.InlargenetworksthatdeploydynamicroutingprotocolsoverFrameRelay,itismoreefficienttoexchangeroutinginformationbetweenonlythespecificroutersconfiguredtorunthedynamicroutingprotocols.TheroutingupdatesaresentusingframesonthemulticastDLCIs.
帧中继网络规划
A.规划帧中继网络需考虑的因素
●了解用户要求：
了解用户网络流量，清楚每个主要应用最小带宽要求，清楚用户关键业务（如：
ERP/Voice/Video）；
●了解网络协议类型（ProtocolMIX?
?
）:
Acommonprotocolmixonanetworkincludesthefollowing:
HTTP/Multicast/Routingprotocolupdates/InteractiveTelnet/SSH/FTPtransfers/SMTP/Real-timevoiceandvideo
●理解流量模式：
突发的数据流量（如FTP）会导致关键业务应用（如ERP）和实时应用（如电话会议、视频点播）出现延时、抖动、超时中断等问题；
●使用75%规则规划带宽
B.拓扑选择
StarTopology（Hub-and-Spoke）
简单易于管理，最少PVC需求，费用最低；单点故障，不可扩展，核心路由器限制了性能；
部分网状结构
帧中继网络一个常见问题--SplitHorizon
帧中继术语及简单描述：
Term
Definition
Description
CIR
CommittedInformationRate
Rateinbpsatwhichthenetworkcommitstotransferuserdataundernormalconditions,averagedoveraminimumtimeinterval,Tc.
EIR
ExcessInformationRate
ThemaximumrateinbpsinexcessoftheinsuredCIRatwhichthenetworkattemptstotransferuserdataundernormalconditions,averagedoveraminimumtimeinterval,Tc.
Bc
CommittedBurst
Themaximumamountofdatainbitsthatthenetworkcommitstotransfer,undernormalconditions,duringaTcinterval.
Be
ExcessBurst
ThemaximumamountofdatainbitsinexcessoftheBcthatthenetworkattemptstotransfer,undernormalconditions,duringaTcinterval.
Tc
CommittedRateMeasurementInterval
TciscomputedasTc=Bc/CIR.
DLCI
DataLinkConnectionIdentifier
Aunique10-bitnumberassignedtoaPVCendpointinaFrameRelaynetworktoidentifyaPVCconnection.
LMI
LocalManagementInterface
AsuiteofmaintenanceprotocolsforFrameRelay.
PVC
PermanentVirtualCircuit
Alogicalconnectionthatispermanentlyestablished.
SVC
SwitchedVirtualCircuit
Alogicalconnectionthatisestablishedonlyondemand.
VC
VirtualCircuit
Alogicalconnectionestablishedbetweentwoendpointsinanetwork.
DCE
DataCircuit-TerminatingEquipment
DCEdevicesprovideaclockingsignalusedtosynchronizedatatransmissionbetweenDCEandDTEdevices.
DTE
DataTerminalEquipment
DTEdevicesuseclockingprovidedbyDCEdevices.
2.帧中继实验
实验一：
点对点子接口，物理接口，三个网段
R0配置3个点到点子接口，连接到R2/R3/R4，R1和R3之间VC使用cisco封装；R2/R3在物理接口上配置，R4配置一个点到点子接口；
R0#conft
R0（config）#interfaces1/0.103point
R0（config-subif）#ipadd172.31.3.1255.255.255.0
R0（config-subif）#frame-relayinterface-dlci103?
ciscoUseCISCOEncapsulation
ietfUseRFC1490/RFC2427Encapsulation
pppUseRFC1973EncapsulationtosupportPPPoverFR
protocolOptionalprotocolinformationforremoteend
R0（config-subif）#frame-relayinterface-dlci103cisco
R0（config-fr-dlci）#exit
为什么R2/R3平不通自己的串口地址了？
？
R0#shframmap
Serial1/0.102（up）:
point-to-pointdlci,dlci102（0x66,0x1860）,broadcast,IETF
statusdefined,active
Serial1/0.103（up）:
point-to-pointdlci,dlci103（0x67,0x1870）,broadcast,CISCO
statusdefined,active
Serial1/0.104（up）:
point-to-pointdlci,dlci104（0x68,0x1880）,broadcast,IETF
statusdefined,active
R0#shints1/0.102
Serial1/0.102isup,lineprotocolisup
HardwareisM8T-X.21
Internetaddressis172.31.2.1/24
MTU1500bytes,BW1544Kbit,DLY20000usec,
reliability255/255,txload1/255,rxload1/255
EncapsulationFRAME-RELAYIETF
Lastclearingof"showinterface"countersnever
R0#shrun
interfaceSerial1/0
noipaddress
encapsulationframe-relayIETF
serialrestart-delay0
!
interfaceSerial1/0.102point-to-point
ipaddress172.31.2.1255.255.255.0
frame-relayinterface-dlci102IETF
!
interfaceSerial1/0.103point-to-point
ipaddress172.31.3.1255.255.255.0
frame-relayinterface-dlci103IETF
!
interfaceSerial1/0.104point-to-point
ipaddress172.31.4.1255.255.255.0
frame-relayinterface-dlci104IETF
!
R2#shframemap
Serial0/0（up）:
ip172.31.2.1dlci201（0xC9,0x3090）,dynamic,
broadcast,
IETF,statusdefined,active
R2#shins0/0
Serial0/0isup,lineprotocolisup
HardwareisM4T
Internetaddressis172.31.2.2/24
MTU1500bytes,BW1544Kbit,DLY20000usec,
reliability255/255,txload1/255,rxload1/255
EncapsulationFRAME-RELAYIETF,crc16,loopbacknotset
Keepaliveset（10sec）
Restart-Delayis0secs
LMIenqsent444,LMIstatrecvd445,LMIupdrecvd0,DTELMIup
LMIenqrecvd0,LMIstatsent0,LMIupdsent0
LMIDLCI0LMItypeisANSIAnnexDframerelayDTE
Broadcastqueue0/64,broadcastssent/dropped9/0,interfacebroadcasts0
Lastinput00:
00:
01,output00:
00:
01,outputhangnever
R2（config-if）#frame-relayinterface-dlci201
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