Download - Lecture Week 3 Frame Relay Accessing the WAN. 3.1 Basic Frame Relay Concepts Accessing the WAN
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Lecture Week 3
Frame Relay Accessing the WAN
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3.1 Basic Frame Relay Concepts
Accessing the WAN
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Basics• Frame Relay is a data link layer packet-switching protocol
that uses digital circuits.
• It is used for medium to longer distances and for longer
connectivity.
• Leased lines also provide longer connectivity but a physical
circuit is used to make connection between 2 sites and the
same circuit path is used always.
• Frame Relay connections use logical circuits to make
connections between 2 sites. These logical circuits are
referred to as Virtual Circuits(VCs).
• Multiple VCs can exist on the same physical connection.
• VCs are Full duplex.
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Advantages of Frame Relay
• VCs overcome the scalability problems of
leased lines by providing multiple logical circuits
over the same physical connection.
• Only one serial interface of a router is needed to
handle the VC connections to multiple sites Whereas
using leased lines multiple serial interfaces are needed
to connect to multiple sites.
• VCs provide full connectivity at a much lower price
compared to leased lines.
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• Sub-interfaces
• Uses Shared bandwidth
• Local Management interface(LMI):
– used between the Frame relay DTE(eg.Router) and the
Frame Relay DCE(eg. Frame Relay switch)
– Defines how the DTE interacts with the DCE
– Locally significant
– Provides VCs status information(a keep-alive
mechanism)
– LMI standards : Cisco, ANSI, Q933a
The DTE and DCE must have the same LMI signaling type
Frame Relay Terminology
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• Data Link Connection Identifier(DLCI) :
– used to identify each VC on a physical interface (i.e.)
Each VC has a unique local address called a DLCI number.
– switch will map to the destination depending on
the DLCI number
– Inverse ARP is used to map DLCIs to next hop
addresses.
– Mapping can also be done manually.
– Its Locally significant.
– These numbers are given by the Frame relay
service providers, Service providers assign DLCIs in
the range of 16 to 1007.
Frame Relay Terminology
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• Virtual circuits are of two types:
– Permanent Virtual Circuits – PVCs
– Switched Virtual Circuits –SVCs
• Permanent Virtual Circuit :
– similar to a dedicated leased line , permanent connection.
– used when constant data is being generated.
• Switched Virtual Circuit :
– also called as Semi-permanent virtual circuit
– similar to a circuit switched connection where the VC is dynamically built and then torn down once the data has been sent.
– used when data has to be sent in small amounts and at periodic intervals.
Frame Relay Terminology
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• Committed Information Rate(CIR) :
– Average data rate measured over a fixed period
of time that the carrier guarantees for a VC.
– committed bandwidth
• Burst Rate(BR) :
– Average data rate provider guarantees for a VC.
– Excess bandwidth
Frame Relay Terminology
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• FECN and BECN :
– Forward Explicit Congestion Notification
– Backward Explicit Congestion Notification
• When congestion occurs switch marks the FECN
and BECN bits in the frame header.
• FECN is sent to the destination
• BECN is sent to the source
• Thereby notifying both source and destination
about the congestion.
• FECN = 0 and BECN =0 implies no congestion.
Frame Relay Terminology
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Frame Relay: An Efficient and Flexible WAN TechnologyFrame Relay reduces network costs by using less equipment, less complexity, and an easier implementation.
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The Frame Relay WAN
Frame Relay provides access to a network, delimits and delivers
frames in proper order, and recognizes transmission errors
through a standard Cyclic Redundancy Check.
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Virtual CircuitsThe connection through a Frame Relay network between two DTEs is called a virtual circuit (VC).
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The Frame Relay Encapsulation Process
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Frame Format
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Frame Relay Topologies
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Frame Relay address maping
• Dynamic Mapping – Inverse ARP
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Frame Relay address mapping• Static Mapping
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Local Management Interface • LMI is a keepalive mechanism that provides
status information about Frame Relay connections between the router (DTE) and the Frame Relay switch (DCE)
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LMI extensions
• VC status messages - Provide information about PVC integrity by communicating and synchronizing between devices.
• Multicasting - Allows a sender to transmit a single frame that is delivered to multiple recipients.
• Global addressing - Gives connection identifiers global rather than local significance, allowing them to be used to identify a specific interface to the Frame Relay network.
• Simple flow control - Provides for an XON/XOFF flow control mechanism that applies to the entire Frame Relay interface.
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LMI status messages
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3.2 Configure a Basic FR
Accessing the WAN
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Configuring FR
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Configuring FR (contd.)
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Configure a static Frame Relay map
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3.3 Advanced FR Concepts
Accessing the WAN
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Frame Relay NBMA topology
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Frame Relay Subinterfaces
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Bandwidth control
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Implementing flow control
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3.4 Configuring Advanced FR Accessing the WAN
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Point-to-point subinterfaces
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Point-to-point subinterfaces
Use the frame-relay interface-dlci for local DLCI on the subinterface
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Verifying FR configuration.
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Troubleshooting FR.
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Summary
• Frame relay is the most widely used WAN technology because it:– Provides greater bandwidth than leased line– Reduces cost because it uses less equipment– Easy to implement
• Frame relay is associated with layer 2 of the OSI model and encapsulates data packets in a frame relay frame
• Frame relay is configured on virtual circuits– These virtual circuits may be identified by a DLCI
• Frame relay uses inverse ARP to map DLCI to IP addresses
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Summary• Configuring frame relay requires
– Enable frame relay encapsulation– Configuring either static or dynamic mapping– Considering split horizon problems that develop
when multiple VCs are placed on a single physical interface
• Factor affecting frame relay configuration– How service provider has their charging scheme
set up• Frame relay flow control
– DE– FECN– BECN
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Summary
• The following commands can be used to help verify frame relay configuration– Show interfaces– Show frame-relay lmi– Show frame-relay pvc ###– Show frame-relay map
• Use the following command to help troubleshoot a frame relay configuration– Debug frame-relay lmi
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Thank You