10.wide area networks (wans)

8/13/2019 10.Wide Area Networks (WANs)

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The Ultimate CCNA Study Package - ICND 1

Chris Bryant, CCIE #12933 http://www.thebryantadvantage.com Back To Index

Wide Area Networks (WANs)

Don't Miss The "Recommended Video Viewing" Section At The End OfThis Section!

Overview

NOTE: As of July 1, 2009, it is strongly recommended to all CCENT test-takers that you study BOTH the Frame Relay material in the ICND1 andICND2 sections of this course. The ICND1 material is in this section; theICND2 material is in the "Point-to-Point And Frame Relay" section.

Go get 'em! :) - Chris B.

Up to this point in the course, we've concentrated on the Local AreaNetwork (LAN), and with good reason! Both your CCENT exam and the

average network admin's duties focus on the local network. However, weshould certainly know a little about the network that connects ourLANs....the Wide Area Network (WAN).

The Physical Side Of WANs

Directly Connecting Routers Via Serial Interfaces

HDLC And PPP

Introduction To Frame Relay

RFC 1918 Private Addresses, NAT, and PAT

Introduction To ATM

Modems And DSL Variations


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I used a cloud to symbolize the WAN, and you'll hear that term more thanonce relating to WANs, as you'll see during the Frame Relay discussionlater in this section. We're going to take a look at the physical connectivityof the WAN that we as network admins (and future CCENTs and CCNAs)need to know about, and follow that up with a discussion of different WANprotocols we can use to make the inter-LAN connections possible.

The Physical Side Of WANs

Going back to the previous illustration, one reason we refer to the WANconnection as a "cloud" is that we don't know exactly what hardware is inuse in the WAN, and we're not responsible for it - that's up to the serviceprovider, the company that sells WAN services such as Frame Relay.

What we areresponsible for are the routers you see in that diagram, andour routers are going to have to communicate with one of the serviceprovider's devices. The service provider's half of this communication is an

external channel service unit / data service unit, which thankfully isreferred to as a CSU/DSU.

At some point, the responsibility for the physical devices passes from usas network admins to the service provider. This is the demarcation point,typically referred to as the demarc point. In theory, the demarc point iseasy to define; when you're arguing with the service provider on a Friday

afternoon when everyone wants to go home, the exact location of thedemarc point suddenly becomes a huge point of contention.

Theoretically, the demarc point is found at the CSU/DSU. The cableleading from the CSU/DSU to the router and the CSU/DSU itself isconsidered to be the customer's equipment and responsibility. That's us!

All cabling on the "other side" of the CSU/DSU, along with the hardware inthe WAN cloud, is the service provider's equipment.

The CSU/DSU fills the router in on a very important piece of information,the clock rate. When the CSU/DSU does this, it's basically telling the

router "here's how quickly you can send and receive data". Later in thissection, we'll simulate a point-to-point link on a Cisco router and you'll seethe command that allows a Cisco router to give another router this vital


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information.

When it comes to the clockrate:

The Data Communications Equipment(DCE)provides the clockrate The Data Terminal Equipment (DTE) receives the clockrate. By

default, a Cisco router acts as a DTE.

We're going to discuss two common WAN protocols in just a moment, and

I want you to see the showcommands that verify these protocols. To doso, we're going to use a configuration that isn't common in real life, but isvery common in home labs. We're going to use two Cisco routers that aredirectly connected at their Serial0 interfaces, which means that one mustserve as the DCE. We also need a special cable, the aptly-namedDTE/DCE cable.

Connecting Two Cisco Router Serial Interfaces Directly

For these demos, I've got R1 and R3 directly connected at their S0(Serial0) interfaces via a DTE/DCE cable.

To tell the DTE end from the DCE end of the cable before connecting it,look for a small label wrapped around one or both of the cable ends. Thatlabel will indicate whether that is the DCE or DTE end. If there is no label,the connector itself may have DTE or DCE imprinted on it. Most newerDTE/DCE cables have the imprint rather than the label.

