Frame relay and mapping ip question

[John Blakley]

All,

I've been playing around with FR mappings and noticed that the mappings require the address and LOCAL dlci, not the remote.

frame-relay map ip 192.168.1.1 201

The 192.x.x.x address is the remote router, but the 201 is the local router's dlci. Why is this? Am I mapping a way for 192.168.1.1 to get back to me through my dlci?

Thanks,

John

[Edison Ortiz]

(Please say yes!)

No !

j/k :)

[John Blakley]

LOL! I'll take that as a yes :)

[Richard Burts · Accepted solution]

John

What you are suggesting here is exactly correct if the network is to be full mesh. In full mesh each router needs a unique DLCI for every peer in the Frame Relay network.

HTH

Rick

[Edison Ortiz · Accepted solution]

*EDIT* I went back and looked, and when configuring the router as a FRSW, you don't even need addresses on the FRSW interfaces, which makes my question even more confusing.

Why? It actually answers your question. You keep mixing IPs with DLCI.

The FRSW swaps|switches DLCI from incoming to outgoing. The CE routers will map a DLCI to an IP and send the packet out towards the FRSW. The FRSW does not care about the IP address information, just the DLCI.

And you want me to post more complicated FR configs? LOL

__

Edison.

[John Blakley]

And you want me to post more complicated FR configs? LOL

That not what I meant by that statement :)

[Jon Marshall · Accepted solution]

John

Think of it like this. Frame Relay like ATM can multiplex many different virtual circuits on the same physical link. With frame-relay the virtual circuits (vc) are identified with DLCIs.

So if the physical interface has many vc's using the frame-relay map ip command allows you to tell the router that to get to a specific remote address the data needs to be sent over specific vc and that vc has to be significant to the local router. That vc is a connection between the local router and the Frame switch. And you tell the Frame switch that data arriving on that vc is switched to the remote destination.

Hope this makes sense.

Jon

[John Blakley]

Actually Jon, it doesn't :)

I could understand if I'm telling the router to get to 192.168.1.1 to go to the dlci that 192.168.1.1 owns, but not coming from the dlci that I own. I guess I'm trying to compare the term mapping to something like a hosts file.

I don't have any experience with frame-relay in the field. I've always worked on T1s and DS3s, and the whole concept of frame-relay switching is a bit different. I'm reading the comprehensive guide on FR from Cisco now.

Thanks,

John

[Giuseppe Larosa]

Hello John,

DLCIs are like MPLS labels they are local and defined on each interface.

DLCIs change at each FR switch hop.

So it can be 101 then changes to something else

suppose you have

RA --- SW1 -- SW2 -- SW3 -- SW4 -- RB

the pvc is built by associating incoming interface, DLCI pair with an outgoing interface, DLCI pair

a switch can say

if a frame with DLCI 101 arrives on interface 8.1 switches the frame to interface 10.1 using DLCI 512

next switch in path will do the same and so on up to reach the last link to remote router RB.

The DLCI is used in both directions to send and to receive frames on a link between a DTE (router interface) and a DCE (FR switch interface).

The path is pre-established and so by specifying the initial DLCI on RA interface to SW1 there is no ambiguity.

This is totally different then using a MAC address on a LAN where you actually have a global identifier of the remote end device.

Here you just need to identify on which logical circuit a frame has to be sent out depending on the remote end device you want to reach.

Frame relay introduced this idea of logical circuit and DLCI to perform multiplexing/demultiplexing.

It is all about switching frames by changing the DLCI at each FR switch hop.

So what you see is correct and the only possible way to have it working.

The local router doesn't know and cannot use the DLCI as defined in the last link between remote FR switch and remote router.

It can only use the local DLCIs as defined on the UNI interface between its interface and the the FR switch port.

Hope to help

Giuseppe

[John Blakley]

Giuseppe,

The local router doesn't know and cannot use the DLCI as defined in the last link between remote FR switch and remote router. It can only use the local DLCIs as defined on the UNI interface between its interface and the the FR switch port.

So, the router, even though I have a common subnet defined across wan links, can't see it because it only knows how to get to the switch based off of it's DLCI? And because of that, any address that I need to get to out of my wan interface would need to be mapped to my local dlci, and the switch would handle it from there, and return the traffic back to my dlci? (Aside from running a routing protocol for my internal subnets.)

I've searched for documents for FR on Cisco's site, but the only thing I'm finding are configuration guides and nothing discussing functionality.

Thanks,

John

[Richard Burts]

John

There is an analogy that I sometimes use in explaining Frame Relay and the DLCI. Perhaps it will help you to understand the relationship between the DLCI (local) and the remote IP address.

Consider a multiline phone on my desk: perhaps it has 3 buttons that correspond to 3 working phone lines and has a local phone number associated with each line/button. The identifier (line1, line2, line3) and the phone number of the buttons is a bit like the DLCI - it tells me which connection to the phone switch is which (or the DLCI says which connection to Frame Relay is which - assuming that you may have more than 1 DLCI).

Now lets assume that a connection has been established from my phone to your phone (so the phone company established a switched connection from me to you - which is similar to the way that Frame Relay establishes a connection from my router to your router). That connection is made on one of the buttons on my phone and uses that phone number.

If you are with me so far then the rest is easy. To communicate with your remote address I need to remember which button/which phone number on my phone is the connection to you. I am less concerned with what your phone number was, I need to remember which of my local identifiers is the way to connect to you. Frame Relay DLCI works the same way. To communicate from your router to the remote subnet you need to use the DLCI of your local connection and do not care what is the DLCI assigned to the connection at the remote end.

HTH

Rick

[John Blakley]

That's a great explanation Rick :)

Thanks!

John

[Giuseppe Larosa]

Hello John,

FR is standards based

FR forum has joined ATM forum that later joined MPLS forum in the ip/mpls forum.

the official FR specifications can be found here

http://www.ipmplsforum.org/tech/fr_ia.shtml

download the uni for pvc spec

Hope to help

Giuseppe

[Jon Marshall]

John

Apologies if i haven't explained it well enough altho it looks like Giuseppe has expanded on what i was trying to explain.

Your router connects to a frame relay switch with a serial interface. However on that physical serial interface you can have multiple virtual connections. Each virtual connection is logically separate from all the other virtual connections.

Now all packets from your router to the frame-relay switch will come from the same physical serial interfaced but the frame-relay switch has to have some way of knowing which logical connection this traffic is on. Remember that the frame-relay switch operates at L2 so it does not do L3 lookups.

So a mechanism is needed between the local router and the FR switch to work out which logical connection is being used. That is what DLCI's are for. They are locally significant because the DLCI is only used between the local router and the FR switch.

Now as described by Edison and Giuseppe the FR switch has mappings so that traffic arriving on port x with a DLCI of x gets sent out on port y with a DLCI of y. Again the FR switch has no knowledge of the IP addressing. The FR switch only deals in DLCI's, bit like MPLS as Guiseppe described. So it receives the packet with DLCI x and forwards it out on DLCI y.

Only when the packet arrives at the destination router does the IP address become relevant ie. once the packet has left the local router the IP address is never referenced until it gets to it's destination.

Jon