Chapter 6. WAN Protocols and Technologies: Voice over X

Configuring Frame Relay

You need to follow only two physical steps when configuring Frame Relay on Cisco routers. In this text, we list a four-step process; some of these steps do not require configuration. Regardless, you should always be aware of all the components or steps needed to configure the complete frame service. A couple of these commands are set by default, and no additional key-ins are necessary. Further additional steps can be added to the basic steps, but they are not required to get frame service running on your router.

Step 1. Enable Frame Relay encapsulation on an interface or subinterface. This is done with the following interface command:

 router(config-if)#  encapsulation frame-relay  [  cisco   ietf  ] 

cisco is the default encapsulation type and should be used when connecting to another Cisco device or an RFC 1490 “compliant device.

ietf should be used when connecting to non-Cisco devices.

Step 2. Set the LMI type. All Cisco routers running Cisco IOS Software Release 11.2 and later support LMI autosense and require no additional configuration. You can statically configure the LMI with the following interface command:

 Router(config-if)#  frame-relay lmi-type  [  ansi   cisco   q933i  ] 

Refer back to the descriptions on different LMI types in the "Frame Relay Terminology" section if you need to refresh your memory. Cisco is the default LMI type.

Step 3. Configure static or dynamic protocol and address mapping. Next , determine what type of address mapping is needed for the specific Frame Relay interface. Your choice to use frame-relay map command versus frame-relay interface-dlci or no command at all depends on how you have the Frame Relay interface configured and whether the remote device supports Frame Relay Inverse ARP. Subinterfaces are logical divisions of the physical interface. Dynamic address mapping uses Frame Relay Inverse ARP, as previously mentioned. Because you are now splitting the physical interface into multiple subinterfaces, you must provide additional configuration information that ties a specific subinterface to a specific DLCI. Two types of subinterfaces exist in Frame Relay networks, point-to-point and multipoint. If you are creating a point-to-point subinterface, use the following command:

 Router(config-if)#  frame-relay interface-dlci   dlci_number  

When you are creating a multipoint interface, you need to use static addressing. This is not as much for Frame Relay purposes as it is for general routing issues. Inverse ARP will still be resolved; however, routed protocols will not be capable of forwarding packets to the appropriate next-hop address without the aid of static addressing. Use the following command on multipoint interfaces:

 Router(config-if)#  frame-relay map   protocol  [  ip   dec   appletalk   xns   ipx   vines   clns   bridge   llc2   dlsw  ]  next_hop_address   dlci  [  broadcast  ]   [  ietf   cisco  ] 

The frame-relay map statement creates a static map between the local DLCI and the next-hop network address. The broadcast keyword is required to forward specific broadcasts, such as the ones needed for OSPF. This keyword should be used at all times. The ietf and cisco keywords allow for different frame encapsulation types on a PVC basis. The frame-relay map statement also can be used to load-share traffic over a frame network. For example, IPX traffic can be mapped to one DLCI, while IP can be mapped to the other. This command also is used to transport protocols in accordance with RFC 1490, such as Spanning-Tree frames and Data Link Switching frames . This command has many uses; refer to the Cisco IOS Software Configuration Guide for all frame-relay map command options.

Table 5-1 illustrates the recommended use of Inverse ARP address mapping type for each interface.

Table 5-1. Recommend Address Mapping and Inverse ARP Pairings

Natural or Standard Interface	Multipoint Subinterface	Point-to-Point Interface	Connecting to a Device Without Inverse ARP
Add a network layer address for each protocol.	Add a network layer address for each protocol. Use frame-relay map statements.	Add a network layer address for each protocol. Use the frame-relay interface-dlci command.	Add a network layer address for each protocol. Use frame-relay map statements.
Static or dynamic addressing	Static addressing	Dynamic addressing	Static addressing

Step 4. Address protocol-specific issues. You need to be aware of certain issues when configuring routing protocols over Frame Relay. For example, OSPF works properly only with the network type changed or with neighbor statements. All multipoint networks running distance vector protocols, or EIGRP, are subject to split horizons. We discuss these issues in more detail in upcoming chapters. Table 5-2 lists common issues to address with Frame Networks.

Table 5-2. Common Issues with Frame Networks

Protocol	Multipoint Subinterface	Point-to-Point Interface
OSPF	Must use neighbor statements, or use the ip ospf network type command on the interface. Use a priority of 1 to set the DR router. This should be the router with a PVC to all of its neighbors.	Must use neighbor statements, or use the ip ospf network type broadcast, or ip ospf network type point-to-point command on the interface.
EIGRP	Disable IP or IPX split horizons. Add bandwidth command.	Add bandwidth command.
RIP	Disable IP or IPX split horizons.	None.
IGRP	Disable IP or IPX split horizons. Add bandwidth command.	Add bandwidth command.
BGP	None.	None.
Bridging	Set the root bridge to the router that has PVCs to all leaves of the bridge.	Set the root bridge to the router that has PVCs to all leaves of the bridge.

