Internetworking Technologies Instructor: Prabul, CCNA

	prev	index	next

12. Frame Relay

Top 10 Graphics:

1. Frame Relay Overview
2. Frame Relay Terminology
   • FRS - Frame Relay Switch
   • VC - Virtual Circuit
   • DLCI - Data-Link Connection Identifier
   • LMI - Local Management Interface
3. Frame Relay Terminology (cont.)
   • FRAD - Frame Relay Access Device
   • CIR - Committed Information Rate
   • FECN - Forward Eplicit Congestion Notification
   • BECN - Backward Explicit Congestion Notification
4. Frame Relay Addressing Example
5. Purpose of LMI (Local Management Interface)
6. Frame Relay Mapping
7. Frame Relay Operation
8. Frame Relay Operation (cont.)
9. Configuring Basic Frame Relay
10. Configuring Basic Frame Relay (cont.)

Frame Relay - Industry-standard, switched data-link layer protocol that handles multiple virtual circuits using HDLC encapsulation between connected devices. Frame Relay is more efficient than X.25, the protocol for which it is generally considered a replacement. By using a simplified framing with no error correction mechanisms over high-quality digital facilities, Frame Relay can transmit data very rapidly, compared to other WAN protocols. Frame Relay uses Layer 2 identifiers (DLCIs) and Permanent Virtual Circuits (PVCs).

Frame Relay WAN technology provides a flexible method of connecting LANs over Frame Relay WAN links:

• A ITU-T (CCITT) & American National Standards Institute (ANSI) standard
• Defines the process for sending data over a Public Data Network (PDN)
• Uses Virtual Circuits (VCs) to establish connections across the WAN
• A connection-oriented service - the complete path to the destination is established prior to the sending of the first frame
• DLCIs are used to identify Virtual Circuits
• Provides a means for multiplexing many logical data conversations (referred to as virtual circuits) by assigning each pair of DTEs connection identifiers
• Can divide a single Physical WAN Interface into multiple Subinterfaces
• Operates at the Physical an Data Link Layers of the OSI model but it relies on upper-layer protocols such as TCP for error correction
• A data-link technology that is streamlined (little error checking) to provide high performance and efficiency
• Defines the interconnection process between the Customer Premises Equipment (CPE) (also known as data terminal equipment - DTE), such as a router, and the service provider’s local access switching equipment (known as data communications equipment - DCE)

Packet/Cell Switched Connections

Frame Relay (key points):

• Simpler and faster than X.25
• PVC only, SVC has been specified
• Access is at 56 kbps, 64 kbps, or 1.544 Mbps
• Typically operates up to T1
• Very streamlined (little error checking)
• Has none of the reliability features/complexity of X.25
• Uses:
  • LAN-to-LAN connectivity
  • Remote access
• Logical DTE (router) to logical DCE (Frame Relay switch)
• Virtual Circuits (VC) are end-to-end

VC (Virtual circuit) - Logical circuit created to ensure reliable communication between two network devices. A virtual circuit is defined by a VPI/VCI pair, and can be either permanent (a PVC) or switched (an SVC). Virtual circuits are used in Frame Relay and X.25. {In ATM, a virtual circuit is called a virtual channel.}

PVC (Permanent Virtual Circuit) - Virtual circuit that is permanently established. PVCs save bandwidth associated with circuit establishment and tear down in situations where certain virtual circuits must exist all the time. {Called a permanent virtual connection in ATM terminology.}

SVC (Switched Virtual Circuit) - Virtual circuit that is dynamically established on demand and is torn down when transmission is complete. SVCs are used in situations where data transmission is sporadic. {Called a switched virtual connection in ATM terminology.}

FRAD (Frame Relay Access Device) - Any network device that provides a connection between a LAN and a Frame Relay WAN.