After connecting the cable to the respective routers, use show controllerserial xto ensure the router sees the cable as a DCE or DTE. You will seea great deal more output than this when you run this command, but theinformation that's important to us right now is at the very top.R3#show controller serial 1

HD unit 1, idb = 0x1C44E8, driver structure at 0x1CBAC8

buffer size 1524 HD unit 1,V.35 DCE cable

It's the DCE end of the cable that must provide the clockrate.

Here's what show interface serial 1 on R1 reveals before the clockratecommand is configured:


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R1#show interface serial 1

Serial1 is up, line protocol is down

Hardware is HD64570

Internet address is 172.12.13.1/24

MTU 1500 bytes, BW 1544 Kbit, DLY 20000 usec,

reliability 255/255, txload 1/255, rxload 1/255

Encapsulation HDLC, loopback not set

When you see the physical interface up and the line protocol down, there'ssome kind of logical problem with the interface. In this case, the DTE sideis not receiving the required clockrate. Once we do configure theclockrate on the DCE's Serial1 interface, the line protocol comes up andstays up. No reset or reload is needed.

R3(config)#interface serial1

R3(config-if)#clockrate 56000

13:06:40: %LINEPROTO-5-UPDOWN: Line protocol on Interface Serial1, changed

state to up

Verify with show interface serial1on R1.

R1#show interface serial1Serial1 is up, line protocol is up

Hardware is HD64570





I know I've mentioned this several times during the course, but this trulybears repeating as it's a fundamental rule of troubleshooting:

If the interface shows as administratively down, it's simply shut downmanually and needs to be opened.

If the interface shows as down, there's a physical problem, perhaps aloose cable

If the interface is up but the line protocol is down, that means theinterface is physically fine but there's a logical issue, generally anencapsulation mismatch or missing clockrate.

When youre at your practice rack, youll find out that you cant put theclockrate on the DTE, because the router wont let you!

R1(config)#interface serial0R1(config-if)#clock rate 56000

%Error: This command applies only to DCE interfaces


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It's a good idea to have that memorized for the exam, though. :)

Real-world hint: If you're troubleshooting a line protocol issue and yousee the line protocol come up, make sure to stick around for a minute andmake sure itstaysup. Also, the line protocol may show as up for about20 seconds or so after you first open a Serial interface, but stick aroundand make sure it staysup.

A moment ago, I mentioned that an encapsulation mismatch can bring theline protocol down. Let's take a look at two popular encapsulation optionsand what happens when they're used together.

HDLC and PPP

HDLC and PPP are the two data-link (Layer 2) protocols to consider whenchoosing an encapsulation method across a serial point-to-point link.

The version of HDLC that runs on Cisco routers is Cisco-proprietary,making it unsuitable for multivendor environments. If RouterA is a Ciscorouter running HDLC, the only way the line protocol can come up is if theremote router is also a Cisco router running HDLC.

There are major points of distinction between the two. First, HDLC is thedefault encapsulation for a Cisco serial interface. Here's the output ofshow interface serial 1 from the previous discussion. The encapsulationis defaulting to HDLC.R1#show interface serial1

Serial1 is up, line protocol is up

Hardware is HD64570





PPP allows data compression to be configured, where HDLC does not.Compression is performed on data before it's sent across the WAN, andthe data then uses less bandwidth to send the data across the WAN.

PPP multilink allows multiple physical channels to be bundled into a singlelogical channel. HDLC offers no multilink capability.

PPP allows the use of two authentication schemes for point-to-point links(PAP and CHAP), which HDLC does not support either of these.

To review:

HDLC is the default encapsulation on a Cisco router's Serial


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interface.

PPP has features that allow the use of authentication and datacompression.

PPP also allows multilink bundling, where HDLC does not.