NOTE

Split horizon refers to the rule that information about a route will not be sent out the same interface or subinterface from which it was received. Split horizon rears its ugly head most predominantly in multipoint configurations. Here, routing updates flow into one subinterface but also must be sent out that same subinterface to reach the other routers on the multipoint network. Split horizon is on by default and prevents routing updates for EIGRP, IGRP, and RIP from being propagated properly in a multipoint configuration. Disable this with the following interface key-in: no ip split-horizon for RIP or IGRP networks, and no ip split-horizon eigrp autonomous_system for EIGRP networks. These commands have similar forms for IPX and AppleTalk. If you have two point-to-point subinterfaces configured, routing updates flow in one subinterface and are forwarded out the other subinterface because each subinterface is on a different network. Therefore, there is no need to disable split horizon when using point-to-point subinterfaces.

Practical Example: Configuring Hybrid Frame Relay Networks

The example that follows walks you through the complete configuration of a Frame Relay network by using the different types of interfaces. Figure 5-4 illustrates a hybrid Frame Relay network.

Figure 5-4. Frame Relay Hybrid Network

In this example, you configure a Frame Relay multipoint network between the marlin, glock, and sig routers. You also configure a Frame Relay point-to-point network between the marlin and the bushmaster routers. The routing protocol is IGRP.

Let's begin with the marlin router. Following the four-step Frame Relay configuration process, start by setting the encapsulation to Frame Relay on the serial interface. You define two types of subinterfaces. A multipoint is needed for the subnet 172.16.1.0/24, which connects the glock and sig routers. You can use a point-to-point or a multipoint for subnet 172.16.16.0/24 to connect to the bushmaster router.

In this example, you use a point-to-point network. Example 5-2 demonstrates this configuration.

Example 5-2 Setting Encapsulation and Defining Subinterfaces

 marlin#  conf t  Enter configuration commands, one per line.  End with CNTL/Z. marlin(config)#  int s0  marlin(config-if)#  encapsulation frame-relay  marlin(config-if)#  int s0.1 multipoint  marlin(config-subif)#  ip address 172.16.1.1 255.255.255.0  marlin(config-subif)#  exit  marlin(config)#  int s0.2 point-to-point  marlin(config-subif)#  ip address 172.16.16.1 255.255.255.0  marlin(config-subif)#  ^Z  

You can follow the same steps defining a multipoint subinterface on the glock router and a point-to-point subinterface on the bushmaster router. You will not use any subinterfaces on the glock router, and you should treat it as a multipoint router. At this time, you need only to define the Frame Relay encapsulation on the glock router's s0 interface.

The next step is to configure LMI. As previously mentioned, Frame Relay autosense detects and configure the LMI automatically. No additional configuration is needed. For practice, you will statically configure the LMI on the bushmaster router to ANSI. This is accomplished with the frame-relay lmi-type ansi command under the s0 interface.

The third step is to configure static or dynamic addressing. On the marlin router, use a static address on the s0.1 interface, the multipoint interface. The s0.2 interface is a point-to-point interface, so you can use dynamic addressing. You need one frame-relay map statement pointing to each remote router on the 172.16.1.0/24 subnet. Example 5-3 demonstrates the configuration for static mapping.

Example 5-3 Configuring Static Mapping

 marlin(config)#  int s0.1 multipoint  marlin(config-subif)#  frame-relay map ip 172.16.1.3 110 broadcast  marlin(config-subif)#  frame-relay map ip 172.16.1.5 120 broadcast  marlin(config-subif)#  exit  

Example 5-4 demonstrates the configuration of dynamic addressing needed for the marlin router.

Example 5-4 Configuring Dynamic Mapping

 marlin(config)#  int s0.2 point-to-point  marlin(config-subif)#  frame-relay interface-dlci 130  marlin(config-fr-dlci)#  ^Z  marlin# 

The glock router's serial interface is a natural interface on a multipoint network; therefore, you use static addressing. Here, you need two frame-relay map statements. You configure one frame-relay map statement pointing at DLCI 111 for IP address 172.16.1.1, and one pointing at the same DLCI, 111, for IP address 172.16.1.5. Example 5-5 shows the configuration on the glock router serial interface.

