Adjusting LMI Options

Problem

You want to configure different LMI options on your Frame Relay circuit.

Solution

There are several different LMI options. The first specifies which version of LMI protocol you wish to use:

Branch1#configure terminal
Enter configuration commands, one per line. End with CNTL/Z.
Branch1(config)#interface Serial0
Branch1(config-if)#encapsulation frame-relay
Branch1(config-if)#frame-relay lmi-type ansi
Branch1(config-if)#exit
Branch1(config)#end
Branch1#

By default, LMI sends keepalive packets through every PVC every 10 seconds to verify that the path is still available. You can adjust this value with the keepalive command:

Branch1#configure terminal
Enter configuration commands, one per line. End with CNTL/Z.
Branch1(config)#interface Serial0
Branch1(config-if)#encapsulation frame-relay
Branch1(config-if)#keepalive 5
Branch1(config-if)#exit
Branch1(config)#end
Branch1#

LMI is not supported on all networks. If this is the case in your network, you must configure the router to announce its own DLCI number with the local-dlci command:

Branch1#configure terminal
Enter configuration commands, one per line. End with CNTL/Z.
Branch1(config)#interface Serial0
Branch1(config-if)#encapsulation frame-relay
Branch1(config-if)#frame-relay local-dlci 50
Branch1(config-if)#exit
Branch1(config)#end
Branch1#

 

Discussion

The first example in this recipe sets an alternative LMI type. By default, Cisco routers will attempt to detect the LMI type. There are three options. The first is called cisco on the router, although this is slightly confusing because it is not a Cisco proprietary standard, but rather was developed jointly by Cisco and other vendors. In fact, this default setting is usually the one you want because it is the default setting in many Frame Relay switches. The second LMI-type option is called ANSI, and the third is Q933A.

The example shows how to set the ANSI standard LMI type:

Branch1(config-if)#frame-relay lmi-type ansi

Another LMI option, q933a, is also available. This option configures the router to use the Annex A of the ITU-T standard. This is sometimes called the CCITT LMI standard. The Q933A LMI type is most commonly used for SVCs, although it can also support Frame Relay PVCs:

Branch1(config-if)#frame-relay lmi-type q933a

You must ensure that the LMI type you use matches what your carrier uses. It is not even necessary that all routers in the network use the same LMI type, as long as each router matches the settings on its respective Frame Relay switch. In most cases, the router is able to successfully auto-detect the LMI type, so you would only use these commands if there is a problem with this process.

The second example sets the LMI keepalive time. Once again, this parameter depends on what is configured on the switch. In fact, as long as you use a keepalive setting that is less than the period on the switch, you should have no problems. In networks that do not use LMI, you should disable this polling as follows:

Branch1#configure terminal
Enter configuration commands, one per line. End with CNTL/Z.
Branch1(config)#interface Serial0
Branch1(config-if)#encapsulation frame-relay
Branch1(config-if)#no keepalive
Branch1(config-if)#exit
Branch1(config)#end
Branch1#

The last option defines a local DLCI:

Branch1(config-if)#frame-relay local-dlci 50

Here we have set the router's serial interface to use DLCI number 50. This command is only required on networks that do not use LMI, but there is no harm in configuring it for networks that do use LMI. In fact, some network engineers opt to configure this statement on all of their Frame Relay circuits as a mnemonic to remind them of the DLCI numbers that other devices use to reach this circuit. However, if you use the local DLCI number for this purpose, you should not disable keepalives because LMI requires them.