HDLC is the default on a Cisco router's serial interface. Changing theencapsulation type to PPP is simple enough, but be prepared for the lineprotocol to go down when doing so.R1(config)#interface serial1

R1(config-if)#encapsulation ppp

4d19h: %LINEPROTO-5-UPDOWN: Line protocol on Interface Serial1, changed

state to down

The line protocol went down almost immediately, but there's no messageabout the physical interface going down. The issue is that the opposite

end of the connection, interface Serial1 on R3, is still running HDLC.Mismatched encapsulation types will result in the line protocol going downand staying down.

Let's compare the output of show interface serial 1 on both R1 and R3after PPP was configured on R1.

R1#show int serial1


Hardware is HD64570




Encapsulation PPP,loopback not set

R3#show interface serial 1


Hardware is HD64570





Changing the encap type to PPP on R3 will result in the line protocolcoming back up dynamically - no reload or resetting the interface isnecessary. When troubleshooting a line protocol issue, make sure that the

line protocol STAYS up after coming up!

R3#conf t


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Enter configuration commands, one per line. End with CNTL/Z.

R3(config)#int serial1

R3(config-if)#encap ppp

13:16:48: %LINEPROTO-5-UPDOWN: Line protocol on Interface Serial1, changed

state to up

An Introduction To Frame Relay

Frame Relay has 3 things going for it that endears it to network admins:

it's cheap it's reliable it's cheap and reliable

It also happens to be very prevalent in today's WANs, so it's a good ideato know about it for working with real-world production networks and your

CCENT and CCNA exams!

You'll learn all about configuring and fine-tuning Frame Relay during yourCCNA studies, but let's get the fundamental concepts down now.

In the following WAN, we have two routers that are hundreds of milesapart, and we need them to talk to each other. Simple enough! We knowthat we're going to use the Serial interfaces on the routers for a WANconnection... now what?

In the case of Frame Relay, we call our friendly Frame Relay ServiceProvider and tell him where our routers are, and how much bandwidthwe're willing to pay for. The provider then configures some of his framerelay switches, gives us a few numbers to add to our router configuration,and we're all set!

The frame relay service provider guarantees a certain amount ofbandwidth will be available to a given user at any time. The moreguaranteed bandwidth desired, the more it costs, but its still cheaper than


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a dedicated point-to-point link. This guaranteed bandwidth is referred toas the committed information rate(CIR).

Naturally, the more guaranteed bandwidth we need, the more money wehave to give the frame provider!

The frame provider's collection of frame relay switches has a curiousname - frame relay cloud. You'll often see the frame provider's switchesrepresented with a cloud drawing in network diagrams, much like this:

Again, the frame switches that make up that cloud belong to the frameprovider. We don't configure them, and we don't want to - we've gotenough to do!

All of the frame switches in that cloud are DCEs, and the routers areDTEs. The frame switch that's actually connected to the router on eachend of the connection will supply clockrate to the router - otherwise, theline protocol will come down, as discussed in another section.

Now, those two frame switches shown in that diagram are not going to bethe only switches in that cloud. Quite the contrary, there can be hundredsof them! For simplicity's sake, the following diagram will have less thanthat.

Don't worry, we don't need to list or even know every possible path in thatcloud! The key here is to know that not only will there be multiple pathsthrough that cloud from Router A to Router B, but data probably willtake


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different paths through that cloud. That's why we call this connectionbetween the routers a virtual circuit- we can send data over it anytime weget ready, but data will not necessarily take the same path through theprovider's switches every time.

Frame relay is apacket-switchingprotocol. The packets may take differentphysical paths to the remote devices, at which point they will bereassembled and will take the form of the original message. In contrast,circuit-switchingprotocols have dedicated paths for data to travel from onepoint to another.

There are two types of virtual circuits, one much more popular than theother. Apermanent virtual circuit(PVC) is available at all times, where aswitched virtual circuit(SVC) is up only when certain criteria are met.You're going to see PVCs in most of today's networks, and we'll buildsome during your CCNA studies.

You're not responsible for configuring SVCs on the CCENT or CCNAexams.