Example 5-5 Configuring the glock Router's Serial Interface

  Interface serial0   ip address 172.16.1.3 255.255.255.0   no ip directed-broadcast   encapsulation frame-relay   no ip mroute-cache   no fair-queue   frame-relay map ip 172.16.1.5 111 broadcast   frame-relay map ip 172.16.1.1 111 broadcast  

The sig router has a multipoint subinterface on s0; therefore, this router also needs two static frame-relay map statements. One frame-relay map statement is for the glock router, and one is for the marlin router. Example 5-6 shows the configuration for the serial interface for the sig router.

Example 5-6 sig Router's Serial Interface

  interface serial0.1 multipoint   ip address 172.16.1.5 255.255.255.0   no ip directed-broadcast   no ip mroute-cache   frame-relay map ip 172.16.1.3 121 broadcast   frame-relay map ip 172.16.1.1 121 broadcast   !  

Returning to the marlin router, you can complete Step 3 for the point-to-point side of the link. The subinterface s0.2 is a point-to-point interface to the bushmaster router. Therefore, you can use dynamic addressing on this interface. To accomplish this, use the f rame-relay interface-dlci dlci_number command under the s0.2 interface, such as in Example 5-7.

Example 5-7 Configuring marlin Router's serial 0.2 Subinterface

  interface serial0.2 point-to-point   ip address 172.16.16.1 255.255.255.0    frame-relay interface-dlci 130    !  

Repeat this same process for the point-to-point subinterface on the bushmaster router; this time, however, it points toward DLCI 131.

Now, you can move on to Step 4 in the configuration process: address any protocol-specific issues. As previously mentioned, a split-horizon issue occurs on a multipoint network running IGRP, such as this one. With the default of split horizon set to on, the marlin router will not forward sig's Ethernet network of 172.16.5.0/24 back out the s0.1 port toward the glock router. It also will not forward the glock router's Token Ring network 172.16.3.0/24 back out its s0.1 port toward the sig router. To resolve this, use the no ip split-horizon command on the marlin router's s0.1 port. You now have full IP connectivity across the Frame Relay network. IGRP uses bandwidth to influence routing decisions. To further tune the network, assign bandwidth statements to all serial interfaces to make routing decisions more accurate. Example 5-8 lists the relevant portions of all the router configurations.

Example 5-8 Relevant Configuration Listing for the Routers in Figure 5-4

  hostname marlin   !   interface Ethernet1   ip address 172.16.2.1 255.255.255.0   media-type 10BaseT   !   interface Serial0   no ip address   encapsulation frame-relay   no ip mroute-cache   bandwidth 1544   no fair-queue   !   interface Serial0.1 multipoint   ip address 172.16.1.1 255.255.255.0   no ip split-horizon   frame-relay map ip 172.16.1.3 110 broadcast   frame-relay map ip 172.16.1.5 120 broadcast   !   interface Serial0.2 point-to-point   ip address 172.16.16.1 255.255.255.0   frame-relay interface-dlci 130   !   router igrp 2001   network 172.16.0.0   !  ________________________________________________________________  hostname glock   !   <<<text omitted>>>   !   interface Serial0   bandwidth 64   ip address 172.16.1.3 255.255.255.0   no ip directed-broadcast   encapsulation frame-relay   no ip mroute-cache   no fair-queue   frame-relay map ip 172.16.1.5 111 broadcast   frame-relay map ip 172.16.1.1 111 broadcast   !   interface TokenRing0   ip address 172.16.3.3 255.255.255.0   no ip directed-broadcast   ring-speed 16   !   router igrp 2001   network 172.16.0.0   !  ________________________________________________________________  hostname sig   !   <<<text omitted>>>   !   interface Ethernet0   ip address 172.16.5.5 255.255.255.0   no ip directed-broadcast   !   interface Serial0   no ip address   no ip directed-broadcast   encapsulation frame-relay   no ip mroute-cache   no fair-queue   !   interface Serial0.1 multipoint   bandwidth 64   ip address 172.16.1.5 255.255.255.0   no ip directed-broadcast   no ip mroute-cache   frame-relay map ip 172.16.1.3 121 broadcast   frame-relay map ip 172.16.1.1 121 broadcast   !   router igrp 2001   network 172.16.0.0   !  ________________________________________________________________  hostname bushmaster   !   interface Ethernet0   ip address 172.16.6.6 255.255.255.0   !   interface Serial0   no ip address   encapsulation frame-relay   frame-relay lmi-type ansi   !   interface Serial0.1 point-to-point   ip address 172.16.16.6 255.255.255.0   bandwidth 64   frame-relay interface-dlci 131   !   router igrp 2001   network 172.16.0.0  

To verify that your Frame Relay network is operational, you can use standard ping s and traces tests; however, sometimes you might want to require more information about the operational status of the Frame network. The "Big show " and "Big D" commands for Frame Relay can provide a lot of useful information, as described in the next section.