Frame relay and mapping ip question

Answered Question
Apr 22nd, 2009

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

I have this problem too.
0 votes
Correct Answer by Richard Burts about 7 years 7 months ago

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

Correct Answer by Edison Ortiz about 7 years 7 months ago

Bingo!

That's the reason your mappings didn't work yesterday.

If you see my GNS3 FRSW mappings, it should make sense to you. You had fewer mappings than mine. Check both .NET files and compare.

__

Edison.

Correct Answer by Edison Ortiz about 7 years 7 months ago

*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.

Correct Answer by Jon Marshall about 7 years 7 months ago

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

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Edison Ortiz Wed, 04/22/2009 - 05:50

At the Frame-Relay switch, on the interface facing your router, there is a group of commands that instruct the switch what to do in terms of DLCI switching.

The FRSW sends LMI with DLCI 201 to your router and any packets coming from your router towards DLCI 201 will be switched to another DLCI that corresponds the 192.168.1.1 device.

To recap, the DLCIs are locally significant between the FRSW and the router connected to it.

HTH,

__

Edison.

John Blakley Wed, 04/22/2009 - 07:04

Edison,

So the only DLCI the router will ever know about is the local one that was assigned by the switch?

I know that a FR switch can route packets from one to another dlci like:

frame-relay route 150 inter s0/0 140

I believe this is saying that a router assigned dlci 150 gets routed out of s0/0 to dlci 140. Does this complete one pvc?

If it does, then take the following on the same switch:

frame route 130 inter s0/1 120

This does this same thing for incoming 130 to outgoing 120? Now if I had another router at dlci 110, and I mapped:

frame map ip 192.168.1.2 110 broadcast

Let's assume that 192.168.1.2 is out s0/0. How does the switch know to route out of that interface instead of s0/1? Is it based off of the ip address that's assigned to that interface?

Thanks,

John

Edison Ortiz Wed, 04/22/2009 - 08:24

So the only DLCI the router will ever know about is the local one that was assigned by the switch?

It can be multiple DLCIs advertised by the FRSW switch towards your router but each DLCI is switched to a different destination.

I believe this is saying that a router assigned dlci 150 gets routed out of s0/0 to dlci 140. Does this complete one pvc?

Not always, your local FRSW will be accepting your incoming flow but it can route within the frame-relay network to other FRSWs, if necessary. Once it reaches the final destination, then you can define a complete PVC.

frame map ip 192.168.1.2 110 broadcast

Let's assume that 192.168.1.2 is out s0/0. How does the switch know to route out of that interface instead of s0/1?

The command above is done at the router, not the FRSW. The FRSW needs the frame-relay route command, not the frame map ip command.

__

Edison.

John Blakley Wed, 04/22/2009 - 09:15

Edison,

Warning: May get confusing.

I think my main question is that if a router is configured as a FRSW, and it knows to route:

RTR A is configured as a FRSW:

192.168.1.1 (dlci 110) --> 210

192.168.1.3 (dlci 120) --> 220

192.168.1.5 (dlci 130 --> 230

RTR B 192.168.1.2 is 210

RTR C 192.168.1.4 is 220

RTR D 192.168.1.6 is 230

Rtr B, if inverse arp is enabled, will have a mapping like:

serial0/0: 192.168.1.1, 210, broadcast

Right?

For me to get to 192.168.1.6, on Router B I would need to create a static mapping like:

int s0/0

frame-relay map ip 192.168.1.6 210 broadcast

I *should* be able to ping 192.168.1.6, but how does the switch know to go out the 192.168.1.5 address to get to .6 if it's only caring about the L2 mapping? I think that's the question I'm trying to ask. Thank you for your patience :)

*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.

If RTR B is trying to get to RTR C, and RTR B has the same mapping, how does the switch know to go out RTR C's serial interface if there's no address assigned to it? Does the switch keep a table also?

John

Richard Burts Wed, 04/22/2009 - 09:47

John

I must admit that I am a bit confused in your question. I get RTRA as the Frame Relay switch. But I am confused about 192.168.1.1 (dlci 110) --> 210. Clearly it says that it receives DLCI 110 but on what interface? And who is 192.168.1.1? And similar confusions for the other two lines:

192.168.1.3 (dlci 120) --> 220

192.168.1.5 (dlci 130 --> 230

Perhaps one thing that may help is to clarify that Frame Relay switching operates only at layer 2. So the configuration of the Frame Relay switch says to expect DLCI x incoming on interface A and it should be switched as outgoing DLCI y on interface B. All of the Frame Relay switching has no knowledge of the IP addressing and operates independently of the IP addressing.

[edit] I just reread your post and would like to make an additional point. You say:"if a router is configured as a FRSW, and it knows to route: ". This seems to imply that you see routing and Frame Relay switching as related. They are not related. Frame Relay switching is just creating virtual circuits, so an incoming virtual circuit has a DLCI from RTRA and it is switched to an outgoing DLCI to RTRB (for example). It does all the Frame Relay switching without any reference to what IP addresses are being used.

