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Frame relay question

Posted on 2005-03-06
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Router A------Dlci 200--------------------------------------------------------Dlci 201 Router B
            ------ Dlci 201------------------------
                                                              |
                                                              |
                                                         Dlci 201  Router C


(from my cisco book):  When router A wants to reach Router B, he uses DLCI200
                                  When router B wants to reach Router A, he uses DLCI 201
                                  When router C wants to reach RouterA, he uses DLCI201

When router A sends to router B, the switches swap the DLCIs and deliver the packet to router b with a dlci of 201. Router B this came from Router A

When router C sends  to router A, the switches swap the DLCIs and deliver the packet to router a with a dlci of 201. Router A knows this came from Router C.

My questions:

1. What happens if Router B wants to send something to RouterC? Does he have to go through Router A to do this?  If so, how would the DLCIs change in this instance.

2.Is this an example (my diagram) of point to point frame relay?  Where you have one router with two DLCIs kind of inter-routing between all the networks.

thanks
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Question by:dissolved
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by:Merlin740500
ID: 13475147
1. You need to create a subinterface in wich are router b and router c.  Communication will not occur if not set.  I can also say you should add a point to point subinterface including router b and router c.

2. it is well explained (more than i would) at : http://www.cisco.com/univercd/cc/td/doc/product/software/ios121/121cgcr/wan_c/wcdfrely.htm  Under understanding frame relay subinterfaces.

Also, those questions are covered in CCNA book.  If it interest you.  Todd Lammle does very good CCNA books.

Note : if it does not answer enough, i'm gonna be outside city for 4 days, i might not reply.
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lrmoore earned 800 total points
ID: 13475244
1. What happens if Router B wants to send something to RouterC? Does he have to go through Router A to do this?  If so, how would the DLCIs change in this instance.
If RB wants to talk to RC, then RA has to route L3 traffic between the two DLCI's

2.Is this an example (my diagram) of point to point frame relay?  Where you have one router with two DLCIs kind of inter-routing between all the networks.
Yes, this is an example of non-mesh point-to-point FR. To create a full-mesh, there would be another point-to-point DLCI between RB and RC

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Assisted Solution

by:skpruett
skpruett earned 1200 total points
ID: 13475589
Hi dissolved,
Answers for your questions:

1) Separate yourself from the very common mistake of associating DLCI numbers with needing a match on each side. DLCI's are unique to ONE SIDE ONLY and very commonly won't have numbers that in any way correspond to the opposite side. If router B needs to traverse to C this is easily done in a hub and spoke setup for frame-relay. He leaves on DLCI 201, arrives on 200, enters the sub-interface on router A, gets pointed out a different sub-interface toward router C, leaves on DLCI 201, arrives on DLCI 201, reaches his destination.

2) No, this isn't point to point exactly. It's usually referred to as "Hub and Spoke". It's an extremely common example of a main office communicating to satellite offices.

-skpruett
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Expert Comment

by:skpruett
ID: 13475621
To expand on my answer to number 2 above this is usually called point to multipoint.

If this is homework be sure and tell us what grade you make. =)

-skpruett
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