Download - CCNA4 M5 Frame Relay
CCNA – Semester4
Module 5Frame Relay
Objectives
• Components of a Frame Relay network
• The technology of Frame Relay and topology of a Frame Relay network
• Configuring Frame Relay
• Issues of a non-broadcast multi-access network
Frame Relay Concepts
Introducing Frame Relay
• An ITU-T and ANSI standard.
• A packet-switched, connection-oriented, WAN service.
• It operates at the data link layer of the OSI reference model.
• Uses a subset of HDLC protocol LAPF.
Frame Relay operation
• Frames carry data between user devices called DTE, and the DCE at the edge of the WAN.
FRAD
• Computing equipment that is not on a LAN may also send data across a Frame Relay network.
• The computing equipment will use a Frame Relay access device (FRAD) as the DTE.
Frame Relay Toll network
• May be privately owned, but it is more commonly provided as a service by a public carrier.
• Typically consists of many geographically scattered Frame Relay switches interconnected by trunk lines.
Frame Relay terminology
• Connection through the Frame Relay network between two DTEs is called a virtual circuit (VC).
• VCs established dynamically by sending signaling messages to the network are SVCs.
• PVCs are preconfigured by the carrier
Terminologies: Access Rate
• The clock 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.
Terminologies: DLCI
• Data-link connection identifier.
• A number that identifies the end point in a Frame Relay network.
• Significant only to the local network.
• The Frame Relay switch maps the DLCIs between a pair of routers to create a permanent virtual circuit.
Terminologies: LMI
• Local management interface.
• A signaling standard between the CPE device and the Frame Relay switch.
• Responsible for managing the connection and maintaining status between the devices.
Terminologies: CIR
• Committed information rate (in bps).The average rate at which you want to transmit in periods of noncongestion.
• The CIR is the guaranteed rate, that the service provider commits to providing.
• While a frame is being transmitted, each bit will be sent at the port speed.
Terminologies: Tc
• Committed Rate Measurement Interval. The time interval over which the rates are calculated is called the committed time.
• The time interval shouldn’t exceed 125 ms, almost always 125 ms.
Terminologies: Bc
• The number of committed bits in Tc is the committed burst.
• Bc=CIR x Tc
Terminologies: Excess burst
• The maximum number of uncommitted bits that the switch attempts to transfer beyond the CIR.
• Dependent on the service offerings available by the vendor, but is typically limited to the port speed of the local access loop.
Terminologies
MinCIR
Frame Relay flow control
• The switch maintains a bit counter for each VC.
• An incoming frame is marked DE if it puts the counter over Bc.
• An incoming frame is discarded if it pushes the counter over Bc + Be.
• At the end of each Tc seconds the counter is reduced by Bc.
Terminologies: FECN
• Forward explicit congestion notification.
• When a switch recognizes congestion in the network, it sends a FECN packet to the destination device.
Terminologies: BECN
• Backward explicit congestion notification.
• When a 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.
Frame Relay congestion
Terminologies: DE
• Discard eligibility indicator.
• A set bit that indicates the frame may be discarded in preference to other frames if congestion occurs.
• The DE bit is set on the oversubscribed traffic.
Frame Relay bandwidth
• The serial connection or access link to the Frame Relay network is normally a leased line.
• The speed of the line is the access speed or port speed.
• Port speeds are typically between 64 kbps and 4 Mbps. Some providers offer speeds up to 45 Mbps.
Frame Relay frame format
• DLCI: Indicates the DLCI value. Consists of the first 10 bits of the Address field.
• Congestion Control: The last 3 bits in the address field. These are the FECN, BECN, and discard eligible (DE) bits.
Frame Relay addressing
• DLCI address space is limited to 10 bits. possible 1024 DLCI addresses.
• The usable portion of these addresses are determined by the LMI type: – The Cisco LMI type supports a range of DLCI addresses from DLCI
16-1007. – The ANSI/ITU LMI type supports the range of addresses from DLCI
16-992.
• The remaining DLCI addresses are reserved for vendor implementation.
Frame Relay Topology
Frame Relay LMI functions
• The heartbeat mechanism, which verifies that a VC is operational
• The multicast mechanism
• The flow control
• The ability to give DLCIs global significance
• The VC status mechanism
LMI types
• The LMI type configured on the router must match the type used by the service provider.
• Three types of LMIs are supported by Cisco routers: – Cisco – The original LMI extensions – Ansi – Corresponding to the ANSI standard T1.617
Annex D – q933a – Corresponding to the ITU standard Q933 Annex
A
LMI frame format
• LMI messages are sent in frames distinguished by an LMI-specific DLCI.
• Cisco specification as DLCI 1023.• The LMI frame contains 4 mandatory bytes:
– The 1st bytes has the same format as the LAPB unnumbered information (UI) frame indicator, with the poll/final bit set to zero.
– The 2nd byte is referred to as the protocol discriminator, which is setto a value that indicates LMI.
– The 3rd byte (call reference) is always filled with zeros.– The final byte is the message type field:
• Status messages• Status enquiry messages
Frame Relay mapping
• Network address DLCI• The routing table is then used to supply the next-hop protocol
address or the DLCI for outgoing traffic.• The resolution is done through a data structure called a Frame Relay
map.• This data structure can be statically configured in the router, or the
Inverse ARP feature can be used for automatic setup of the map.