Frame Relay Service and Circuits:

• Data-Link Connection Identifier (DLCI)
• Committed information rate (CIR)
• Access rate is 56 kbps, 64 kbps, or 1.544 Mbps
• One physical access interface
• One logical hop to any point
• Implemented mostly as a carrier-provided service
• Can also be used for private networks

DLCI (Data-link Connection Identifier) - A 'local identifier' between the DTE and the DCE, it identifies the logical connection that is multiplexed into the physical channel. Value that specifies a PVC or SVC in a Frame Relay network. In the basic Frame Relay specification, DLCIs are 'locally significant' (connected devices might use different values to specify the same connection). In the LMI extended specification, DLCIs are 'globally significant' (DLCIs specify individual end devices). The FR Switch maps the DLCIs between each pair of routers to create a PVC. DLCI values are typically assigned by the Frame Relay service provider (for example, the telephone company).

LMI (Local Management Interface) - A signaling standard between the CPE device and the FR Switch that is responsible for managing the connection and maintaining "status" between the devices. Set of enhancements to the basic Frame Relay specification. LMI includes support for a 'keepalive mechanism', which verifies that data is flowing; a 'multicast mechanism', which provides the network server with its local DLCI and the multicast DLCI; 'global addressing', which gives DLCIs global rather than local significance in Frame Relay networks; and a 'status mechanism', which provides an on-going status report on the DLCIs known to the FR Switch. {Known as LMT in ANSI terminology.}

The main purpose for the LMI process is: (management of the connection)

• PVC status - What is the operational status of the various PVCs that the router knows about?
• Transmission of 'keepalive' packets - Insure that the PVC stays up and does not shut down due to inactivity.

Three types of LMIs are supported:

1. cisco - LMI type defined jointly by Cisco, StrataCom, Northern Telecom, and DEC (frame relay forum)
2. ansi - Annex D defined by ANSI standard T1.617
3. q933a - ITU-T Q.933 Annex A

LMI encapsulation types:

1. IETF Encapsulation Type
2. Cisco Encapsulation Type

CIR (Committed Information Rate - The rate at which a Frame Relay network agrees to transfer information under normal conditions, averaged over a minimum increment of time. CIR, measured in bits per second, is one of the key negotiated tariff metrics.

Local access rate - The clock speed (port speed) of the connection (local loop) to the Frame Relay cloud. It is the rate at which data travels into or out of the network.

Committed Burst (Bc) - The maximum number of bits that the switch agrees to transfer during any Committed Rate Measurement Interval (Tc).

Excess Burst - The maximum number of uncommitted bits that the Frame Relay switch will attempt to transfer beyond the CIR. Excess Burst is dependent on the service offerings available by your vendor, but is typically limited to the port speed of the local access loop.

FECN (Forward explicit congestion notification) - When a Frame Relay switch recognizes congestion in the network, it sends an FECN packet to the destination device indicating that congestion has occurred.

BECN (Backward explicit congestion notification) - 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.

DE (Discard Eligibility indicator) - When the router detects network congestion, the FR switch will drop packets with the DE bit set first. The DE bit is set on the oversubscribed traffic; that is, the traffic that was received after the CIR was met.

Connection-Oriented Services: (Three Phases)

1. Connection Establishment
   • a single path between source and destination devices is determined
   • resources reserved to ensure a consistant rate of service
2. Data Transfer
   • data transmtted sequentialy over the established path
   • packets arrive at the destination in the order sent
3. Connection Termination
   • terminate the connection between source and destination

Frame Relay Frame Format:

length:    1            2                       Var     2     1
 field: [Flags][Address: DLCI FECN BECN DE EA][ Data ][FCS][Flags]

Inverse ARP (Eliminates manual configuration)

• Router learns DLCIs that are in use from the switch
  (during initial LMI exchange)
• Sends Inverse ARP request to each DLCI
  (for each protocol configured)
• Return information used to build the Frame Relay map

Frame Relay Mapping:

[ ------- Routing Table -------- ]  [  Frame Relay Map  ]
[Network ][Next Router][Interface]  [Next Router][ DLCI ]
[10.0.0.0][172.16.1.2 ][   S0    ]  [172.16.1.2 ][ 100  ]

The router next-hop address determined from the routing table must be resolved to a Frame Relay DLCI. The resolution is done through a data structure called a Frame Relay map. This data structure may be statically configured in the router, or the Inverse ARP feature can be used for automatic setup of the map.

