Download What is FRAME RELAY

What is FRAME RELAY ?
• Frame Relay is a way of sending information over a WAN
by dividing data into packets
• It operates at the Physical and Data Link layers of the OSI
reference model
• It relies on upper-layer protocols such as TCP for error
correction
• Frame Relay is a switched data link-layer protocol that
handles multiple virtual circuits using (HDLC)
encapsulation
• Frame Relay interface can be either a carrier-provided
public network or a network of privately owned
equipment, serving a single enterprise
Benefits of FRAME RELAY
 Reduced internetworking costs
• Statistically multiplexed traffic from multiple sources over private backbone
networks can reduce the number of circuits and corresponding cost of bandwidth
• Lower Equipment Costs
• Lower cost than dedicated leased lines
 Increased performance & reduced network complexity
• Reduces the amount of processing (as compared to X.25)
• Efficiently utilizing high speed digital transmission lines, frame relay can improve
performance and response times of applications.
 Increased interoperability via international standards
• Frame relay can be implemented over existing technology
• Access devices often require only software changes or simple hardware
modifications to support the interface standard
• Existing packet switching equipment and T1/E1 multiplexers often can be upgraded
to support frame relay over existing backbone networks.
FRAME RELAY Overview
 Packet Switched
 Uses Virtual Circuits (Connection Oriented Service)
Logical connection created between two (DTE) devices across a
Frame Relay packet-switched network (PSN)
FRAME RELAY Technology
 Access rate
• The clock speed (port speed) of the connection (local loop) to the
Frame Relay cloud
 Data-link connection identifier (DLCI)
• DLCI number identifies the end point in a Frame Relay network
• Each Virtual Circuit is uniquely identified by a DLCI number
• The Frame Relay switch maps the DLCIs between a pair of routers to
create a permanent virtual circuit
 Local management interface (LMI)
• Signaling standard between the customer premises equipment (CPE)
device and the Frame Relay switch. It includes:
1. A keepalive mechanism, which verifies that data is flowing
2. Multicast mechanism, which conserves bandwidth
FRAME RELAY Technology
 DLCI
 LMI
Frame Relay Addressing
Frame Relay DLCIs have local significance
The values themselves are not unique in the Frame Relay WAN
Two DTE devices connected by a virtual circuit might use a different
DLCI value to refer to the same connection
Functions of LMI (Local Management Interface)
• Determines the operational status of the various PVCs that the router
knows about
• To transmit keepalive packets to ensure that the PVC stays up and
does not shut down due to inactivity
• Three LMI types can be invoked by the router: ansi, cisco, and q933a
LMI EXTENSIONS
 Are set of enhancements to the basic Frame Relay
specifications
 It offers a number of features (called extensions) for
managing complex internetworks
 Key Frame Relay LMI extensions include:
- Global addressing
- Virtual-circuit status messages
- Multicasting
LMI EXTENSIONS
• Virtual circuit status messages (common)
PVC Integrity and information about new and existing PVC.
• Multicasting (optional)
Allows a sender to transmit a single frame but have it delivered by the network to
multiple recipients.
• Global addressing (optional)
Gives connection identifiers global rather than local significance, allowing them to
be used to identify a specific interface to the Frame Relay network. Global
addressing makes the Frame Relay network resemble a local-area network (LAN) in
terms of addressing; address resolution protocols therefore perform over Frame
Relay exactly as they do over a LAN.
• Simple flow control (optional)
Provides for an XON/XOFF flow control mechanism that applies to the entire
Frame Relay interface. It is intended for devices whose higher layers cannot use the
congestion notification bits and that need some level of flow control
FRAME RELAY Technology
• Committed information rate (CIR)
The CIR is the guaranteed rate, in bits per second, that the service
provider commits to providing.
• Committed burst
The maximum number of bits that the switch agrees to transfer during
a time interval.
• Excess burst
The maximum number of uncommitted bits that the Frame Relay
switch attempts to transfer beyond the CIR.
Excess burst is typically limited to the port speed of the local access
loop (Your router’s connection to the Frame Relay Switch).
• Discard eligibility (DE) indicator
A set bit that indicates the frame may be discarded in preference to
other frames if congestion occurs.
