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This chapter is from the book

Configuring Frame Relay Subinterfaces

On partially meshed Frame Relay networks, the problem of split horizon can be overcome by using Frame Relay subinterfaces. Frame Relay provides a mechanism to allow a physical interface to be partitioned into multiple virtual interfaces. In a similar way, using subinterfaces allows a partially meshed network to be divided into a number of smaller, fully meshed point-to-point networks. Generally, each point-to-point subnetwork is assigned a unique network address. This allows packets received on one physical interface to be sent out from the same physical interface, albeit forwarded on VCs in different subinterfaces.

There are two types of subinterfaces supported by Cisco routers: point-to-point and multipoint subinterfaces. Each of these types will be described in the next sections.

Point-to-Point Subinterfaces

In general, to configure a subinterface on a Frame Relay interface, follow the configuration commands listed below beginning in global configuration mode:

Step 1

Go to the interface configuration mode of the interface on which you want to create Frame Relay subinterfaces and enable Frame Relay encapsulation.

Step 2

Configure and create a point-to-point or multipoint subinterface.


Example 4-20 shows an example of the configuration steps required to create a point-to-point subinterface. Example 4-21 shows an example of the configuration steps required to create a multipoint subinterface. Both configuration commands are performed on the hub router R3 in Figure 4-2.

Example 4-20 Creating Point-to-Point Subinterfaces on a Physical Interface Example 4-21 Creating Multipoint Subinterfaces on a Physical Interface

R3#configure terminal
00:41:34: %SYS-5-CONFIG_I: Configured from console by console
Enter configuration commands, one per line. End with CNTL/Z.
R3(config)#interface serial3/1
R3(config-if)#encapsulation frame-relay
R3(config-if)#interface serial3/1.1 point-to-point
R3(config-subif)#

NOTE

Once you create a specific type of subinterface, you cannot change it without reloading the router. For example, you cannot create a multipoint subinterface serial0.2 and then change it to point-to-point. To change it, you need to either reload the router or create another subinterface. This is the way the Frame Relay code works in Cisco IOS software.

An example is used to illustrate how multiple subinterfaces can be created under the same physical interface. A fourth router, R4, is added to Figure 4-2. The resultant topology is shown in Figure 4-3.

Figure 3Figure 4-3 Frame Relay Network Using Subinterfaces

At the hub router R3, two subinterfaces are created on the physical serial interface 3/1. One multipoint subinterface is created, and an IP address for the 172.16.1.0/29 subnet is assigned. Under the same physical serial interface 3/1, another point-to-point subinterface can be created for the point-to-point connection to spoke router R4 for the IP subnet 192.168.1.0/30. Example 4-22 shows the configuration files of all four routers shown in Figure 4-3.

Example 4-22 Configuration Files of Routers in Figure 4-3

R1#show running-config
Building configuration...
!
hostname R1
!
<output omitted>
!   
interface Serial4/2
 ip address 172.16.1.1 255.255.255.248
 encapsulation frame-relay
 frame-relay map ip 172.16.1.2 103 broadcast

R2#show running-config
Building configuration...
!
hostname R2
!
<output omitted>
!
interface Serial3/0
 ip address 172.16.1.2 255.255.255.248
 encapsulation frame-relay
frame-relay map ip 172.16.1.1 203 broadcast
!
<output omitted>

R3#show running-config 
Building configuration...
!
hostname R3
!
<output omitted>
!
interface Serial3/1
 no ip address
 encapsulation frame-relay
!
interface Serial3/1.304 point-to-point
 ip address 192.168.1.1 255.255.255.252
 frame-relay interface-dlci 304  
!
interface Serial3/1.301 multipoint
 ip address 172.16.1.3 255.255.255.248
 frame-relay interface-dlci 301
 frame-relay interface-dlci 302

R4#show running-config
Building configuration...
!
hostname R4
! 
<output omitted>
!
interface Serial1/2
 no ip address
 encapsulation frame-relay
!
interface Serial1/2.403 point-to-point
 ip address 192.168.1.2 255.255.255.252
 frame-relay interface-dlci 403  
!
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