Configuring Frame Relay SVCs

Problem

You want to configure the router to support Frame Relay SVCs.

Solution

Frame Relay SVCs are not extremely common, but some carrier networks support them. The advantage to using SVCs is that the router can add and remove inactive virtual circuits dynamically in a lightly used network. Because of the extra complexity and the management problems associated with dynamic network topologies, most network engineers will use this feature only if it offers significant cost advantages.

You can configure SVCs to use subinterfaces, as in Recipe 10.1:

Central#configure terminal
Enter configuration commands, one per line. End with CNTL/Z.
Central(config)#interface Serial0
Central(config-if)#encapsulation frame-relay
Central(config-if)#frame-relay lmi-type q933a
Central(config-if)#frame-relay svc
Central(config-if)#exit
Central(config)#interface Serial0.10 point-to-point
Central(config-subif)#ip address 192.168.1.129 255.255.255.252
Central(config-subif)#frame-relay interface-dlci 100
Central(config-subif)#map-group SVCMAP
Central(config-fr-dlci)#class SVCclass
Central(config-fr-dlci)#exit
Central(config-subif)# exit
Central(config)#map-list SVCMAP source-addr X121 1234 dest-addr X121 4321
Central(config-map-list)#ip 192.168.55.6 class SVCclass ietf
Central(config-map-list)#exit
Central(config)#map-class frame-relay SVCclass
Central(config-map-class)#frame-relay traffic-rate 56000 128000
Central(config-map-class)#exit
Central(config)#end
Central#

And you can also configure Frame Relay SVCs without subinterfaces, similar to the map configuration for PVCs, which we discussed in Recipe 10.3:

Central#configure terminal
Enter configuration commands, one per line. End with CNTL/Z.
Central(config)#interface Serial0
Central(config-if)#ip address 192.168.55.1 255.255.255.0
Central(config-if)#encapsulation frame-relay
Central(config-if)#frame-relay lmi-type q933a
Central(config-if)#frame-relay svc
Central(config-if)#map-group SVCMAP
Central(config-if)#frame-relay interface-dlci 50
Central(config-fr-dlci)#class SVCclass
Central(config-fr-dlci)#exit
Central(config-if)#exit
Central(config)#map-list SVCMAP source-addr X121 1234 dest-addr X121 4321
Central(config-map-list)#ip 192.168.55.6 class SVCclass ietf
Central(config-map-list)#exit
Central(config)#map-class frame-relay SVCclass
Central(config-map-class)#frame-relay traffic-rate 56000 128000
Central(config-map-class)#exit
Central(config)#end
Central#

Discussion

You can enable Frame Relay SVCs on an interface simply by including the frame-relay svc command. This is required whether you use maps or subinterfaces:

Central(config-if)#frame-relay svc

However, this doesn't tell the network how to actually build the virtual circuits. To do that, you need to define a map-list and a map-class as follows:

Central(config)#map-list SVCMAP source-addr X121 1234 dest-addr X121 4321
Central(config-map-list)#ip 192.168.55.6 class SVCclass ietf
Central(config-map-list)#exit
Central(config)#map-class frame-relay SVCclass
Central(config-map-class)#frame-relay traffic-rate 56000 128000
Central(config-map-class)#end

The map-list associates an IP address with either X.121 or E.164 source and destination addresses. In the example, we have used X.121 addresses, but if your carrier's network uses E.164 addressing instead, you would simply replace the keyword X121 with E164, and specify the appropriate E.164 addresses:

Central(config)#map-list SVCMAP  source-addr E164 1234  dest-addr E164 4321

The map-class command tells the router about the actual SVC parameters such as CIR and EIR. In this example, we want the network to create SVCs with CIR of 56000 and total burst rate (CIR+EIR) of 128000 bits per second.

By default the router will keep an idle SVC for 120 seconds before tearing it down. You can change this period using the frame-relay idle-timer command. There are three ways to specify an idle time. You can have the router tear down an idle PVC if there is no traffic in either direction for a specified time period like this:

Central(config)#map-class frame-relay SVCclass 
Central(config-map-class)#frame-relay idle-timer 60

Or you can specify the inbound and outbound directions separately:

Central(config)#map-class frame-relay SVCclass
Central(config-map-class)#frame-relay idle-timer in 20
Central(config-map-class)#frame-relay idle-timer out 30

In each case, the argument is the time period specified in seconds.

You can view the SVC map information on a router with the show frame-relay svc command:

Central#show frame-relay svc maplist SVCMAP
Map List : SVCMAP
Address : Source X121 1234 <----> Destination X121 4321

Protocol : ip 192.168.55.6 Encapsulation : IETF

FMIF (Frame Mode Information Field Size), bytes
Configured : In = 1500, Out = 1500

CIR (Committed Information Rate), bits/sec
Configured : In = 56000, Out = 56000,

Minimum Acceptable CIR, bits/sec
Configured : In = 56000, Out = 56000,

Bc (Committed Burst Size), bits
Configured : In = 56000, Out = 56000,

Be (Excess Burst Size), bits
Configured : In = 56000, Out = 56000,

Central#

It is useful to remember that whether you use maps or subinterfaces, you can combine SVCs and PVCs on the same physical interface.