RFC 1918 Private Addresses, NAT, And PAT

You were introduced to the three RFC 1918 private address ranges in theIP Addressing section, but let's review them here:

Class A: 10.0.0.0 - 10.255.255.255 (10.0.0.0 /8) Class B: 172.16.0.0 - 172.31.255.255 (172.16.0.0 /12) Class C: 192.168.0.0 - 192.168.255.255 (192.168.0.0 /16)

You also learned that these addresses are not routable - without anyadditional help, hosts with these addresses will not be able tocommunicate with any other hosts outside their private network. Thatmeans no internet access and no communication with other hosts acrossthe LAN!

In the following example, a host attempts to send packets out to the WAN.The router will see the source IP address of 10.2.2.2, realizes that thisaddress is a private address, and does not attempt to send the packet toits destination.

We have two options that will allow this host to communicate with WAN-based hosts, Network Address Translation (NAT) and Port Address

Translation(PAT). The words address translationare the key here, sincethe host's private address will be translated to an IP address that canberouted.


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You'll see the actual CLI configurations and some options for both NATand PAT in your CCNA studies, but for now let's take an overview of thesevital network services.

A common NAT deployment method is to set aside a pool of routable IP

addresses, and when hosts with private addresses attempt to sendpackets to WAN-based hosts, their private IP address is translated to oneof those routable addresses.

The router will keep a NAT table that maps the private addresses to theassigned NAT address, so when packets come back in with the NATaddress, the router translates that particular address back to theappropriate private address.

NAT works beautifully, but there's one drawback - many organizationsdon't have enough routable IP addresses to set aside for NAT users. Port

Address Translation allows a company to use a single routable IP addressfor multiple hosts. The routable IP address will be the same, but the portnumber will be different, and it's theport numberthat the router will use tokeep the different translations straight.


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We have two translations here, and the source IP address for bothtranslations is the same, 210.1.1.2. It's the port number that's different.The router will keep a PAT table much like the NAT table mentionedearlier, with the port numbers mapped to the appropriate private address.When the WAN-based hosts respond to a particular IP address and portnumber, the router will translate it back to the correct private IP address.

Whether you choose NAT or PAT, the entire process is transparent to thehosts with the private addresses and the WAN-based hosts they'recommunicating with. The only device that even knows that NAT or PAT isin use is the router itself.

The actual NAT and PAT translation table can be viewed with thecommand show ip nat translation. Even if you're running PAT, thecommands will still reference NAT.

R3#show ip nat translationsPro Inside global Inside local Outside local Outside global--- 210.1.1.2 10.5.5.5 --- ------ 210.1.1.3 10.5.5.6 --- ------ 210.1.1.4 10.5.5.7 --- ---

Notice those four terms in the translation table - "inside global", "insidelocal", and so forth? Here's what they mean:

Inside local addresses are used by hosts on the inside network tocommunicate with other hosts on that same network. These are theaddresses that are actually configured on the hosts, and generally theyare RFC 1918 private addresses.

These inside local addresses are translated into inside global addresses.Inside global addresses are routable addresses. In the followingexample, 10.2.2.2 is the inside local address and 210.1.1.2 is the insideglobal address.


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Outside global addresses are the addresses that are configured on theoutside hosts. These are fully routable addresses used by Internet-basedhosts.

Finally, outside local addresses are the actual addresses of remotehosts. These can be (and probably are) RFC 1918 addresses as well.

These terms can take a little getting used to, but just remember - theinside addresses are the ones used by your network, and the outsideaddresses are the ones used by the hosts at the remote end of thecommunication (assuming they're using NAT as well).

Most of the NAT and PAT deployments currently in use were configuredat the CLI, and you'll learn how to do that in your CCNA studies. In thefollowing lab, we're going to use the Security Device Manager (SDM) toconfigure a router for PAT. We'll then test the translation as well!

You will not be tested on SDM in the CCENT or CCNA exam, but it's goodfor you to see a Cisco GUI in action.

