Frame Relay DLCI question?

abdigulaid · ‎09-20-2007

Hello ladies & gentlement,

I have a question that you might be able to help me with. It is regarding Frame-Relay, i seem to have some gaps in my study material... I understand how it mostly works but i feel that something is missing. The question is as follows:

When a router sends a Frame Relay packet accross a VC how does the router know what the correct DLCI iS for the router at the other end of that VC (I understand that mapping is involved but just dont know how this is done), would the Frame Relay Switch (DCE) make changes to the packet?

Answers or guidance would be much appreciated.

rodman.frowert · ‎09-20-2007

I'm by no means an expert at all, but from what I remember from my CCNA material, the telco (or whoever your service provider is) talks to your router via LMI. After you call and setup two PVC's and connect them end to end via your T1 or whatever you are using, the router works out what DLCI (via IARP I believe) it needs to use to talk to the other router as soon as you turn on Frame Relay encapsulation.

If, for some reason, your service provider can not talk to your router (perhaps LMI isn't working properly), then you have to map the DLCI's to the IP on the other end...

I am sure you will get a better response than this from other more knowledgable people... But this is laymens turms..

Rodman

abdigulaid · ‎09-20-2007

Rodman,

Thank you for responding so quickly, your input is much appreciate and is already beginning to help me understand this technology alittle better. Thanks again.

Richard Burts · ‎09-20-2007

Abdi

The router establishes which DLCI to use to get to various destination addresses. There are several methods that it uses. If it learns it on a point to point subinterface it knows automatically that there is a one to one mapping of remote address to DLCI. If the router is configured is manual frame relay mapping then the router uses the contents of the manual mapping to relate local DLCI to remote address. Or the router can use inverse ARP to establish the relationships.

One way to think about it is that the router needs to create a mapping of layer 3 addresses to layer 2 addresses. This is quite similar to the process used on Ethernet to map layer 3 addresses to layer 2 addresses (which is ARP).

The Frame Relay switch connected to the router will make some changes in the frame. It receives a frame with the local DLCI of the connection to the router and the switch rewrites the frame with the DLCI of the next link in the path. The switch may also rewrite the header to indicate DE (discard eligible) or to rewrite the frame to indicate FECN or BECN.

HTH

Rick

HTH

Rick