Wan protocols question

cisconetguy · ‎03-18-2010

Hi,

I want to know, Why do we use WAN protocols instead of ethernet, apart from cabling limitations?

Also, how exactly WAN works, like How the router forwards the packet over the WAN? e.g using ppp

Do WAN protocols also rely on TCP for guarantee delivery?

I also read somewhere, PPP does not use destination address over WAN. Why is that?

Sorry for all newbie questions.

I would be happy if somebody describes to me clearly.

Tharak Abraham · ‎03-18-2010

Sandy,

can give a very brief answer here.

It all depends on what your SP has to offer.

Ethernet, Leased lines, Frame Relay, ATM, Sonet et al..

Now coming on to TCP, it depends on what kinda applications you are running.

eg: for Http, yes you need TCP but for VoIP, not required !

WAN protocols dont really need to know or know what they have at the transport layer !

They just do their job at Layer 2 and thats all !!

i.e Ethernet wont even know whether its carrying http or ftp or even voice traffic.

PPP and destination address trivia, am not aware !

This should also help ya

http://www.comptechdoc.org/independent/networking/cert/netwan.html

cisconetguy · ‎03-19-2010

Tharak, I am aware of what you said already. thats not what i'm looking for.

I want to know, Why do we WAN protocols just because they can connect sites that spans geographical area?

What if ethernet had supported long cabling for connecting sites that spans geographical area?

Again, How does router forwards packet when we are using WAN protocol like PPP? Like, Ethernet looks up at layer 2 mac address when forwarding.

What about WAN protocols?

Tharak Abraham · ‎03-19-2010

See, it all depends (borrowing Scott Morris's qoute)

Do you think spanning a cable between Geographies is a viable soln.?

(Metro ethernet is a close call for your question though)

Ethernet is a shared media and PPP is NOT !

Since its shared we need to exactly specify an L2 address in the frame to whom its destined for.

In PPP we tell the router that its a point to point connection i.e there is only one endpoint to what and where it sends !

Hence it doesnt carry any address as such (please check for the frame format)

The address field in a PPP frame would be all 1's meaning 0xFF or 255 in decimal.

WAN protocols on a general note means what is going to carry my IP traffic ?

There are indeed reasons for choosing the WAN protocol.

eg: PPP for dialup etc.

Giuseppe Larosa · ‎03-19-2010

Hello Sandy,

>> Again, How does router forwards packet when we are using WAN protocol like PPP? Like, Ethernet looks up at layer 2 mac address when forwarding.

Being PPP a point-to-point protocol there is no need for L2 MAC address mapping a fixed PPP header is used to encapsulate the IP packet in a PPP frame.

In other cases like in a Frame-Relay network other forms of mappings are used:

each remote Frame-Relay device IP address has to be associated to the correct Frame-Relay DLCI.

So the FR DLCI could be the equivalent of a MAC address but actually it has only local value (it changes at each switch hop).

Dynamic association of IP address and FR DLCI can be performed by Inverse ARP: special frames that travel end to end telling who is on the other end of a PVC.

Hope to help

Giuseppe

Ronit Bhattacharjee · ‎03-19-2010

Hi Sandy,

1. I want to know, Why do we use WAN protocols instead of ethernet, apart from cabling limitations?

Ans. WAN protocols is a broad term, no one would have the patience to sing the virtues of each one of them. However, for PPP in particular, here're the reasons

a. Authentication support before bringing line protocol up

b. Link failure detection (does not work for ethernet if intermediate device is down)

c. Negotiation of parameters like MTU etc. before bringing the link up

d. Support for numerous physical media (T1/T3, E1/E3, POS, SONET, ATM etc.)

e. Support for seamless/smart load-balancing using PPP multilink

2. Also, how exactly WAN works, like How the router forwards the packet over the WAN? e.g using ppp

Ans. Try google

http://en.wikipedia.org/wiki/Point-to-Point_Protocol

3. Do WAN protocols also rely on TCP for guarantee delivery?

Ans. These are all Layer 2 protocols. They don't care for upper layer protocols. They work the same way for ppp/Fr as for ethernet. Layer 3 and above work the same way

4.I also read somewhere, PPP does not use destination address over WAN. Why is that?

Ans. What destination address? If you;re talking about Layer 2, PPP doesn't need this because as the name says, it's point to point. It expects only 1 device on the other end of the wire so why would it need a Layer 2 destination address? Layer 3 is the same as ethernet

Try getting a good book (BCRAN would do), or install GNS, capture packets and see for yourself.