Enabling Frame Relay Encapsulation
On a Cisco router, Frame Relay can be configured only on the supported interfaces; it's most commonly supported on synchronous serial interfaces. A single Cisco IOS command is all that is required to enable Frame Relay on the serial interface. The encapsulation frame-relay interface configuration command, as follows, is used to enable Frame Relay encapsulation and to allow Frame Relay processing on the supported interface.
To enable Frame Relay on a serial interface, follow the configuration steps listed below beginning in the global configuration mode:
Go into the interface configuration mode of the interface on which you want to enable Frame Relay.
(optional) Configure Frame Relay encapsulation to use either Cisco or IETF encapsulations. If the encapsulation type is not specified, by default Cisco encapsulation is used.
The no form of the encapsulation frame-relay command removes the Frame Relay encapsulation on the interface, as shown in Example 4-2. On a serial interface, the no encapsulation frame-relay command causes the interface to revert to the default High-Level Data Link Control (HDLC) encapsulation. Moreover, all preexisting Frame Relay configurations on the serial interface are automatically removed.
Example 4-2 Unconfiguring Frame Relay Encapsulation
R1#config terminal Enter configuration commands, one per line. End with CNTL/Z. R1(config)#interface serial4/2 R1(config-if)#no encapsulation frame-relay
Readers should note that Frame Relay can be configured only on certain supported interfaces, which presently include synchronous serial interfaces, High Speed Serial Interfaces (HSSI), and packets over SONET (POS) interfaces on the Cisco 12000 Series Gigabit Switch Router. It is not possible to configure Frame Relay on specialized interfaces such as Ethernet or ATM. An error message is returned by the CLI every time a user attempts to configure Frame Relay on nonsupported interfaces, as demonstrated in Example 4-3. In this chapter, the term Frame Relay interface refers to a Frame Relay-enabled interface, which can belong to any of the supported interfaces mentioned.
Example 4-3 Error Message Shown When Attempting to Configure Frame Relay Encapsulation on Nonserial Interfaces
R1#config terminal Enter configuration commands, one per line. End with CNTL/Z. R1(config)#interface Ethernet1/0 R1(config-if)#encapsulation frame-relay ^ _% Invalid input detected at '^' marker.
Cisco supports two different Frame Relay encapsulation types. The default Frame Relay encapsulation enabled on supported interfaces is the Cisco encapsulation. Cisco also supports the IETF Frame Relay encapsulation type, which is in conformance with RFC 1490 and RFC 2427. RFC 2427 supercedes RFC 1490. Both RFC specifications define standards allowing multiple routed protocols to be carried over Frame Relay. Readers can refer to http://www.faqs.org/rfcs/rfc2427.html and http://www.faqs.org/rfcs/rfc1490.html for references on both RFCs.
In general, the IETF Frame Relay encapsulation should be used when connecting a Cisco router to non-Cisco equipment across a Frame Relay network. The IETF Frame Relay encapsulation allows interoperability between equipment from multiple vendors. Example 4-4 describes the steps for enabling Frame Relay on a serial interface using the IETF encapsulation. The keyword ietf specifies IETF Frame Relay encapsulation to be used on the serial interface. If the encapsulation frame-relay command is entered without specifying the optional ietf keyword, the router defaults to using the Cisco encapsulation type on that interface.
Example 4-4 Configuring Frame Relay IETF Encapsulation at the Interface Level
R1#configure terminal Enter configuration commands, one per line. End with CNTL/Z. R1(config)#interface serial 4/2 R1(config-if)#encapsulation frame-relay ? ietf Use RFC1490/RFC2427 encapsulation <cr> R1(config-if)#encapsulation frame-relay ietf
Both Cisco and IETF encapsulations for Frame Relay can be configured on a per-virtual-circuit (VC) basis. This gives greater flexibility when configuring Frame Relay in a multivendor environment. A user can specify the Frame Relay encapsulation types to be used on different virtual circuits configured under the same physical interface.
Example 4-5 Configuring Frame Relay Cisco and IETF Encapsulation at the DLCI Level
R1#configure terminal Enter configuration commands, one per line. End with CNTL/Z. R1(config)#interface serial 4/2 R1(config-if)#encapsulation frame-relay R1(config-if)#frame-relay map ip 172.16.1.1 102 broadcast ietf R1(config-if)#frame-relay map ip 192.168.1.1 103 broadcast cisco
After enabling Frame Relay encapsulation on the interface, it might be necessary to perform a no shutdown command at the interface level to bring up the interface if it was previously in the shutdown mode. Verify the status of the Frame Relay interface with the show interface type slot/port privileged EXEC mode command. When the Frame Relay interface is operational, the interface is in the Interface is up, line protocol is up state. Both configuration changes and the associated command output are illustrated in Example 4-6.
Example 4-6 Bringing Up the Interface and Displaying the Configured Frame Relay Encapsulation
R1#configure terminal Enter configuration commands, one per line. End with CNTL/Z. R1(config)#inter serial4/2 R1(config-if)#no shutdown R1(config-if)# 02:46:09: %LINK-3-UPDOWN: Interface Serial4/2, changed state to up 02:46:10: %LINEPROTO-5-UPDOWN: Line protocol on Interface Serial4/2, changed state to up R1#sh 02:46:10: %SYS-5-CONFIG_I: Configured from console by conso R1#show interface serial 4/2 Serial4/2 is up, line protocol is up Hardware is M4T Internet address is 172.16.1.1/24 MTU 1500 bytes, BW 2048 Kbit, DLY 20000 usec, reliability 255/255, txload 1/255, rxload 1/255 Encapsulation FRAME-RELAY, crc 16, loopback not set Keepalive set (10 sec) LMI enq sent 76, LMI stat recvd 78, LMI upd recvd 0, DTE LMI up LMI enq recvd 0, LMI stat sent 0, LMI upd sent 0 LMI DLCI 1023 LMI type is CISCO frame relay DTE FR SVC disabled, LAPF state down Broadcast queue 0/64, broadcasts sent/dropped 9/0, interface broadcasts 0 Last input 00:00:09, output 00:00:09, output hang never Last clearing of "show interface" counters 00:14:03 Input queue: 0/75/0/0 (size/max/drops/flushes); Total output drops: 0 Queueing strategy: weighted fair Output queue: 0/1000/64/0 (size/max total/threshold/drops) Conversations 0/1/256 (active/max active/max total) Reserved Conversations 0/0 (allocated/max allocated) Available Bandwidth 1536 kilobits/sec 5 minute input rate 0 bits/sec, 0 packets/sec 5 minute output rate 0 bits/sec, 0 packets/sec 79 packets input, 1163 bytes, 0 no buffer Received 0 broadcasts, 0 runts, 0 giants, 0 throttles 0 input errors, 0 CRC, 0 frame, 0 overrun, 0 ignored, 0 abort 101 packets output, 1525 bytes, 0 underruns 0 output errors, 0 collisions, 4 interface resets 0 output buffer failures, 0 output buffers swapped out 4 carrier transitions DCD=up DSR=up DTR=up RTS=up CTS=up
As shown in Example 4-6, the output of the show interface command also reveals the Frame Relay encapsulation type configured on that interface. Encapsulation FRAME-RELAY in the output indicates that Cisco Frame Relay encapsulation type is enabled. Encapsulation FRAME-RELAY IETF shows that IETF Frame Relay encapsulation type is in use.