Cisco Presentation GuideOSPF • Frame-relay cloud represented with NBMA network node type in OSPF topology graph • Non-Broadcast Multi Access = multiple neighbors accessible via

© 2005 Petr Grygarek, Advanced Computer Networks Technologies 1

Frame RelayFrame Relay

Petr GrygPetr Grygáárekrek

2© 2005 Petr Grygarek, Advanced Computer Networks Technologies

Basic characteristicsBasic characteristics• Used to implement WAN links over shared infrastructure Used to implement WAN links over shared infrastructure

• infrastructure provided by independent operatorinfrastructure provided by independent operator• based on Frame-Relay switches or other technology (ATM, IP)based on Frame-Relay switches or other technology (ATM, IP)

• Frame-Relay may be implemented as access technology onlyFrame-Relay may be implemented as access technology only

• Layer 2 technologyLayer 2 technology• Assumes fast and reliable links between switching elements in FR cloudAssumes fast and reliable links between switching elements in FR cloud

• No hop-by-hop error-control and flow-controlNo hop-by-hop error-control and flow-control• upper layers of user devices have to solve it by themselvesupper layers of user devices have to solve it by themselves

• Fast and efficient switchingFast and efficient switching• Independent of layer 3 protocol(s) usedIndependent of layer 3 protocol(s) used

• Virtual-circuit basedVirtual-circuit based• Advantage: fast (manual) VC creation/deletionAdvantage: fast (manual) VC creation/deletion

• comparing to installation of leased linecomparing to installation of leased line• QoS-enabled (by provider cloud, end-to-end)QoS-enabled (by provider cloud, end-to-end)

• Per-VC CAR, Bc, BePer-VC CAR, Bc, Be


Infrastructure and DTEsInfrastructure and DTEs

Most-common DTEs• Router• Frame Relay Access Device (FRAD)


Local access loopLocal access loop

Subscriber connected by single leased line•Typical access line rates 56kbps to (approx) 54 Mbps


Virtual circuitsVirtual circuits• Set-up over FR infrastructure (cloud)Set-up over FR infrastructure (cloud)• Simulate “real” circuits with predefined parametersSimulate “real” circuits with predefined parameters

• Permanent Virtual Circuit (PVC)Permanent Virtual Circuit (PVC)• requires (software based) preconfiguration by infrastructure providerrequires (software based) preconfiguration by infrastructure provider• used most oftenused most often

• Switched Virtual Circuit (SVC)Switched Virtual Circuit (SVC)• created on user-device (DTE) demandcreated on user-device (DTE) demand• need for signalling standard between DTE and FR switch and unique need for signalling standard between DTE and FR switch and unique

DTE addressesDTE addresses• Q.922/Q.933 – similar to ISDN signalling (QoS-aware)Q.922/Q.933 – similar to ISDN signalling (QoS-aware)

• not used very oftennot used very often


VC PrincipleVC Principle

FR SwitchFR Switch

FR SwitchFR Switch FR SwitchFR Switch

II11

II22

DTEDTE A A

II11 II11

II11

II22

II22II22

II33

II33 1717

110110

5656 121121DLCIin IIinin DLCIDLCIoutout IIoutout --------------------------------------------------------------------------------56 56 II3 3 121 121 II22

121 121 II22 56 56 II33

DLCIin IIinin DLCIDLCIoutout IIoutout --------------------------------------------------------------------------------121 121 II1 1 110 110 II22

110 110 II22 121 121 II11

FR SwitchFR SwitchDTEDTE B B

DLCIin IIinin DLCIDLCIoutout IIoutout --------------------------------------------------------------------------------110 110 II2 2 17 17 II33

1717 II33 110 110 II22


Data Link Connection IdentifierData Link Connection Identifier(DLCI)(DLCI)

• Identifies individual virtual circuits on local loopIdentifies individual virtual circuits on local loop• and on inter-FR-switch links, if network cloud and on inter-FR-switch links, if network cloud

implemented using FR switchesimplemented using FR switches

• Assigned by FR cloud operatorAssigned by FR cloud operator• Locally uniqueLocally unique

• can be reused on internal linkscan be reused on internal links

• Frame Relay switches may change DLCI along Frame Relay switches may change DLCI along PVC pathPVC path