HTH

Rick

Richard Burts Wed, 04/22/2009 - 10:10

John

I believe that it might be helpful to rethink the example that you propose. You suggest 3 routers and a router/Frame Relay switch. First lets be clear that the Frame Relay switch might route IP, but that is not common and it complicates the discussion. So let us assume that the Frame Relay switch will switch Frame Relay and not route.

So there are RTRA, RTRB, and RTRC which are the Frame Relay network. Will this network operate as a full mesh network where each router has a connection to each other router? If so how many DLCIs would each router have?

Or will the network operate as a hub and spoke network? If so which router is the hub? How many DLCIs will the hub router have? How many DLCIs will each spoke router have?

After you think through these things, then think about what IP addressing you would use on each router for its Frame Relay interface. Is it all in the same subnet or are there multiple subnets involved?

HTH

Rick

Richard Burts Wed, 04/22/2009 - 10:36

John

I have looked at your drawing and believe that it does much of what I was asking in my previous post.

The biggest conceptual difficulty with your drawing (and perhaps this explains the difficulty you are having explaining your question) is that it shows each router with a single DLCI to the Frame Relay switch. At least one of the routers must have 2 DLCIs.

Perhaps we should clarify in your drawing and in your question whether you intend the Frame Relay network to operate as a full mesh network or as a hub and spoke network.

If it is to be a full mesh then each router needs 2 DLCIs (a DLCI for each peer to whom it communicates). Or if it is hub and spoke then the hub router needs 2 DLCIs and the spokes need a single DLCI. (and bear in mind that for hub and spoke when a spoke wants to communicate with the other spoke it must send to the hub and the hub must forward to the other spoke).

If you can clarify the nature of the network we may be closer to getting explanations that work.

HTH

Rick

John Blakley Wed, 04/22/2009 - 10:43

Rick,

There's not really a problem. I'm just trying to get a grasp on the FR concept because I'm studying for BSCI and ran across it with the way OSPF interacts. I set up a lab in gns3, and I realized that true frame is something that I don't think I've ever worked with.

Thanks!

John

Richard Burts Wed, 04/22/2009 - 10:50

John

It is good to have a grasp on the FR concept - especially the concept that a DLCI represents a connection to a single Frame Relay peer. and that Frame Relay switching is a layer 2 process and is separate from the layer 3 IP addressing.

And to have a good grasp of FR it is also helpful to understand the differences in Frame Relay between full mesh networks and hub and spoke networks.

HTH

Rick

Edison Ortiz Wed, 04/22/2009 - 10:36

Based on your diagram, the mappings are incorrect. For instance, Router B has 2 FR mappings but they are both pointing to the same DLCI.

The FRSW will switch any packets coming from 202 out to the same egress interface.

If you want the FRSW to switch packets intended for 192.168.1.1 to go up to Router A, then within the FRSW, it will have a FR route from 202 to 201 but you can't reuse 202 for Router C so you need something like 302 to 203, thus your Layer3 mappings in router B would look like:

frame map ip 192.168.1.1 202 bro

frame map ip 192.168.1.3 302 bro

John Blakley Wed, 04/22/2009 - 10:42

AH!!!! That's what I'm NOT understanding! So, there needs to be a one-to-one mapping for EVERY incoming/outgoing interface? So, for this to work, I would need two mappings per router? And because it sees what interface the incoming is coming in on, the switch knows what interface to go out on?

Correct Answer
Edison Ortiz Wed, 04/22/2009 - 10:43

Bingo!

That's the reason your mappings didn't work yesterday.

If you see my GNS3 FRSW mappings, it should make sense to you. You had fewer mappings than mine. Check both .NET files and compare.

__

Edison.

John Blakley Wed, 04/22/2009 - 10:50

Well, I want to thank ALL of you for sticking with me on this! I understand the concept now:

If I have a mapping on my router to two other routers:

My local dlci's are 201 and 202:

frame map ip 192.168.1.1 201

frame map ip 192.168.1.2 202

The switch is then mapped from:

(incoming) 201 --> 101

(incoming) 202 --> 102

Two other routers:

Router A has ip 192.168.1.1 and it's local dlci is 101

Router B has ip 192.168.1.2 and it's local dlci is 102.

The switch changes the dlci from 201 to 101 and 202 to 102 in order for the two devices to talk.

Does that sound right?? (Please say yes!)

John

Correct Answer
Richard Burts Wed, 04/22/2009 - 10:46

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

Correct Answer
Edison Ortiz Wed, 04/22/2009 - 10:23

*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 Wed, 04/22/2009 - 10:39

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

That not what I meant by that statement :)

Correct Answer
Jon Marshall Wed, 04/22/2009 - 05:50

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 Wed, 04/22/2009 - 06:07

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 Wed, 04/22/2009 - 06:18

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 Wed, 04/22/2009 - 06:58

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 Wed, 04/22/2009 - 08:20

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

Jon Marshall Wed, 04/22/2009 - 08:22

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

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