Frame Relay mapping

Unanswered Question
Oct 20th, 2009

I am having trouble understanding how frame relay gets passed on through a frame relay network. What happens at each hop? I understand how each frame has DCLI for the local interface. But how does the frame travel accross the cloud? How does the network build a map to the other end of the PVC? In ethernet MAC and IP addresses are used to move across a inter-networks. How is it done with Frame relay? I hope that is clear..; )

Thanks

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Giuseppe Larosa Tue, 10/20/2009 - 21:20

Hello Scott,

Frame Relay requires that an end-to-end path has to be built before frames can be sent.

Most of Frame Relay implementations use permanent Virtual Circuits.

That is the path is built once and it is ready to carry frames.

The big difference between a FR network and a LAN is that DLCI is a form a local addressing and MAC address is a form of global addressing.

FR uses Virtual circuits, each virtual circuit is associated to a DLCI value.

but the DLCI has local validity and can change at each hop.

so each FR switch looks at (incoming interface, incoming DLCI) to find out what (outgoing interface, outgoing DLCI) to use.

So if you have

R1--SW1--SW2---SW3---R2

a PVC can be

DLCI 30 between R1 and SW1

DLCI 51 between SW1 and SW2

DLCI 823 between SW2 and SW3

DLCI 77 between SW3 and R2

R1 has to use DLCI 50 to send a frame to R2 that receives it on DLCI 77 on interface between R2 and SW3

FR SVC are instead setup dynamically when a need for sending traffic arises.

This requires more complex activities in the signalling plane that is some form of global addressing is needed to designate the intended destination, some protocols find out where this DTE is connected and a path again with DLCI changing is built end-to-end.

PVCs are manually built directly or using management suites.

SVCs require initial setup.

see

http://www.cisco.com/en/US/docs/ios/12_2/wan/configuration/guide/wcffrely.html

Hope to help

Giuseppe

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