How to use a Cisco router as Frame-Relay switch

For this tutorial you can use  a low cost Cisco router and of course you need some serial interfaces available on this router. I will use a 7206 with 3 serial interfaces. This router I will call R6 and the rest of the three routers connected to R6, will be R2, R5 and R9. In this way it will be easier for you to understand how the frame-relay routing is achieved.

If we have a look to R6’s (the router used as frame-relay switch) interfaces:

r6-c7206#sh int desc
Interface                      Status         Protocol       Description
Fa0/0                            up                down
Fa0/1                             up                down
Se5/0:1                         up                up                    TO_R2
Se5/1:2                         up                up                    TO_R5
Se6/0                            down          down
Se6/1                             up                up                    TO_R9

you’ll notice that we have 3 active serial interfaces, each being connected to one of the three routers R2, R5 and R9.

Very important, before you begin define a scalable range for your DLCI numbers, otherwise you will have a complete mess when troubleshooting is needed. I like to define them after formula Rx0Ry. In the middle you have the number zero. In this idea, we will have something like R20R5 and from this resul the DLCI 205 for the Frame-Relay connection between R2 and R5. Below you have the DLCI numbers used in this tutorial:

R2 -> R5: DLCI 205
R2 -> R9: DLCI 209
R5 -> R2: DLCI 502
R5 -> R9: DLCI 509
R9 -> R2: DLCI 902
R9 -> R5: DLCI 905

Now that we have defined the DLCI numbers lets configure R6 router as frame-relay switch.

First of all, you need to enable frame relay switching on the router:

r6-c7206#conf t
Enter configuration commands, one per line.  End with CNTL/Z.
r6-c7206(config)#frame-relay switching

This command enable the switching  of packets based on the data?link connection identifier (DLCI) inside your router.

Next, we have to configure the frame-relay routing on the physical interface. We will start with interface S5/0:1 where R2 router is connected and apply the following configuration

interface Serial5/0:1
description TO_R2
no ip address
encapsulation frame-relay
! the frame-relay switch is the DCE and the other end is the DTE
frame-relay intf-type dce
frame-relay route 205 interface Serial5/1:2 502
frame-relay route 209 interface Serial6/1 902

With frame-relay route command we tell to frame-relay switch where to forward the packets based on the DLCI number. If we take a look to the first frame-relay route command, than the path to DLCI 502 is interface Serial5/1:2. If you feel confuse, please check again above the interface connection to the router and the assigment scheme for DLCI numbers.

The same like for interface S5/0:1, we will configure the interfaces connected to R5 and R9:

interface Serial5/1:2
description TO_R5
no ip address
encapsulation frame-relay
frame-relay intf-type dce
frame-relay route 502 interface Serial5/0:1 205
frame-relay route 509 interface Serial6/1 905

interface Serial6/1
description TO_R9
no ip address
encapsulation frame-relay
frame-relay intf-type dce
frame-relay route 902 interface Serial5/0:1 209
frame-relay route 905 interface Serial5/1:2 509

Having a look to S5/1:2, first frame-relay route command, here we configure the path back from R5 to R2, by telling the frame-relay switch to chose interface S5/0:1 to send packets to DLCI 205 (R2).

This is the basics of how to configure a Cisco router as a frame-relay switch. If you need help, please don’t hesitate to use the comment form below.

Cisco: Frame-Relay back-to-back routers in simple steps

In one of my earlier posts I have presented how to connect 3 routers in a Hub and Spoke Frame-Relay topology. Now I want to show you how to connect 2 routers back to back, in a Frame Relay topology. With a back to back connection and without any FR switch, things are a little bit different than in the Hub and Spooke topology.

First let’s have a look to the topology:

We have 2 routers, connected back to back. The interface status on both routers:

sh run int s0/0
interface Serial0/0
no ip address

First let set up the encapsulation to Frame-Relay and to bring the interfaces UP:

conf t
interface S0/0
encapsulation frame-relay
no shutdown

Everything should be fine now, but it’s not, as if you check your interfaces you will see that they are in a Up/Down status on both routers:

sh int s0/0
Serial0/0 is up, line protocol is down

Even this is messing up a little bit with our brains, the Up/Down status is normal in this phase. Why? Remember that we do not have a FR switch, both interface consider themselved DTE side and LMI is not working. You can check if LMI like this:

R1#sh frame-relay lmi

LMI Statistics for interface Serial0/0 (Frame Relay DTE) LMI TYPE = CISCO
Invalid Unnumbered info 0             Invalid Prot Disc 0
Invalid dummy Call Ref 0              Invalid Msg Type 0
Invalid Status Message 0              Invalid Lock Shift 0
Invalid Information ID 0              Invalid Report IE Len 0
Invalid Report Request 0              Invalid Keep IE Len 0
Num Status Enq. Sent 6                Num Status msgs Rcvd 0
Num Update Status Rcvd 0              Num Status Timeouts 5
Last Full Status Req 00:00:04         Last Full Status Rcvd never

You will see Sent packages on both routers, but nothing received as there is no FR switch. In this conditions we have to disable LMI and to assign DLCIs manually. To disable LMI, issue the following command under Frame-Relay interface:

interface S0/0
no keepalive

Now interfaces should be in a Up/Up status:

sh int s0/0
Serial0/0 is up, line protocol is up

As in this moment everything looks fine, let’s start to configure the Frame-Relay back to back connections. Obvious, at least for me, when you have a back to back connection, first things that comes into your mind is a point-to-point interface. Let’s start with this configuration (we will use from diagram the black line connection with subnet /24) . I will show only the configuration on the primary router, but it’s the same on the secondary one, just with a different IP address in the 4th octet.

interface S0/0.100 point-to-point
ip address
frame-relay interface-dlci 100

Remember DLCIs are only local significant so you can define whatever number you want there, but of course the same DLCI number on both sides. Let’s check if everything is fine:

R1#show frame-relay map
Serial0/0.100 (up): point-to-point dlci, dlci 100(0x64,0x1840), broadcast

R1#show frame-relay pvc | i STATUS

We can see a point-to-point dlci in frame-relay map, and a static defined PVC. If you ping from R1 to R2 and viceversa it should work.

This was the straight forward solution, but going a little bit more into details, you can be required in some situations that you have to use a multipoint Frame-Relay interface. Actually what is a multipoint interface more than multiple point-to-point interfaces. Let’s take the sencond line (red one, with subnet /24 in the topology) and configure this back to back connection using multipoint interfaces:

interface s0/0.110 multipoint
ip address
frame-relay map ip 110 broadcast
frame-relay map ip 110

As you see the configuration is different from the point-to-point interface. Actually under multipoint interface you can issue the frame-relay interface-dlci 110 command, but this will not help too much. Remember that we have disable LMI in the first steps because we do not have a FR switch. No FR switch means no automatic L3 to L2 mapping. In other words even if you specify the interface-dlci, the interface being a multipoint will not know where to forward packets. Why this didn’t happen in the point-to-point scenario, you may ask. Well, because there the keyword is interface sx/x point-to-point, so by it’s nature the interface knows that there is only one destination possible, meaning the other end (or point if you want).

In this scenario we had to manually map L3 to L2 with the command frame-relay map. Actually you only need the first frame-relay map, pointing to the other router IP address, but I add the last command just in case you want to ping your own interface.

Now maybe you ask which is the third scenario (blue line). Well, this is not very common in the real environment, but maybe you have to deal with it in a special condition like lab environment, Cisco exam and so on. Let’s say that you have a request that you need to have 3 virtual PVC connections, but only 2 subinterface. Now, you already have 2 subinterface configured, so how can you achieve the third PVC connection. The answer is that you configure the main interface with the same configuration like in multipoint subinterface scenario. By it’s nature and interface is described point to multipoint, so in a Frame Relay scenario like this you have to manually map L3 to L2:

interface Serial0/0
ip address
encapsulation frame-relay
no keepalive
frame-relay map ip 120
frame-relay map ip 120 broadcast

If you followed this tutorial, at the end you should have reachability over the 3 subnets.

Frame-Relay: PVC test with ping to own interface

Let’s say that you have a link configured with Frame-Relay and from time to time you observe that the link is having some strange problems. Since you cannot be sure that the link is provided 100% error free you want to do your own tests. You want to see if the Frame-Relay between R0 and R1 is correctly configured. In order to verify this you want to set up the R0 to test the PVC to R1, by sending traffic to its own IP address.

Please download the topology here. The Frame-Relay between R0 and R1 is already configured.

See the tutorial below: