Exploration Accessing WAN Chapter3-1

8/23/2019 Exploration Accessing WAN Chapter3-1

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2006 Cisco Systems, Inc. All rights reserved. Cisco Public 1Version 4.0

Frame Relay

Accessing the WAN Chapter 3


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2006 Cisco Systems, Inc. All rights reserved. Cisco Public 2

Introduction to Frame Relay

Frame Relay is a WAN protocol that operates at Layers 1 and2 of the OSI Model

It is often used by Network Providers to encapsulate voice

and data between LANs over the WAN

It is one of the most extensively used protocols in the worldbecause it is inexpensive in comparison to dedicated lines andeasy to configure


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Frame Relay Example

One of the benefits of Frame Relay over leased lines is the ability to pay fora (Committed Information Rate) CIR rate and still be able to burst to a

maximum speed

- Essentially, the customer is paying for access to the local loop and thebandwidth they are guaranteed

In the example below, you can see that ourdifferent locations havedifferent bandwidth requirements and Frame Relay is a perfect fit


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The Frame Relay WAN The example below shows the Frame Relay WAN and how Frame Relay

works

Frame Relay is based on X.25, but doesnt have the overhead for errorcorrection

- If there is an error, packets are dropped

- Frame Relay relies on upper layer protocols forerror recovery

- The service providers networks are more reliable today


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Frame Relay Operation When Frame Relay is used to connect over the WAN

- DTEs are the routers on the LAN. The connection between two DTEs via

Frame Relay is called a virtual circuit- A serial connection like a T1 leased line connects the router to a FrameRelay switch (the DCE) of the carrier at the nearest point-of-presence(POP) for the carrier

- Network switches are used to carry the frames from one DTE to the otherDTE by way of the DCEs


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Questions!!!

At what layers does Frame Relay operate?

What does Frame Relay do if there is an error?

What is the connection between the two DTEs in aFrame Relay network called?


7/38 2006 Cisco Systems, Inc. All rights reserved. Cisco Public 7

Virtual Circuits

The connection through a Frame Relay network between two

DTEs is called a Virtual Circuit The bandwidth is shared between multiple companies and any

single site can communicate with any other site without usingmultiple dedicated lines

- You only need one connection to connect to many locations



SVCs vs PVCs

Virtual Circuits are further classified into Switched Virtual

Circuits and Permanent Virtual Circuits Switched Virtual Circuits are established dynamically by

sending signaling messages to the network (CALL SETUP,DATA TRANSFER, IDLE, and CALL TERMINATION)

Permanent Virtual Circuits are preconfigured by the carrierand after theyre set up only operate in DATA TRANSFER andIDLE



Data Link Connection Identifiers

DLCIs are used to identify the link between the DTE and the

service providers equipment They are only locally significant, so it doesnt matter if the

routers have different DLCIs configured

The service providerwill provide you with DLCI information

- They will typically be numbers between 16 to 1007



Multiple Virtual Circuits The beauty of Frame Relay is that you can have multiple

connections through the same serial port because thesignificance of the connection is local

You can see that R1 has two DLCIs assigned to the sameport because it has two separate virtual circuits set up



Questions!!!

What type of virtual circuit is established dynamically?

What type is preconfigured by the carrier?

What is used to identify the link between the DTE andthe service providers equipment?

Do the DLCIs have to be the same on both routers?



Frame Relay Encapsulation

Frame Relay takes in the Layer 3 packet and encapsulates itinside of a frame

Flags are used to indicate the beginning and end of the frame

The address field contains the DLCI information

The Frame Check Sequence is a checksum



Frame Relay Frame Format Forward Explicit Congestion Notification (FECN) is used to inform the

router receiving the frame that congestion was experienced in the path

from source to destination

Backward Explicit Congestion Notification (BECN) is set in framestraveling in the opposite direction of the congestion to notify the sendingrouterthat there is congestion

The DE bit (Discard Eligible) is turned on when a frame being transmittedexceeds the CIR

- If the Frame Relay switch detects an excessive build up of frames in its queue, it will dropany frames with the DE bit turned on



Star Topology (Frame Relay)

One central office (Chicago) has Virtual Circuit connectionswith all of the satellite offices