When the router detects network congestion, the Frame Relay switch
will drop packets with the DE bit set first. The DE bit is set on the
oversubscribed traffic (Anything over the CIR).
FRAME RELAY Congestion
• Forward explicit congestion notification (FECN)
A bit set in a frame that notifies a DTE that congestion avoidance
procedures should be initiated by the receiving device. When a Frame
Relay switch recognizes congestion in the network, it sends a FECN
packet to the destination device, indicating that congestion has
occurred.
• Backward explicit congestion notification (BECN)
A bit set in a frame that notifies a DTE that congestion avoidance
procedures should be initiated by the receiving device. When a Frame
Relay switch recognizes congestion in the network, it sends a BECN
packet to the source router, instructing the router to reduce the rate
at which it is sending packets. If the router receives any BECNs
during the current time interval, it decreases the transmit rate by 25%.
FRAME RELAY Congestion
FRAME RELAY Multiplexing
Statistical Time Division Multiplexing (STDM)
Multiplexes multiple virtual circuits, through a shared physical
medium by assigning DLCIs to each DTE/DCE pair of devices
INVERSE ARP
Allows the router to automatically build the Frame Relay map
The router learns the DLCIs that are in use from the switch during the
initial LMI exchange. The router then sends an Inverse ARP request to
each DLCI for each protocol configured on the interface if the protocol is
supported. The return information from the Inverse ARP is then used to
build the Frame Relay map.
FRAME RELAY Mapping
Frame relay maps (which bind next router hop IP addresses
to DLCIs and work together with standard routing tables)
can be statically configured, or can be dynamically created
by the invocation of inverse ARP
FRAME RELAY Switching tables
The Frame Relay switching table consists of four entries:
• 2 for incoming port and DLCI
• 2 for outgoing port and DLCI
Subinterfaces
• A single physical interface can be split into multiple logical
interfaces
• Subinterfaces can resolve split horizon issues
• Routing updates can be sent out subinterfaces as if they
were separate physical interfaces
• Overall cost of implementing a Frame Relay network can be
reduced.
FRAME RELAY Implementation without
subinterfaces
Router (DTE device) have a WAN serial interface for
every PVC
Basic FRAME RELAY Configuration
BASIC FRAME RELAY CONFIGURATION ASSUMES THAT:
• Configure Frame Relay on one or more physical interfaces
• LMI and Inverse ARP are supported by the remote router(s)
Basic FRAME RELAY Configuration
1. Select the interface (S0, S1) & get into the interface
configuration mode
2. Configure network layer address (IP address)
3. Configure the encapsulation type (cisco is default, ietf is
used if connecting to non-cisco routers)
4. Cisco IOS release 11.1 or earlier, specify the LMI type
used by the Frame relay switch {ansi | cisco | q933a}
5. Cisco IOS 11.2 or later, the LMI type is autosensed
6. Configure bandwidth for the link (Affects many routing
protocols which uses it for a metric)
7. Inverse ARP is on by default
Verifying FRAME RELAY Operation
After configuring Frame Relay, you can verify that the
connections are active by using the show commands
• Show interface serial (Displays DLCI used on the configured
interface, LMI DLCI used for the LMI)
• Show frame-relay pvc (Displays status of each configured
connection & view the number of BECN & FECN packets received
by the router)
• Show frame-relay map (Displays the IP address & associated
DLCI for each remote destination to which the router is connected to)
• Show frame-relay lmi (Displays LMI traffic statistics- it will
show the number of status messages between the router & the FR
Switch)
Configuring Subinterfaces
1. Select interface & get into interface configuration mode
2. Remove any existing network-layer address assigned to
the physical interface
3. Configure Frame Relay encapsulation
router(config-if)#encapsulation frame-relay
4. Select the subinterface you want to configure
router(config-if)#interface serial 0.1 {multipoint | point to point}
5. Configure the network-layer address on the subinterface
6. Configure the DLCI for the subinterface to distinguish it
from the physical interface
router(config-if)#frame-relay interface-dlci dlci-number
Multipoint Subinterfaces
Point to point Subinterfaces