Here's the network topology:

Our PC needs to connect to that web server, but there's a problem. ThePC has a Class A RFC 1918 private address, which is not a routableaddress. We'll use SDM to configure PAT to allow that PC connectivity tothe web server, plus I'll sneak in an extra point about pings and PAT. (Try

saying thatthree times really fast!)

In another section, we use SDM to configure DHCP, and that was found


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under "Additional Tasks" at the very bottom of the screen. For PAT, wejust click on "Configure" and then "Interfaces And Connections" to bringthis screen up.

We'll select the second radio button and click "Create New Connection".

We're going to assign a static IP address of 172.20.21.1 /16 to the FastEthernet 0/1 interface. Note that this is the interface facing the Internet,as Fast Ethernet 0/0 has an RFC 1918 private address as well.


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The next screen gives us a chance to configure Advanced Options, andit's here that we'll enable PAT on this interface by checking the "Port

Address Translation" box.

When doing so, make sure to choose the correct interface. The firstchoice here is Fast Ethernet 0/0, but that's the inside interface.

I simply clicked on the drop-down box and chose Fast Ethernet 0/1.


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Finally, SDM shows you a summary of what you've chosen. Note the PATinside interface is Fast Ethernet 0/0 and the outside interface is Fast 0/1,

just what we want.

By clicking "Finish" at the bottom of the screen, you'll see a CommandDelivery Status window that verifies the configuration has been written tothe router. Note that it takes 13 commands to configure what we'vechosen!

SDM verifies that FastEthernet 0/1 has been configured with an IPaddress of 172.20.21.1 and that the interface is the outside NAT interface.


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This all looks good, but we better test it! Let's revisit the network topology:

Let's send a ping from the router to the web server to verify connectivity,and then check the NAT translation table.

RouterA#ping 172.20.21.254

Type escape sequence to abort.

Sending 5, 100-byte ICMP Echos to 172.20.21.254, timeout is 2 seconds:

!!!!!

Success rate is 100 percent (5/5), round-trip min/avg/max = 1/2/4 ms

RouterA#show ip nat translation

< nothing shows up and we're taken back to the prompt >

RouterA#

When you run a showcommand and you get nothing in return, that meansthere's nothing to show you. In this case, there have been no NATtranslations. Why? Because we sent the ping from the router, and by


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default the source IP address of the ping will be the exit interface's IPaddress. The source IP address of the ping from the router is172.20.21.1, and that address requires no translation.

To truly test PAT, we need to do one of two things:

Send an extended ping from the router and specify a source IPaddress for the ping of 10.1.1.11

Go to the PC and send a ping from there

Since it's the PC that needs connectivity to the web server, let's send aping from the PC to that server and then check the NAT translation table.

C:\>ping 172.20.21.254

Pinging 172.20.21.254 with 32 bytes of data:

Reply from 172.20.21.254: bytes=32 time=4ms TTL=127

Reply from 172.20.21.254: bytes=32 time=1ms TTL=127Reply from 172.20.21.254: bytes=32 time=1ms TTL=127

Reply from 172.20.21.254: bytes=32 time=1ms TTL=127

The ping from the PC is successful. Let's check the translation table:

RouterA#show ip nat translationPro Inside global Inside local Outside local Outside globalicmp 172.20.21.1:512 10.1.1.1:512 172.20.21.254:512 172.20.21.254:512

Not all NAT tables list the protocol, but this one does, and you can seethat it's an ICMP translation since that's what ping packets are. You cansee the port translation as well, and the inside global address of

172.20.21.1 is the PAT interface on the router! That's just what weexpected to see.

Other WAN Technologies

What follows is strictly an overview of some other WAN communicationmethods, and with ATM, it's really an overview! Configuration of ATM isfar beyond the scope of the CCENT and CCNA exams, but it's a goodidea to know the basics.

The Asynchronous Transfer Mode (ATM) is unique in that it does nothandle frames, as Frame Relay does. ATM places data into cells, and all

ATM cells are exactly the same size, 53 bytes - 48 bytes of data and a 5-byte header.

