Cisco Routing & Switching

Router as Frame Relay Switch

The first step before playing with the topology is setting up the router acting as frame relay switch, this is a very straight forward process, so I will show you how to do it in a few minutes.

If you take a look at the topology you will see that R1, R4, R7 and R9 are directly connected to the Frame Relay cloud using their Serial0/0 interfaces. Behind the cloud is R3, another Cisco 3725 router like the others. Watching the corresponding sh cdp neigh  we can check which router is connected to R3 serial interfaces.

R3(config-line)#do sh cdp ne
Capability Codes: R - Router, T - Trans Bridge, B - Source Route Bridge
                  S - Switch, H - Host, I - IGMP, r - Repeater
Device ID        Local Intrfce     Holdtme    Capability  Platform  Port ID
R1               Ser 0/0            134         R S I     3725      Ser 0/0
R7               Ser 0/2            136         R S I     3725      Ser 0/0
R4               Ser 0/1            137         R S I     3725      Ser 0/0
R9               Ser 0/3            125         R S I     3725      Ser 0/0

Before start typing, we have to write down the corresponding bindings between routers and DLCIs because we will configure the router based on this table.  Our table will look like this:

R1 -> DLCI 104 -> R4
R1 -> DLCI 107 -> R7
R1 -> DLCI 109 -> R9
R4 -> DLCI 401 -> R1
R4 -> DLCI 407 -> R7
R4 -> DLCI 409 -> R9
R7 -> DLCI 701 -> R1
R7 -> DLCI 704 -> R4
R7 -> DLCI 709 -> R9
R9 -> DLCI 901 -> R1
R9 -> DLCI 904 -> R4
R9 -> DLCI 907 -> R7

 Once the table has been built we are ready to start configuring. The first step is enabling frame-relay switching on the router, change the encapsulation type to frame relay and declare the interfaces facing the neighbors as DCE for providing clocking.

interface Serial0/0
 description ToR1
 no ip address
 encapsulation frame-relay
 clock rate 512000
 frame-relay intf-type dce
interface Serial0/1
 description ToR4
 no ip address
 encapsulation frame-relay
 clock rate 512000
 frame-relay intf-type dce
interface Serial0/2
 description ToR7
 no ip address
 encapsulation frame-relay
 clock rate 512000
 frame-relay intf-type dce
interface Serial0/3
 no ip address
 encapsulation frame-relay
 clock rate 512000
 frame-relay intf-type dce

Secondly, we need to translate the mapping created in above table to a understandable IOS language. The key command for this purpose is frame relay route <input-dlci> interface <output-interface> <output-dlci> With this command we are instructing the router to do something like this: “Every frame received for this interface with input-dlci, forward to the output-interface and replace the initial-dlci with the output-dlci”.

And this results in the following output:

interface Serial0/0

description ToR1

no ip address

encapsulation frame-relay

clock rate 512000

frame-relay intf-type dce

frame-relay route 104 interface Serial0/1 401

frame-relay route 107 interface Serial0/2 701

frame-relay route 109 interface Serial0/3 901

interface Serial0/1

description ToR4

no ip address

encapsulation frame-relay

clock rate 512000

frame-relay intf-type dce

frame-relay route 401 interface Serial0/0 104

frame-relay route 407 interface Serial0/2 704

frame-relay route 409 interface Serial0/3 904

interface Serial0/2

description ToR7

no ip address

encapsulation frame-relay

clock rate 512000

frame-relay intf-type dce

frame-relay route 701 interface Serial0/0 107

frame-relay route 704 interface Serial0/1 407

frame-relay route 709 interface Serial0/3 907

interface Serial0/3

no ip address

encapsulation frame-relay

clock rate 512000

frame-relay intf-type dce

frame-relay route 901 interface Serial0/0 109

frame-relay route 904 interface Serial0/1 409

frame-relay route 907 interface Serial0/2 709

With the configuration applied on R3 and the endpoints serial interfaces with frame relay encapsulation enabled we are ready to check connectivity between them. First take a look to the R1 pvc table to find out if we are receiving the correct pvc information:

R1(config-if)#do sh fram pvc summ
Frame-Relay VC Summary
              Active     Inactive      Deleted       Static
  Local          2            0            0            0
  Switched       0            0            0            0
  Unused         1            0            0            0
R1#sh fram pvc | i PVC
PVC Statistics for interface Serial0/0 (Frame Relay DTE)
DLCI = 104, DLCI USAGE = LOCAL, PVC STATUS = ACTIVE, INTERFACE = Serial0/0
DLCI = 107, DLCI USAGE = LOCAL, PVC STATUS = ACTIVE, INTERFACE = Serial0/0
DLCI = 109, DLCI USAGE = UNUSED, PVC STATUS = ACTIVE, INTERFACE = Serial0/0

Notice DLCI 109 shows as UNUSED while the other two are in LOCAL state. The reason is quite simple, based on our currently configuration, R1, R4 and R7 have an ip assigned to the serial 0/0 and inarp has run over the pvcs, changing their states. R9, however, hasn’t an ip assigned and any packet traversed the pvc yet. So let’s fix it and check that ping works.

R9(config-if)#int se0/0
R9(config-if)#ip add 85.0.146.9 255.255.255.0
R1#sh fram pvc | i PVC
PVC Statistics for interface Serial0/0 (Frame Relay DTE)
DLCI = 104, DLCI USAGE = LOCAL, PVC STATUS = ACTIVE, INTERFACE = Serial0/0
DLCI = 107, DLCI USAGE = LOCAL, PVC STATUS = ACTIVE, INTERFACE = Serial0/0
DLCI = 109, DLCI USAGE = LOCAL, PVC STATUS = ACTIVE, INTERFACE = Serial0/0
R1#ping 85.0.146.4

Type escape sequence to abort.

Sending 5, 100-byte ICMP Echos to 85.0.146.4, timeout is 2 seconds:
!!!!!
Success rate is 100 percent (5/5), round-trip min/avg/max = 4/28/68 ms
R1#ping 85.0.146.7

Type escape sequence to abort.

Sending 5, 100-byte ICMP Echos to 85.0.146.7, timeout is 2 seconds:
!!!!!
Success rate is 100 percent (5/5), round-trip min/avg/max = 1/29/80 ms
R1#ping 85.0.146.9

Type escape sequence to abort.

Sending 5, 100-byte ICMP Echos to 85.0.146.9, timeout is 2 seconds:
!!!!!
Success rate is 100 percent (5/5), round-trip min/avg/max = 4/34/64 ms

 Perfect! We succesfully configured the frame relay swith and tested end to end connectivity.

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