Redes Multimedia Copyright 2004M.C. Juan Antonio Guerrero Ibáñez CAPÍTULO III Frame Relay M. En...

Copyright 2004 M.C. Juan Antonio Guerrero Ibáñez Redes Multimedia

CAPÍTULO III

Frame Relay

M. En C. Juan Antonio Guerrero Ibáñez


Objectives• Describe the operation of Frame Relay• Describe the functions of DLCIs in Frame

Relay• Describe Cisco’s implementation of Frame

Relay• Describe the process for configuring and

verifying Frame Relay• Describe the Frame Relay subinterfaces• Describe how Frame Relay uses

subinterfaces to solve the problem of split horizon


Frame Relay Technology Overview

• Frame Relay Terminology• Frame Relay Operation• Frame Relay Multiplexing• Frame Relay Frame Format• Frame Relay Addressing


What Is Frame Relay?

Frame Relay is a packet-switched, connection-oriented, WAN service.

FR defines the interconnection process between a

router and the service provider’s local access switching equipment.


• Frame Relay uses a subnet of the HDLC protocol called LAPF (Link Access Procedure for Frame Relay)

• Originally FR was designed to allow ISDN equipment to have access to a packet-switched service on a B channel

• FR is now a stand-alone technology

What Is Frame Relay?


Connection-Oriented Service

Connection-oriented protocols use an acknowledgement process between source and destination.


Frame Relay stack layered support

• Takes data packets from a network layer protocol, such as IP or IPX

• Encapsulates them as the data portion of a Frame Relay frame • Passes them to the physical layer for delivery on the wire


Terminology• VC can be:

– established dynamically by sending signaling messages to the network. (SVC)

– Preconfigured by the carrier (PVC)

• The switching information for a VC is stored in the memory of the switch.

• FR provides no error recovery mechanism because it was designed to operate on high-quality digital lines.

• The FRAD or router connected to FR network may have multiple virtual circuits connecting it to various end points.


LMI and Local Access

The DLCI (Data Link Channel Identifier)value identifies the logical connection that is multiplexed into the physical channel.

LMI (Local Management Interface)


Frame Relay Terminology #1• Local access rate

– The maximum physical media speed of the link used by the Frame Relay interface. Frame Relay may or may not use this full bandwidth, but cannot exceed it

• Common local access rates include T1 (1.544 Mbps) and 56 Kbps – could be up to T3 (44.476 Mbps).

• Committed information rate (CIR)– The transmission rate that the frame relay provider

guarantees without frame drops• Any frames sent above this rate has the DE bit set to one

allowing them to be dropped if congestion occurs.• Monthly service charge is based heavily on this rate.


Frame Relay Terminology #2• Committed burst

– The maximum number of bits per second that the Frame Relay provider agrees to transmit during any CIR measurement time interval.

• While in most contracts, generally isn’t used very much .

• Excess burst– The maximum bits that the Frame Relay switch will

attempt to transfer beyond the CIR. Can not exceed the local access rate .


Frame Relay Terminology #3• Oversubscription

– A link is oversubscribed when the combined traffic rate of all VCs (often customers) exceeds the local access link speed. The bursty nature of data traffic allows some VCs to burst while others are idle.

• Port speed– The maximum burst rate in bits per second.

Basically, CIR is what you’re guaranteed, port speed is what you can burst up to, and local access rate is the capacity of the physical interface to the cloud.


BECN and FECN

A Frame Relay switch sends FECN and BECN packets to reduce congestion.

FECN packets are sent to the destination device, BECN packets are sent to

the source device.


Frame Relay Traffic-Control Functions

• Error Control with Frame Relay– Relies on upper-layer protocols to provide

retransmission of damaged or missing frames. This is a concession to increase speed, but it works only because the quality and reliability of the underlying circuits.

• Flow/Congestion Control with Frame Relay– Pseudo flow-control capability via the FECN,

BECN, and DE bits in the protocol frame’s address field. Not true flow control because upper-layer protocols are still required to actually carry out the task of reducing transmission rates


Frame Relay Operation

Interconnection of Frame Relay switches and routers.


Frame Relay DLCIs

The end devices at two different ends of a connection can use

different DLCI numbers to the same connection. DLCIs are

locally significant.


Frame Relay Frame Format

• Flag - Beginning and the end of the Frame Relay frame (01111110)

• Address - Contains the following information: – DLCI VALUE – First 10 bits: Indicates the DLCI value.

– Congestion CONTROL - Last 3 bits control congestion-notification mechanisms. These bits are:

• FECN (forward explicit congestion notification)

• BECN (backward explicit congestion notification)

• DE (discard eligible)

• Data - Contains encapsulated upper-layer data

• FCS - Frame check sequence


Frame Relay Addressing


LMI Operation

LMI provides for management of connections. Includes operational status of PVCs, transmit keepalives, and tell the router the available PVCs.


LMI Frame Format

• Flag• DLCI - Defined in the specification as DLCI = 1023.• Unnumbered information indicator - Same format as LAPB unnumbered

information (UI) frame indicator, with the poll/final bit set to 0.• Protocol discriminator - Value indicates LMI.• Call reference - Always filled with 0s.• Message type - Two message types:

– Status messages (respond to status-enquiry messages). Examples:• Keepalives• Status message of each DLCI defined for the link

– Status-enquiry messages– Together help verify the integrity of logical and physical links

• Information element (IE) - Consists of one or more: 1-byte IE identifier, an IE length field, and 1 or more bytes containing actual data.


LMI States

• LMI Connection States. There are three:– Active–Connection between two end routers is up and

capable of transmitting data.

– Inactive–Connection between the local router and local Frame Relay switch is operational, but the remote router’s link is not operational.