The satellite offices only have one Virtual Circuit

Generally, the Hub will be the location with the lowest cost forthe Frame Relay connection



Full Mesh Topology (Frame Relay)

Every site is connected via a Virtual Circuit to every other site

Only one physical connection (port) is required for eachrouter

With leased lines, youd have to use four ports for each router



Frame Relay (Partial-Mesh Topology)

Forlarge networks, full-mesh isnt affordablebecause the number of links increases dramatically

A Partial-Mesh Topology is the answer in this situation

- A partial-mesh has more interconnections than a star topology,but not as many as a full-mesh



Frame Relay Address Mapping

In order for a Cisco router to send data over Frame Relay, it needs to knowwhich local DLCI maps to the Layer 3 address at the remote location

Inverse ARP or IARP is used to get the Layer 3 addresses from the DLCIinformation that it has for the remote router

- IARP combined with Local Management Interface (LMI) status messagesallows for the router to map data link layeraddresses to network layeraddresses in a Frame Relay network

A table is then created that contains address to DLCI mappings

Below, you see that for the IP Address 10.1.1.2 there is a related DLCI 102



Static Frame Relay Address Mapping

If a router doesnt support dynamic mapping, you can statically

create the association between the IP address and the DLCI

To map between the next hop protocol address and the DLCIdestination address, use the frame relay map protocolprotocol address dlci [broadcast] [ietf] [cisco] command

- [broadcast] is used to ensure routing protocols canbroadcast information over the link

- [ietf] is used when you connect to a non-cisco router



Local Management Interface (LMI) LMIs are keepalive mechanisms that provide status information about

Frame Relay connections between the router and the Frame Relay Switch

Approximately every 10 seconds, the end device polls the network to verifythe connection is still up

The show frame-relay lmi command will show you whether or not theconnection between the router and the Frame Relay switch is up



Frame Relay LMI Types There are 3 different types of LMI:

- Cisco- Ansi Also called Annex D

- q933a Also called Annex A

The type used will depend on the service provider Starting with Cisco IOS release 11.3, LMI is autosensed and

will automatically detect the LMI type supported by the FrameRelay Switch

If you need to manually set the LMI type youll use theframe-relay lmi-type [cisco] [ansi] [q933a] command



Questions!!!

What is used to inform the router receiving the frame that congestion hasoccurred?

What is set in frames traveling in the opposite direction of the congestionto notify the sending router that there is congestion?

What type of frame-relay topology has a central hub with spokes that

radiate out to satellite offices?

What will the Frame Relay switch do if there is an excessive build up offrames in its queue?

What is a keepalive mechanism that provides status information aboutFrame Relay connections between the router and the Frame Relay

Switch?



Configuring Frame Relay First, set the IP Addressip address 10.1.1.1 255.255.255.0

Next, configure encapsulationencapsulation frame-relay

- If IOS is older than 11.3, youll need to set the encapsulation type by typingencapsulation frame-relay [cisco] [ietf]

Next, set the bandwidthbandwidth 64



Verify your Frame Relay Configuration Use the show interfaces serial command to verify the configuration



Configure a Basic Frame Relay Map

Configure a static Frame Relay map using the frame-relay map

ip address dlci [broadcast] command Frame Relay, X.25, and ATM are known as nonbroadcast

multiaccess (NBMA) which means they dont supportbroadcast or multicast traffic by default

The broadcast command has to be used with many routingprotocols like RIP, EIGRP, and OSPF in order forroutingupdates to be able to traverse the frame-relay connection


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Frame Relay Subinterface Configuration Due to issues with Split Horizon, broadcasts cant be sent out

of the same interface they came in on

This can be a problem with Frame Relay because broadcastsmay need to be sent back out of an interface (point to multipoint)

To bypass this issue, we create subinterfaces on the serial

ports for our Frame Relay connections


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Point-to-Point With Point-to-Point subinterfaces your router will have one Virtual

Connection to another router

- The routers subinterfaces will be configured to be in the same subnet

- You can set up multiple point-to-point subinterface connectionswhich will allow the interfaces to share routing updates out of thesameserial port thanks to the use of subinterfaces

- Point-to-point subinterfaces act like leased lines and eliminate anysplit horizon routing issues


