How to configure Frame-Relay Point-to-Multipoint

This time we’ll take a look at the configuration of frame-relay point-to-multipoint. If you have no idea what frame-relay is or what a PVC, DLCI or LMI is you should start with my introduction to frame-relay first. Having said that let’s have some fun with frame-relay! This is the topology we’ll use:

frame relay hub and spoke lab

Above is the topology that we’ll use. 3 routers in a hub and spoke model. There are two PVCs and you can see the DLCI numbers in the picture. I’m using a single subnet (192.168.123.0 /24) so we will start with frame-relay point-to-multipoint.

Configuring a frame-relay switch is outside the scope of the CCNA, CCNP and even the CCIE exam. If you use GNS3 you can use the simple-to-configure frame-relay switch emulator.

Let’s prepare the interfaces:

Hub(config)#interface serial 0/0
Hub(config-if)#encapsulation frame-relay
Spoke1(config)#interface serial 0/0
Spoke1(config-if)#encapsulation frame-relay
Spoke2(config)#interface serial 0/0
Spoke2(config-if)#encapsulation frame-relay

We’ll change the encapsulation type to frame-relay for all interfaces. Let’s verify if our PVCs are working first:

Hub#show frame-relay pvc   

PVC Statistics for interface Serial0/0 (Frame Relay DTE)

              Active     Inactive      Deleted       Static
  Local          2            0            0            0
  Switched       0            0            0            0
  Unused         0            0            0            0

DLCI = 102, DLCI USAGE = LOCAL, PVC STATUS = ACTIVE, INTERFACE = Serial0/0

  input pkts 12            output pkts 11           in bytes 1108      
  out bytes 1074           dropped pkts 0           in pkts dropped 0         
  out pkts dropped 0                out bytes dropped 0         
  in FECN pkts 0           in BECN pkts 0           out FECN pkts 0         
  out BECN pkts 0          in DE pkts 0             out DE pkts 0         
  out bcast pkts 1         out bcast bytes 34        
  5 minute input rate 0 bits/sec, 0 packets/sec
  5 minute output rate 0 bits/sec, 0 packets/sec
  pvc create time 00:15:37, last time pvc status changed 00:15:37

DLCI = 103, DLCI USAGE = LOCAL, PVC STATUS = ACTIVE, INTERFACE = Serial0/0

  input pkts 12            output pkts 11           in bytes 1108      
  out bytes 1074           dropped pkts 0           in pkts dropped 0         
  out pkts dropped 0                out bytes dropped 0         
  in FECN pkts 0           in BECN pkts 0           out FECN pkts 0         
  out BECN pkts 0          in DE pkts 0             out DE pkts 0         
  out bcast pkts 1         out bcast bytes 34        
  5 minute input rate 0 bits/sec, 0 packets/sec
  5 minute output rate 0 bits/sec, 0 packets/sec
  pvc create time 00:15:41, last time pvc status changed 00:15:41

The show frame-relay pvc command tells us that the PVCs are active. You can also see the DLCI numbers this way. This tells us that layer 2 of our frame-relay is working. In case of trouble it might be a good idea to verify LMI:

Hub#show frame-relay lmi 

LMI Statistics for interface Serial0/0 (Frame Relay DTE) LMI TYPE = ANSI
  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 147		Num Status msgs Rcvd 148
  Num Update Status Rcvd 0		Num Status Timeouts 0
  Last Full Status Req 00:00:35		Last Full Status Rcvd 00:00:35

Use show frame-relay lmi to see the LMI information. It tells us that we are currently using the ANSI type. It doesn’t matter which one you use as long as it’s the same on all routers.

Since layer 2 is working we’ll configure some IP addresses and see if we can get layer 3 working:

Hub(config)#interface serial 0/0
Hub(config-if)#ip address 192.168.123.1
Spoke1(config)#interface serial 0/0
Spoke1(config-if)#ip address 192.168.123.2
Spoke2(config)#interface serial 0/0
Spoke2(config-if)#ip address 192.168.123.3

Let’s see if we can reach the other side:

Hub#ping 192.168.123.2

Type escape sequence to abort.
Sending 5, 100-byte ICMP Echos to 192.168.123.2, timeout is 2 seconds:
!!!!!
Success rate is 100 percent (5/5), round-trip min/avg/max = 4/8/24 ms
Hub#ping 192.168.123.3

Type escape sequence to abort.
Sending 5, 100-byte ICMP Echos to 192.168.123.3, timeout is 2 seconds:
!!!!!
Success rate is 100 percent (5/5), round-trip min/avg/max = 1/4/8 ms

As you can see the hub router can reach both spoke routers. This is because Inverse ARP is enabled by default.

We can check the frame-relay maps to confirm this:

Hub#show frame-relay map 
Serial0/0 (up): ip 192.168.123.2 dlci 102(0x66,0x1860), dynamic,
              broadcast,, status defined, active
Serial0/0 (up): ip 192.168.123.3 dlci 103(0x67,0x1870), dynamic,
              broadcast,, status defined, active
Spoke1#show frame-relay map 
Serial0/0 (up): ip 192.168.123.1 dlci 201(0xC9,0x3090), dynamic,
              broadcast,, status defined, active
Spoke2#show frame-relay map 
Serial0/0 (up): ip 192.168.123.1 dlci 301(0x12D,0x48D0), dynamic,
              broadcast,, status defined, active

Above you see the mappings between the IP address and the DLCI number. There are two other interesting things to see here. The keyword dynamic means that the entry was learned because of inverse ARP. The keyword broadcast means that we can send broadcast or multicast through our PVC.

Configurations

Want to take a look for yourself? Here you will find the configuration of each device.

Hub

hostname Hub
!
interface Serial0/0
 ip address 192.168.123.1 255.255.255.0
 encapsulation frame-relay
!
end

Spoke1

hostname Spoke1
!
interface Serial0/0
 ip address 192.168.123.2 255.255.255.0
 encapsulation frame-relay
!
end

Spoke2

hostname Spoke2
!
interface Serial0/0
 ip address 192.168.123.3 255.255.255.0
 encapsulation frame-relay
!
end


Let’s disable Inverse ARP and create some mappings ourselves:
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Forum Replies

  1. Hi Rene,

    This is very good explanation about frame realy to understand frame-realy technology.

    Thanks so much for giving this information to all Networking negineers.

    I am doing CCIE Security please help me how to do the study for CCIE security because I am doing job as well in IT MNC company.

    Khandesha Kothale

  2. Hi Ugyen,

    The frame-relay interface-dlci command is meant for point-to-point frame-relay connections and basically it means “This DLCI belongs to this interface”. It is used often on sub-interfaces because the router doesn’t know which DLCI belongs to which sub-interface. By default all DLCI numbers are assigned to the physical interface…

    The frame-relay map command is different, it is used to map layer 2 information (DLCI) to a layer 3 address (IP address). We use this on frame-relay multipoint interfaces where we can reach multiple routers through the same PVC.

    Hope this helps.

    Rene

  3. Hi Dominik,

    The CIR rate is the committed rate, in other words the rate that the ISP guarantees that you will get.

    The DE bit indicates that the frame has a lower importance. When there is congestion, the frames with the DE-bit set can be dropped.

    Normally, all frames that exceed the CIR rate will have their DE-bit set so that’s the relationship between the two.

    Rene

  4. Hi,
    Can you explain the below

    If you use point-to-point it will solve your split-horizon problem but you’ll need to use a different IP subnet per PVC. Point-to-multipoint means you have the split-horizon problem but you can use a single IP subnet for all PVCs.
    Thanks

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