Frame Relay Operation: (Switching)

[ FR  ----- P0 Switching Table ----- ]
[IN Port][IN DLCI][OUT Port][OUT DLCI]
  P0       100      P1        200
                    P2        275

The Frame Relay switching table consists of four entries: two for incoming port and DLCI, two for outgoing port and DLCI. The DLCI could, therefore, be remapped as it passes through each switch; the fact that the port reference can be changed is why the DLCI is "locally significant."

Frame Relay Protocol in Operation:

Frame Relay is a Layer 2 protocol that describes how the DTE device communicates with and connects to a Frame Relay Switch:

1. Order Frame Relay service from a service provider, or you create a private Frame Relay cloud.
2. Each router, either directly or through a CSU/DSU, connects to the Frame Relay switch.
3. When the CPE router is enabled, it sends a Status Inquiry message to the FR switch. The message notifies the switch of the router’s status, and asks the switch for the connection status of the other remote routers.
4. When the FR switch receives the request, it responds with a Status message that includes the DLCIs of the remote routers to which the local router can send data.
5. For each active DLCI, each router sends an Inverse ARP request packet introducing itself and asking for each remote router to identify itself by replying with its network-layer address.
6. For each DLCI that each router receives an Inverse ARP message about, the router will create a map entry in its Frame Relay map table that includes the local DLCI and the remote router’s network-layer address, as well as the state of the connection.

   NOTE: The DLCI is the router's locally configured DLCI, not the DLCI that the remote router is using. Three possible connection states appear in the Frame Relay map table:

   1. Active state - Indicates that the connection is active and that routers can exchange data.
   2. Inactive state - Indicates that local connection to FR Switch is working, but the remote router’s connection to the FR Switch is not working.
   3. Deleted state - Indicates that no LMI is being received from the FR switch or no service between the CPE router and FR Switch is occurring.

7. Every 60 seconds, the routers exchange Inverse ARP messages.
8. Every ten seconds or so (this is configurable), the CPE router sends a 'keepalive' message to the FR Switch. The purpose of the keepalive message is to verify that the FR switch is still active.

Frame Relay 'without' SubInterfaces:

• Early implementation of Frame Relay Technology required that a router (DTE device) must have a WAN serial interface for every (PVC) permanent virtual circuit.

Frame Relay SubInterfaces:

• Logically dividing a single physical WAN serial interface into multiple virtual subinterfaces
• A single router interface can now service may remote locations through individual unique suberinterfaces
• Each subinterface is considered a unique network and a unique DLCI number

Reachability Issues with Routing Updates

Reducing routing loops by:

• Split Horizon - Routing updates received at central router cannot be advertised out the same physical interface to other routers.
  • Not a problem if there is only a single PVC on a physical interface, because this would be more of a point-to-point connection type.
  • When running multiple PVCs over a single physical interface, this can be a big issue.
• A single physical interface can be split into multiple logical interfaces (Subinterfaces).
• Subinterfaces can resolve split horizon issues.
• Routing updates can be sent out suninterfaces as if they were separate physical interfaces.

NBMA (Nonbroadcast Multi-Access) - Term describing a multiaccess network that either does not support broadcasting (such as X.25) or in which broadcasting is not feasible (for example, an SMDS broadcast group or an extended Ethernet that is too large).

Multiaccess network - Network that allows multiple devices to connect and communicate simultaneously.