Virtual circuit advantagesVirtual circuit advantages

• Requires only one physical access line regardless Requires only one physical access line regardless to number of virtual circuits establishedto number of virtual circuits established• doesn’t require multiple leased lines to customer sitedoesn’t require multiple leased lines to customer site• per PVC payment, but smaller fees compared to per PVC payment, but smaller fees compared to

separate leased linesseparate leased lines

• Flexible – new circuits can be quickly added and Flexible – new circuits can be quickly added and unnecessary ones deleted on as-needed basisunnecessary ones deleted on as-needed basis


Frame Relay InterfacesFrame Relay Interfaces

• User-to-Network Interface (UNI)User-to-Network Interface (UNI)• between customer and FR operatorbetween customer and FR operator

• Network-to-Network Interface (UNI)Network-to-Network Interface (UNI)• between two FR operatorsbetween two FR operators


Frame Relay Frame Format Frame Relay Frame Format (Q.922)(Q.922)

• Similar to HDLC• Variable frame length (max 8/16kB)• Header Fields

• C/R, EA – Commonly unused• FECN – Forward Explicit Congestion Notification• BECN – Backward Explicit Congestion Notification• DE – Discard Eligibility


Local Management Interface (LMI)Local Management Interface (LMI)

• Operates between DTE and FR access switchOperates between DTE and FR access switch• FR DTE, FR DCEFR DTE, FR DCE

• VC status messages • DTE-DCE line keepalives

• DTE may obtain a list of VCs from FR switch


Congestion avoidance and recoveryCongestion avoidance and recovery• Congestion avoidanceCongestion avoidance

• ability to inform stations about emerging congestionability to inform stations about emerging congestion

• Congestion recoveryCongestion recovery• actions taken to recover from congestion and return actions taken to recover from congestion and return

to normal operating stateto normal operating state• frame discardframe discard

Separate for every VC and directionSeparate for every VC and direction


Congestion avoidance Congestion avoidance implementationimplementation

• BBackward ackward EExplicit xplicit CCongestion ongestion NNotification otification (BECN)(BECN)• Set by intermediate switching elements to ask source Set by intermediate switching elements to ask source

to slow downto slow down• Forward Forward EExplicit xplicit CCongestion ongestion NNotification otification

((FECNFECN))• Set by intermediate switching elements to inform Set by intermediate switching elements to inform

destination it should ask source to slow downdestination it should ask source to slow down• Destination utilizes upper layer protocol to Destination utilizes upper layer protocol to

accomplish thataccomplish that• (e.g. TCP starts to delay ACKs) (e.g. TCP starts to delay ACKs)


Congestion recoveryCongestion recoveryimplementationimplementation

• Discard Eligibility (DE) bitDiscard Eligibility (DE) bit• Set by access FR switch for nonconforming framesSet by access FR switch for nonconforming frames

• May be also set by source DTE to give priorities to traffic May be also set by source DTE to give priorities to traffic entering FR cloudentering FR cloud

• If congestion occurs on some link, frames with DE If congestion occurs on some link, frames with DE set should be dropped firstset should be dropped first

• If congestion persists even when DE frames If congestion persists even when DE frames discarded, network have to start discarding of discarded, network have to start discarding of non-DE framesnon-DE frames


Frame Relay QoSFrame Relay QoS


FR virtual circuit QoS parametersFR virtual circuit QoS parametersParameters take into account bursty nature of trafficParameters take into account bursty nature of traffic• BcBc – committed burst – committed burst

• Maximum amount of data the network commits to deliver over a VC during Maximum amount of data the network commits to deliver over a VC during appointed interval Tcappointed interval Tc

• Committed information rate (CIR)Committed information rate (CIR)• estimate of normal traffic during a busy periodestimate of normal traffic during a busy period• CIR=Bc/Tc CIR=Bc/Tc • Sum of CIRs of all VCs commonly greater than physical access line speedSum of CIRs of all VCs commonly greater than physical access line speed

• Be Be - excess burst - excess burst• Maximum amount of data by which a user can exceed Bc during an interval TcMaximum amount of data by which a user can exceed Bc during an interval Tc• Delivered at lower probability than the data within BcDelivered at lower probability than the data within Bc

• Excess Information Rate (EIR)Excess Information Rate (EIR)• EIR=Be/TcEIR=Be/Tc• Frame above CIR+EIR (maximum rate) are discardedFrame above CIR+EIR (maximum rate) are discarded