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Point-to-Multipoint With Point-to-Multipoint subinterfaces, your router will have a Virtual

Connection to more than one router

- All interfaces will be in the same subnet, but by default due to split horizonthey cant share routing updates

- Remember, split horizon prevents routing updates to be sent back out of theport in which they were received

You will need to provide a map command to provide the required connections


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Access Rate and CIR

As a customer, youre paying theservice provider for a Frame Relay

connection The Access Rate is the capacity

for the local loop

- For example, the Access Ratespeed could be set to 64 kbps

The Committed Information Rate(CIR) is the amount ofguaranteedbandwidth for the Frame Relayconnection

- For example, the CIR could be setto 48 Kbps

If no other customers are using theconnection, you are able to burst upto the 64 kbps Access Rate


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Questions!!!

What would you follow the encapsulation frame-relay

command with for connecting to a non-Cisco router?

What command would you include at the end of theframe-relay map command to ensure that OSPF is ableto forward updates to the other router?

What is the total bandwidth capacity for the local loopcalled?

What is the amount of guaranteed bandwidth called fora frame-relay connection?


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Configure an Advanced Frame Relay Connection First, remove any IP addresses on the interface using the no ip address command

Next, use the encapsulation frame relay command

Next, for each PVC, create a logical subinterface using the interface serialnumber.subinterface-number [multipoint / point-to-point] command

Next, configure an IP address for the subinterface and then set the bandwidth

Finally, configure the DLCI using the frame-relay interface-dlci command


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Verify an Advanced Frame Relay PVC Use the show interface serial command to verify

You can see that the encapsulation type is listed as Frame-Relay (if youspecified IETF, it would show FRAME-RELAY IETF)


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Verify an Advanced Frame Relay PVC

The show frame-relay lmi command is used to verify any lmi issues

If any of the Invalid items have a number other than 0, it may help you

isolate the problem between the carriers frame-relay switch and your router.Excessive timeouts also indicate a problem with the connection.

In the example below: the LMI type is Cisco which is the default configurationand their are no issues

V if Ad d F R l PVC


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The show frame-relay pvccommand will show you the status for all of the PVCsconfigured on your router

You can view individual PVCs by providing the DLCI number after the command You can then use the clear counters command and then go back 5 minutes later and

see what has changed

High numbers in FECN or BECN tell you about network congestion

V if Ad d F R l PVC


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The show frame-relay map command will show you the IARP mappings betweenIP addresses and DLCIs

-10.140.1.1 is the IP address of the remote router, dynamically learned via theInverse ARP process.

- 100 is the decimal value of the local DLCI number.

- 0x64 is the hex conversion of the DLCI number, 0x64 = 100 decimal.

- 0x1840 is the value as it would appear on the wire because of the way the

DLCI bits are spread out in the address field of the Frame Relay frame.

- Broadcast is enabled on the PVC.

- PVC status is active.

V if Ad d F R l PVC


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Verify an Advanced Frame Relay PVC The debug frame-relay lmi command will allow you to verify that LMI packets are being sent

back and forth properly

Look at the figure and examine the output of an LMI exchange.

- "out" is an LMI status message sent by the router.

- "in" is a message received from the Frame Relay switch.

- An LMI exchange is a "type 1".

- "dlci 100, status 0x2" means that the status of DLCI 100 is active (not shown in figure).

V if Ad d F R l PVC


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Verify an Advanced Frame Relay PVC The map table can show 3 states when an IARP request is made:

- Active Successful circuitDLCI 100, status 0x2

- InactiveSuccessful DTE to DCE on this side, cant detect the other side

- DLCI 100, status 0x0

- Could be a configuration issue

- Deleted DTE is configured for a DLCI not valid for this interfaceDLCI 100, status 0x4

- DLCIs reversed on the router or PVC has been deleted by the ISP


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Questions!!!

What does it mean when you see the word dynamic in the output

from the show frame-relay map command? What does it mean when you see the word IETF in the output

from the show interface interface command?

What do high numbers in the FECN/BECN columns of the showframe relay pvc dlcicommand?

What does output showing dlci 100, 0x4 indicate for a debugframe-relay lmi command?




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Exploration Accessing WAN Chapter3-1

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