Configuring Frame Relay:

Router(config)# interface Serial1

• Select the interface and go into interface configuration mode

Router(config-if)# ip address 10.16.0.2 255.255.255.0

• Configure a network-layer address

Router(config-if)# encapsulation frame-relay [cisco | ietf]

• Select the encapsulation type to encapsulate data traffic end-to-end
• Cisco is the default type, use IETF if connecting to a non-Cisco router

Router(config-if)# bandwidth 56

• Configure the bandwidth for the link (in kilobits)

Router(config-if)# frame-relay lmi-type ansi 
                               {ansi | cisco | q933i}

• If using Cisco IOS Release 11.1 or earlier, specify the LMI-type used by the FR Switch
• Cisco is the default, with IOS Release 11.2 or later, the LMI-type is autosensed so no configuration is needed.

Router(config-if)# frame-relay inverse-arp [protocol] [dlci]

• (OPTIONAL)
• If Inverse ARP was disabled on the router, reenable it.
• Inverse ARP is on by default so it may not appear in config output

Router(config)# router rip
Router(config)# network 10.0.0.0

Verifying Frame Relay Operation:

Router# show interfaces srial

• Displays DLCI and LMI information

Router# show frame-relay pvc

• Displays PVC traffic statistics
• Useful for viewing number of BECN/FECN packets received by the router

Router# show frame-relay map

• Displays the router maps (static or dynamic)

Router# show frame-relay lmi

• Displays LMI information
• Shows number of status messages exchanged between the local router and the Frame Relay switch

Optional Frame Relay commands:

Router(config-if)# keepalive number

• Establishes the interval at which keepalives are sent by the router

Router(config-if)# frame-relay local-dlci number

• Defines the local DLCI number

Router(config-if)# frame-relay map protocol protocol-address
                   dlci [broadcast] [ietf | cisco ] 

• Commands can be used when necessary for enhanced router operation
• These static entries are referred to as static maps

Configuring Subinterfaces

Multipoint:

• Subinterfaces act as a default NBMA network
• Can save subnets because uses single subnet
• Good for full-mesh topology

Point-to-Point:

• Subinterfaces act as leased lines
• Each point-to-point connection requires its own subnet
• Good for star or partial-mesh topologies

Router(config-if)# interface Serial0
Router(config-if)# no ip address
Router(config-if)# encapsulation frame-relay [cisco | ietf]

• Select the encapsulation type to encapsulate data traffic end-to-end
• Cisco is the default type, use IETF if connecting to a non-Cisco router

Router(config-if)# interface Serial'number'.subinterface-number 
                   {multipoint | point-to-point}

• .subinterface-number - Subinterface number range 1 to 4294967293
• The interface number that precedes the period (.) must match the interface number to which this subinterface belongs
• multipoint - Select this if you want the router to forward broadcasts and routing updates that it receives. Select this if routing IP and you want all routers in same subnet
• point-to-point - Select this if you do not want the router to forward broadcasts or routing updates and if you want each pair of point-to-point routers to have its own subnet

NOTE: Remove any network-layer address assigned to the physical interface. If the physical interface has an address, frames will not be received by the local subinterfaces.

Router(config-if)# ip address 10.17.0.1 255.255.255.0
       - OR -
Router(config-if)# ip unnumbered 'interface' 

• Configure a network-layer address on the subinterface If the subinterface is point-to-point, and you are using IP, you can use the 'ip unnumbered' command:

Router(config-if)# bandwidth 64
Router(config-if)# frame-relay interface-dlci 'dlci-number'

• dlci-number - the local DLCI number being linked to the subinterface
• This is the only way to link an LMI-derived PVC to a subinterface because LMI does not know about subinterfaces.

Router(config-if)# interface Serial0.3 point-to-point
Router(config-if)# ip address 10.18.0.1 255.255.255.0
Router(config-if)# bandwidth 64
Router(config-if)# frame-relay interface-dlci 120

Router(config)# router rip
Router(config)# network 10.0.0.0

NOTE: Inverse ARP is enabled. It is not required for multipoint subinterfaces configured with static route maps. Do not use this command on physical interfaces.

Prev	Index	Next

  This page is maintained by:   prabul

---

			aprabul@yahoo.com Copyright © 2003