Service Level AgreementService Level Agreement

Access rate = physical access line bitrateAccess rate = physical access line bitrate


Typical rate limiting usageTypical rate limiting usageOften physical local loop rate at central site

outperforms physical local loop rate at branch site• Because we want to communicate with multiple

branch sites


For Further StudyFor Further Study

•See full version of Frame Relay See full version of Frame Relay presentation for additional detailspresentation for additional details

•Topics concerning TCP/IP over Topics concerning TCP/IP over Frame Relay will be discussed Frame Relay will be discussed during lab workduring lab work


TCP/IP over Frame RelayTCP/IP over Frame Relay

(To be discussed on lab)(To be discussed on lab)


Typical Frame Relay VC TopologiesTypical Frame Relay VC Topologies


Relationship between IP subnets and Relationship between IP subnets and FR virtual circuitsFR virtual circuits

• One physical router interface may correspond to One physical router interface may correspond to one VCone VC

• One physical router interface may accommodate One physical router interface may accommodate multiple VCs (multiple VCs (more obviousmore obvious))• logical subinterface assigned to every VClogical subinterface assigned to every VC

• bound to right VC by DLCI configurationbound to right VC by DLCI configuration

• behaves like physical interface from routing behaves like physical interface from routing protocol’s point of viewprotocol’s point of view


SubinterfacesSubinterfaces


Frame Relay IP (sub)interface TypesFrame Relay IP (sub)interface Types

• Point-to-pointPoint-to-point• separate IP subnet for every subinterfaceseparate IP subnet for every subinterface

• Point-to-multipointPoint-to-multipoint• common IP subnet on all VCscommon IP subnet on all VCs

• similar to broadcast network, but without broadcast similar to broadcast network, but without broadcast capabilitycapability

• router interface “routes” between connected VCsrouter interface “routes” between connected VCs


IP to DLCI mappingIP to DLCI mapping

• Point-to-point interfacePoint-to-point interface• Single IP address and DLCI configuredSingle IP address and DLCI configured

• Point-to-multipoint interfacePoint-to-multipoint interface• IP address of local side (common to all VCs) is IP address of local side (common to all VCs) is

configuredconfigured• Static maps are configuredStatic maps are configured

• for every possible peer IP address we configure DLCI of for every possible peer IP address we configure DLCI of VC connected to that peerVC connected to that peer


Inverse ARP (INARP)Inverse ARP (INARP)INARP allows automatic discovery of peer IP address to INARP allows automatic discovery of peer IP address to

DLCI mappingDLCI mapping

• If no DLCI or static map is configured on FR If no DLCI or static map is configured on FR (sub)interface, router sends INARP FR Frame to every (sub)interface, router sends INARP FR Frame to every known VCknown VC• List of available DLCIs can be obtained from FR switch List of available DLCIs can be obtained from FR switch

using LMIusing LMI• Peer responds with L3 address configured on it’s Peer responds with L3 address configured on it’s

interfaceinterface• DLCI-to-peer-IP-address mapping dynamic entry can DLCI-to-peer-IP-address mapping dynamic entry can

be createdbe created


Frame-relay routing problemsFrame-relay routing problems


Split Horizon problem (RIP)Split Horizon problem (RIP)

Split-horizon rule must be abandoned on physical interface accommodating multiple VCs


OSPFOSPF• Frame-relay cloud represented with NBMA network Frame-relay cloud represented with NBMA network

node type in OSPF topology graphnode type in OSPF topology graph• Non-Broadcast Multi Access = multiple neighbors accessible Non-Broadcast Multi Access = multiple neighbors accessible

via single physical interface, but no single broadcast frame via single physical interface, but no single broadcast frame can be sent to all of themcan be sent to all of them

• Be careful when configuring DR priorities on NBMA Be careful when configuring DR priorities on NBMA network with other than full-mesh topologynetwork with other than full-mesh topology• DR should be directly connected to all other routers using DR should be directly connected to all other routers using

VCVC

• Different OSPF timers used on NBMA networksDifferent OSPF timers used on NBMA networks

Documents

Cisco Presentation GuideOSPF • Frame-relay cloud represented with NBMA network node type in OSPF topology graph • Non-Broadcast Multi Access = multiple neighbors accessible via