Remember the Frame Relay switches that made up the Frame Relaycloud? ATM works along the same lines in that the service providermaintains ATM switches. ATM networks are much faster than FrameRelay networks, but are more expensive to build and maintain.

That includes the need for specialized hardware. You can't just sit downand configure ATM on a Cisco serial interface as you could Frame Relay -you'll need special interfaces to use ATM.

Some ATM documentation says that ATM is a packet-switching servicelike Frame Relay, and technically that's true, but more commonly you'll


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hear ATM referred to as cell-switching. After all, that's what ATMswitches!

Modems

On the other end of the speed spectrum, we've got modems - and yes,they're still out there! In the previous CSU/DSU illustrations, a modemcould and sometimes does take the place of the CSU/DSU.

The word "modem" actually comes from the two operations they carry out:

Modulation, the process of translating digital signals into analogsignals that can be carried over a phone line

Demodulation, the process of translating those analog signals backinto digital signals that the receiving device can understand

In this simple network setup, there are two points where such a translation

is needed.

When data leaves Host A, it's in digital format - a stream of ones andzeroes. Remember, it's all ones and zeroes!

Why do we need the modem? We're going to use a telephone line to getthis signal to Host B, and the telco's phone lines don't know what to dowith a digital signal. That signal's got to be translated into an analogsignal, and that's part of what the modem does.

When the analog signal arrives at the destination, the modem now has to

demodulate the signal by translating it back to the original digital signal.This is necessary because Host B has no idea how to handle an analogsignal. Host B only understands - say it with me - ones and zeroes!


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The two real drawbacks of modems are:

They're not as fast as other methods They tie up the phone line, and other calls cannot be made while the

modem is using the line

And what are those other methods? Glad you asked!

Many of you downloaded this course on a Digital Subscriber Line (DSL)

connection. Or an ADSL line. Or an SDSL line. Or...

Well, you get the idea. There are multiple DSL variations, and here's alook at just a few.

Asymmetrical DSL works under the assumption that the user willdownload more information than they send, and for the average Internetuser, that's a safe assumption. The connection speed from the provider tothe user is going to be 3 - 4 times faster than the speed from the user tothe provider. A typical ADSL connection of 512 kbps will give theuser 384 kbps download capabilities, but only 128 kbps uploadingcapability.

ADSL allows a telephone call and internet access simultaneously.

ADSL uses several different modulation methods, but the most well-known is G.lite (also known as G.922.2), which requires no splitter at thecustomer location. The customer simply hooks up a G.lite modem in thesame way an old-fashioned analog modem would be installed.

G.lite's limitation is speed - where standard ADSL can offer 8 MBPSdownload speed and 1.5 upload speed, G.lite's maximum capability is 1.5MBPS downloading and 512 KBPS uploading. The key is that while G.liteis slower than true ADSL, it's still faster than the dialup options availableto today's home users.

The distance limitation of ADSL must be taken into account as well.


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Officially, there's an 18,000-foot limitation on ADSL services, but mostISPs put a lower limit on ADSL to avoid poor quality service for those nearthe end of the cable. Of course, that limitation is for data transmission, notvoice.

Since we have asymmetric DSL, it makes sense that we'd have

symmetric DSL(SDSL) as well. The term "symmetric" refers to the factthat the sending and receiving speed are the same. The drawback is thatthe phone cannot be used while SDSL is in use.

Two less-common DSL flavors:

Very High Bit-Rate DSL(VDSL) has the capability to deliver speed up to52 MBPS. That's am amazing speed to deliver over copper wire, butthere's a drawback - VDSL over copper has a maximum distance of 4000feet. As more fiber-optic cable is installed by the telephone companies,

VDSL is becoming available in more communities as the distance issue isresolved by the use of fiber.

Rate-Adaptive DSL (RADSL) is just what it sounds like - the softwarecalculates the maximum download and upload speeds on the customer'spreexisting phone line and dynamically adjusts those rates.

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