– Deleted–Indicates a failed link between the local router and local Frame Relay switch.


Multicasting and Inverse ARP

The router learns the DLCIs from the Frame Relay switch and sends an Inverse ARP request to each DLCI.


Multicasting and Inverse ARP

• Multicast groups are designated by a series of four reserved DLCI values (1019 to 1022). Frames sent by a device using one of these reserved DLCIs are replicated by the network and sent to all exit points in the designated set.

• The Inverse ARP allows the router to automatically build the Frame Relay map. The router learns the DLCIs from the switch during the initial LMI exchange. The router then sends an Inverse ARP request to each DLCI. The return information is used to build the Frame Relay map.


Frame Relay Mapping

Responses to Inverse ARP requests are entered in an address-to-DCLI mapping table on the router or access server.


Frame Relay Switching Tables

Routers use Inverse ARP to find the remote IP address and create a

mapping of local DLCIs and their associated remote IP addresses.


Basic Frame Relay Configuration

• Verifying Frame Relay Operation• Configuring the Serial Interface for a Frame

Relay Connection• Verifying Frame Relay Configuration• Configuring Subinterfaces• Configuring Optional Commands


Frame Relay Configuration Steps

• Enabling Frame Relay Encapsulation– Router(config-if)#encapsulation frame-relay [cisco|ietf]

• Default is Cisco; IETF is the standard.

• Specifying the LMI Type– Router(config-if)#frame-relay lmi-type [ansi|cisco|q933a]

– Not necessary since Cisco IOS Release 11.2.

• Configuring the Link Bandwidth (Optional)– Router(config-if)#bandwidth Num kbps

– Default on serial links is 56 kbps.

• Enabling Inverse ARP– Router(config-if)#frame-relay inverse-arp [protocol] [dlci]


Configuring a Static Map (Parameters)

• When not using inverse ARP to dynamically map DLCIs to protocol addresses, you must create a static mapping. The syntax:– Router(config-if)#frame-relay map protocol protocol-address dlci#

[broadcast] [ietf|cisco]• Protocol - Protocol used on the interface

• protocol-address - Protocol address of remote router interface

• dlci# - Local DLCI used to reach the remote router

• Broadcast - Permits the forwarding of broadcasts

• ietf|cisco - Frame Relay encapsulation type– Overrides the encapsulation frame-relay command choice

– Cisco is the default

• Examples– RTA(config-if)#frame-relay map ip 192.168.4.1 16 broadcast

– RTA(config-if)#frame-relay map ip 192.168.4.3 16 broadcast


Basic Configuration Example


Verifying and Troubleshooting Frame Relay

• The show interface serial Command• The show frame-relay lmi Command• The show frame-relay pvc [interface interface

[dlci#]] Command• The show frame-relay map Command• The debug frame-relay Command


The show interfaces serial Command

Lab-B#show interfaces serialSerial 2 is up, line protocol is up Hardware type is MCI Serial Internet address is 131.18.79.1, subnet mask is 255.255.255.0 MTU 1500 bytes, BW 1544 Kbit, DLY 20000 usec, rely 255/255,load 1/255 Encapsulation FRAME-RELAY, loopback not set, keepalive set (10 sec) multicast DLCI 1022, status defined, active source DLCI 20, status defined, active LMI DLCI 1023, LMI sent 10, LMI stat recvd 10, LMI upd recvd 2 Last input 7:21:29, output 0:00:37, output hang never


The show frame-relay lmi Command

Router#show frame-relay lmiLMI Statistics for interface Serial1 (Frame Relay DTE) LMI TYPE = ANSI Invalid Unnumbered info 0 Invalid Prot Disc 0 Invalid dummy Call Ref 0 Invalid Msg Type 0 Invalid Status Message 0 Invalid Lock Shift 0 Invalid Information ID 0 Invalid Report IE Len 0 Invalid Report Request 0 Invalid Keep IE Len 0 Num Status Enq. Sent 9 Num Status msgs Rcvd 0 Num Update Status Rcvd 0 Num Status Timeouts 9


The show frame-relay pvc Command

Router#show frame-relay pvc 100PVC Statistics for interface Serial5/1 (Frame Relay DTE)DLCI = 100, DLCI USAGE = LOCAL, PVC STATUS = ACTIVE, INTERFACE = Serial0.1 input pkts 9 output pkts 16 in bytes 154 out bytes 338 dropped pkts 6 in FECN pkts 0 in BECN pkts 0 out FECN pkts 0 out BECN pkts 0 in DE pkts 0 out DE pkts 0 out bcast pkts 0 out bcast bytes 0 pvc create time 00:35:11, last time pvc status changed 00:00:22 Bound to Virtual-Access1 (up, cloned from Virtual-Template5)


The show frame-relay map Command

Router#show frame-relay mapSerial 1 (up): ip 131.108.177.177dlci 177 (0xB1,0x2C10), static, broadcast, CISCO


The debug frame-relay Command

#debug frame-relaySerial0(i): dlci 500(0x7C41), pkt type 0x0800, datagramsize 24Serial0(i): dlci 1023(0xFCF1), pkt type 0x309, datagramsize 13Serial0(i): dlci 500(0x7C41), pkt type 0x0800, datagramsize 24Serial0(i): dlci 1023(0xFCF1), pkt type 0x309, datagramsize 13

Redes Multimedia Copyright 2004M.C. Juan Antonio Guerrero Ibáñez CAPÍTULO III Frame Relay M. En...

Documents

Transcript of Redes Multimedia Copyright 2004M.C. Juan Antonio Guerrero Ibáñez CAPÍTULO III Frame Relay M. En...