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Alicia Silverstone Discusses Basic ISDN

RouterGod staffer Erwin Chiong recently spent the afternoon with Alicia Silverstone.  Miss Silverstone has starred in many movies such as Clueless and My Gay Life and is well known for appearing in 3 Aerosmith music videos.  Although she has had a very successful film career, few people are aware of the fact that in her leisure time she enjoys radio astronomy, quantum mechanics and most of all, configuring a Cisco 3620 which sits on a nightstand next to her bed.  Let's join Erwin Chiong as he interviews Alicia at her parents home in San Francisco...

 

 

 

 

 

 

 

 

 

Erwin

Well, Alicia, thank you for taking the time from your busy schedule to visit with us.

Alicia

No problem, Erwin, I'm never too busy to talk about Cisco Routers!

Erwin

Great, do you mind telling our readers how you became interested in Internetworking?

Alicia

During the filming of The Crush, production was going slow, so one of my friends thought it would be cool if I got my CNE (editor: Certified Novell Engineer), back then everybody had a CNE and you didn't even need experience on a NetWare server, just a few tests and Bang!  I was a CNE.  Anyway, since then I have a become a Microsoft Certified Professional in Windows Notepad and just 3 months ago I earned the Cisco CCNA.

Erwin

That's great, Alicia.  What's your Favorite routing protocol?

Alicia

I like RIP.  It's easy to configure and there are no tough issues like the "autonomous system" numbers and stuff.  I'm not good at math.

Erwin

But RIP is not really scalable, is it?

Alicia

Whatever!

Erwin

What's your favorite WAN protocol?

Alicia

Ummmmmm...I knew you were going to ask me that!  I really like X.25 but there is too much overhead, ya know?  If I had to say, like fer sure, I'd say that ISDN is my favorite WAN protocol.

Erwin

Oh?  Why do you like ISDN so much?

Alicia

Because it's basically a phone call, except it's digital instead of you know, analog.  ISDN BRI is way cool, it has these 2 channels called B channels, they actually carry the data.  And there is this other baby channel, it's the D channel, it carries signaling for call setup and teardown, that's why ISDN calls connect so quickly.

Erwin

You're right about that, ISDN calls set up quickly.

Alicia

Duh!  I just said that!  There's more to ISDN than what setting up the call ya know.  Like you have to know the "ISDN Switch Type", the guy at the phone company tells you that.  It could be an AT&T 5ESS or a DMS 100 or they have others too.  Some switches require you to configure SPIDs, isn't that a cool sounding name?  On a BRI interface, you would need to configure a SPID for each B channel, but the phone guy will tell you if you need SPIDs.  You also have to configure other stuff too, like a "dialer list" and don't forget you need an IP address too!

Erwin

How would you go about configuring ISDN?

Alicia

Well, if it was me, I'd configure the ISDN switch type first.  You can do that globally or at the interface, it doesn't matter.

Erwin

But isn't it true that if you do it globally, it will be applied to all ISDN interfaces on the router, and if you configure the switch type at the interface, it applies only to that specific interface?

Alicia

Huh?  Yeah, that seems right.

Erwin

What is the command to configure the switch type?

Alicia

That's easy, its "isdn switch-type" followed by the actual switch type, you can enter a question mark if you need help.

Erwin

Super, how would you configure SPIDs?

Alicia

You have to you to the BRI interface, on my router I always enter interface bri 0/0 and hit enter.  Then you are at the BRI interface, it has these little parentheses and inside they say "config-if", anyway you have to configure 2 spids, but only if the telco switch requires it, you go isdn spid1 and enter the SPID number then go isdn spid2 and enter the other SPID number.  It's pretty simple.

Erwin

What's next?

Alicia

Now would be a good time for an IP address, you would enter ip address 172..16.1.1 255.255.0.0 and push down on the enter key again.  Here is where I enter the encapsulation, I go encapsulation ppp and I hit enter.  I like ppp encapsulation because it lets you do stuff like authentication.  Next I enter ppp authentiction chap

Erwin

Authentication?  What for?

Alicia (rolls her eyes)

OH. MY. GOD!  ISDN is essentially just a telephone call. Do you want every gnarly surfer dude from the valley calling your router?  No waa-aay!  Like hello?  Is Alicia home?  I'm sure!  Next thing to configure is a dialer list to define interesting traffic.

Erwin

Interesting traffic?  Do you mind explaining to our readers what you mean by that?

Alicia

No Prob, ISDN is expensive, you pay by the minute and it can really add up.  Your ISDN interface will establish a connection every time it receives traffic destined for whatever networks are at the other end of the link.  Lets say you have a network at a branch office that runs IPX and IP, but you only want IP traffic to bring up the link, so you must define IP as being interesting, get it?  You do it with a "dialer list" it works kinda like an access list.  In global mode you would enter the following:

dialer-list 1 protocol ip permit

Then you go back to the interface mode and apply it like you do an access list:

dialer-group 1

Erwin

But that seems kinda broad, permitting the entire IP suite, is there a way to be more granular about what constitutes interesting traffic?

Alicia

Definitely!  You can make a sub routine call to an extended ip access list, watch: dialer-list 1 protocol ip list 101  Pretty cool, huh?  You can't get much more granular than an extended access list! 

Erwin

What do you configure next?

Alicia

You need to map the layer 3 IP address of the next hop router to the layer 2 address of the ISDN phone numbers that you want the router to dial to reach that IP address.  Pretend that the dialer string you want is 55512121010 and the interface at he other end of the link has an IP address of 172.16.1.2  Also pretend the the router at the other side of the link is called "denver". This uses the dialer map command, here's how to do it:

dialer map ip 172.16.1.2 name denver 55512121010

Erwin

Gee wiz, Alicia, I had no idea you knew so much about ISDN, do you mind showing us your config?

Alicia

This is all easy, it was on the CCNA test, there is lots more to configuring ISDN, but here's my config:

!
isdn switch-type basic-5ess
!
interface bri 0/0
ip address 172.16.1.1 255.255.0.0
encapsulation ppp
dialer map ip 172.16.1.2 name denver 55512121010
dialer-group 1
ppp authentication chap
!
dialer-list 1 protocol ip permit
!

Erwin

Wow, Alicia that was awesome!  You're the best, thanks a lot!

Alicia

Whatever!


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