LMI operation
• LMI status messages combined with Inverse ARP messages allow a router to associate network layer and data link layer addresses.
• When a router that is connected to a Frame Relay network is started, it sends an LMI status inquirymessage to the network.
• The network replies with an LMI status message containing details of every VC configured on the access link.
• Subsequent responses include only status changes.
Frame Relay mapping
Frame Relay switching table
• The Frame Relay switching table consists of four entries: incoming port and DLCI, and outgoing port and DLCI.
• The DLCI may be remapped as it passes through each switch.
Configuring Frame Relay
Frame Relay encapsulation
• Frame Relay is configured on a serial interface and the default encapsulation type is the Cisco proprietary version of HDLC.
• To change the encapsulation to Frame Relay use the
encapsulation frame-relay [cisco | ietf]
LMI type
• The LMI connection is established and configured by the command:
frame-relay lmi-type [ansi | cisco | q933a]
• IOS Release 11.2 or later, the LMI-type is autosensed and no configuration is needed.
• The default LMI type is cisco. • The LMI type is set on a per-interface basis and is
shown in the output of the show interfacescommand.
Configuring basic Frame Relay
Configuring a static Frame Relay map
• The local DLCI must be statically mapped to the network layer address when:– Remote router does not support Inverse ARP. – Broadcast traffic and multicast traffic over the PVC must be
controlled. – Paritial-mesh Frame Relay topology.
router(conf-if)#frame-relay map protocol protocol-address dlci [broadcast]
• Static frame-relay map disables InverseARP, to turn it back on use the command:
router(conf-if)#frame-relay inverse-arp [protocol] [dlci]
Configuring a static Frame Relay map
Split Horizon
• When a single interface is used to interconnect multiple sites, there may be reachability issues as nonbroadcast multiaccess(NBMA) nature of Frame Relay .
• Split horizon does not allow routing updates to be sent out the same interface that was the source of the route information.
Frame Relay subinterfaces
• Use subinterfaces in Frame Relay:– To enable the forwarding of broadcast in a hub-and-spoke Frame
Relay topology.
– To subject problem regarding split-horizon
– To reduce overall cost of many physical interfaces
• Frame Relay subinterfaces can be configured in either point-to-point or multipoint mode: – Point-to-point
– Multipoint
Frame Relay subinterfaces
• The encapsulation frame-relay command is assigned to the physical interface. All other configuration items, such as the network layer address and DLCIs, are assigned to the subinterface.
Configuring Frame Relay subinterfaces
• Configure encapsulation and no shut the physical interface without ip address.
router(config-if)#interface serial number.subinterface-number{multipoint | point-to-point}
• Using major interface as point-to-point, DLCI configuration is not required as it can be learned via LMI from Frame Relay switch.
• With subinterfaces, use this command configure the local DLCI:
router(config-subif)#frame-relay interface-dlci dlci-number [cisco|ietf]
Subinterface configuration sample
Configure Cisco router as Frame Relay switch
• Cisco router can be configured as Frame Relay switch using command:
Router(config)#frame-relay switching
• Then all connections should be DCE type and be specified with the command:
Router(conf-if)#frame-relay intf-type dce|dte
• Configure LMI type on Frame Relay switch:Router(conf-if)#frame-relay lmi-type {ansi | cisco | q933a}
• Configure Frame Relay routes to create switching table:Router(conf-if)#frame-relay route in_dlci interface out_interface out_dlci
Frame Relay routes
Verifying the Frame Relay configuration
• show interfaces command displays LMI type, LMI DLCI, Frame Relay DTE/DCE type
• show frame-relay lmi command to display LMI traffic statistics.
• show frame-relay pvc [interface interface] [dlci] command to display the status of each configured PVC as well as traffic statistics.
• show frame-relay map command to display the current map entries and information about the connections.
• show frame-relay route command on frame relay switch to display switching table.
Show interface
Show frame-relay lmi
Show frame-relay pvc
Show frame-relay map
Troubleshooting the Frame Relay configuration
• debug frame-relay lmi command to determine whether the router and the Frame Relay switch are sending and receiving LMI packets properly.
• debug frame-relay events command to display frame-relay packets.
Debug frame-relay lmi
Summary
• The components of a Frame Relay network
• The technology of Frame Relay
• Point-to-point and point-to-multipoint topologies
• The topology of a Frame Relay network and potential problems
• How to configure a Frame Relay Permanent Virtual Circuit (PVC)
• How to create a Frame Relay Map on a remote network
• Why subinterfaces are needed and how they are configured
• How to verify and troubleshoot a Frame Relay connection
Lab1 Topology
1. PVC(21-42) and PVC(32-41) belong to 1 subnet (partial-mesh)2. All interfaces are multipoint subinterface (full-mesh)3. IP address is 192.168.1.0/24 . Each Loopback interface requires 30
IPs. Rouing protocol is EIGRP with AS 100
Lab2 Topology
Lab2 Requirements
• PVC(111-121), PVC(123-143), PVC(142-112) form 1 subnet
• PVC(122-131) form 1 subnet
• PVC(132-141) form 1 subnet
• Routing protocol is EIGRP, AS 200
• Network address is 172.30.0.0/16, each loopback interface requires 31 IPs.
CCNA4 – Module5