Cisco Back To Back

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.

Firstlet’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
shutdown

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 checkyour 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-relaylmi
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 StatusTimeouts 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 ints0/0
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 subnet100.100.100.0 /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 100.100.100.1 255.255.255.0
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 numberon both sides. Let’s check if everything is fine:

R1#show frame-relay map
Serial0/0.100 (up): point-to-point dlci, dlci 100(0×64,0×1840), broadcast
R1#show frame-relay pvc | i STATUS
DLCI = 100, DLCI USAGE = LOCAL, PVC STATUS = STATIC, INTERFACE = Serial0/0.100

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 shouldwork.
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 110.110.110.0 /24 in the topology) and configure this back to back connectionusing multipoint interfaces:

interface s0/0.110 multipoint
ip address 110.110.110.1 255.255.255.0
frame-relay map ip 110.110.110.2 110 broadcast
frame-relay map ip 110.110.110.1 110

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....