Frame Relay Sub interface Configuration and Verification

The concept of a sub interface is the same now with frame relay; it is just a logical interface that is directly associated with a physical interface. With frame relay sub interface configuration, we can create a sub interface for each of the PVCs coming into a serial interface.

Two types of sub interfaces can be created for frame relay—multipoint and point-to-point. A multipoint sub interface can handle multiple PVCs; its use is similar to that of the NBMA network that we saw in the preceding section. A point-to-point sub interface (the type we are going to use) effectively turns every PVC into a point-to-point network with its own network addressing. Using point-to-point sub interfaces gives us greater control over our frame relay network.

We are going to implement the configuration illustrated in Figure 14. Since Dallas has two PVCs, we are going to create sub interfaces on Dallas. FortWorth and Austin will continue with the original NBMA configuration since they have only one PVC each; however, Austin’s Serial2 IP address will have to change since it will be connected to a different network created by the Dallas point-to-point sub interface.

Figure 14: Frame-relay sub interface internetwork.

Figure 15 shows the commands necessary to change from the NBMA configuration to a point-to-point sub interface configuration. When using sub interfaces, the physical interface normally does not have any network addressing; therefore, we removed the IP address from Serial2 (Line 4). We created the first sub interface, Serial2.1, by referencing it on an interface command (Line 5). The point-to-point keyword is used to create a point-to-point sub interface. The other option is to create a multipoint sub interface by specifying the keyword multipoint. Point-to-point and multipoint sub interfaces cannot be created when the physical interface’s encapsulation is at its default, HDLC; therefore, we had to have the encapsulation frame-relay command on the Serial2 interface to be able to specify that we wanted a point-to-point sub interface. The encapsulation frame-relay command was issued on Serial2 during the NBMA configuration.

1)  Dallas#configure terminal

2)  Enter configuration commands, one per line. End with CNTL/Z.

3)  Dallas(config)#interface serial2

4)  Dallas(config-if)#no ip address

5)  Dallas(config-if)#interface serial2.1 point-to-point

6)  Dallas(config-subif)#ip address 172.16.30.1 255.255.255.0

7)  Dallas(config-subif)#frame-relay interface-dlci 100

8)  Dallas(config-fr-dlci)#interface serial2.2 point-to-point

9)  Dallas(config-subif)#ip address 172.16.31.1 255.255.255.0

10) Dallas(config-subif)#frame-relay interface-dlci 102

11) Dallas(config-fr-dlci)#<Ctrl-Z>

12) Dallas#

Figure 15: Frame-relay sub interface configuration on Dallas.

Sub interface Serial2.1 has its own IP address (Line 6). Instead of mapping a remote network address to a local DLCI, we just need to tell IOS which PVC is supposed to be processed by this sub interface. We use the frame-relay interface-dlci sub interface configuration command to do that. We want Serial2.1 to process the traffic for the PVC going to FortWorth, and that PVC has the local DLCI 100 (Line 7). The second sub interface, Serial2.2, is created and configured similarly. Notice, however, that Serial2.2 also has its own IP address (Line 9), and the IP address is on a different IP subnet than Serial 2.1. Serial2.2 is processing traffic for the Dallas-to-Austin PVC that has DLCI 102 (Line 10).

Serial2 on Austin now needs an IP address on the same IP subnet as that of Dallas’s Serial2.2. Figure 16 shows the commands for changing Austin’s Serial2 IP address to its new value, 172.16.31.2.

1)  Austin#configure terminal

2)  Enter configuration commands, one per line. End with CNTL/Z.

3)  Austin(config)#interface serial2

4)  Austin(config-if)#ip address 172.16.31.2 255.255.255.0

5)  Austin(config-if)#<Ctrl-Z>

6)  Austin#

Figure 16: Austin IP address change for frame-relay sub interface configuration.

We saw the command for verifying frame relay operation after our NBMA configuration; however, the output of the show ip route and show frame-relay map commands changes slightly.

Figure 17 shows the new IP routing table on Dallas. The table now shows the sub interfaces in the paths to networks. Serial2.1 is used in the path to the FortWorth Ethernet LAN (Line 14) and Serial2.2 is used in the path to the Austin Ethernet LAN (Line 16).

1)  Dallas#show ip route

2)  Codes: C - connected, S - static, I - IGRP, R - RIP,
           M - mobile, B - BGP

3)         D - EIGRP, EX - EIGRP external, O - OSPF, IA - OSPF
           inter area

4)         N1 - OSPF NSSA external type 1, N2 - OSPF NSSA
           external type 2

5)         E1 - OSPF external type 1, E2 - OSPF external type 2,
           E - EGP

6)         i - IS-IS, L1 - IS-IS level-1, L2 - IS-IS level-2,
           * - candidate default

7)         U - per-user static route, o - ODR

8)

9)  Gateway of last resort is not set

10)

11)      172.16.0.0/24 is subnetted, 4 subnets

12) C       172.16.30.0 is directly connected, Serial2.1

13) C       172.16.31.0 is directly connected, Serial2.2

14) R       172.16.20.0 [120/1] via 172.16.30.2, 00:00:14, Serial2.1

15) C       172.16.10.0 is directly connected, Ethernet0

16) R    192.168.1.0/24 [120/1] via 172.16.31.2, 00:00:23, Serial2.2

17) Dallas#

Figure 17: Show IP route on Dallas after sub interface configuration.

Figure 18 shows the output of the show frame-relay map command on Dallas. Now that sub interfaces have been implemented, the map entries are neither dynamic nor static. Both entries are listed as point-to-point, and the sub interface is listed for each one.

1)  Dallas#show frame-relay map

2)  Serial2.1 (up): point-to-point dlci, dlci 100(0x64,0x1840),

3)  broadcast status defined, active

4)  Serial2.2 (up): point-to-point dlci, dlci 102(0x66,0x1860),

5)  broadcast status defined, active

6)  Dallas#

Figure 18: Show frame-relay map on Dallas after sub interface configuration.

The one drawback of using the sub interface configuration method for frame relay is that it requires more network address space. However, Cisco network experts still recommend using sub interfaces because of their flexibility and control. For example, if the PVC associated with a point-to-point sub interface goes down, IOS changes the status of the sub interface to down. A downed interface is a little easier to spot than a downed PVC.