Subinterfaces Frame Relay Implementation

You probably know by now that we can have multiple virtual circuits on a single serial interface and yet treat each as a separate interface—I did mention this earlier. We can make this happen by creating subinterfaces. Think of a subinterface as a logical interface defined by the IOS software.

Several subinterfaces will share a single hardware interface, yet for configuration purposes they operate as if they were separate physical interfaces, something known as multiplexing.

To configure a router in a Frame Relay network so it will avoid split horizon issues by not permitting routing updates, just configure a separate subinterface for each PVC, with a unique DLCI and subnet assigned to the subinterface.

You define subinterfaces using a command like int s0.subinterface number. First, you have to set the encapsulation on the physical serial interface, and then you can define the subinterfaces— generally one subinterface per PVC. Here’s an example:

RouterA(config)#int s0
RouterA(config-if)#encapsulation frame-relay
RouterA(config-if)#int s0.?
<0-4294967295> Serial interface number
RouterA(config-if)#int s0.16 ?
multipoint Treat as a multipoint link
point-to-point Treat as a point-to-point link
RouterA(config-if)#int s0.16 point-to-point

Note:Make sure that you don’t have an IP address under the physical interface if you have configured subinterfaces!

You can define a serious amount of subinterfaces on any given physical interface, but keep in mind that there are only about a thousand available DLCIs. In the preceding example, I chose to use subinterface 16 because that represents the DLCI number assigned to that PVC by the carrier. There are two types of subinterfaces:

Point-to-point Used when a single virtual circuit connects one router to another. Each pointto- point subinterface requires its own subnet.

Note:A point-to-point subinterface maps a single IP subnet per DLCI and addresses and resolves NBMA split horizon issues.

Multipoint This is when the router is the center of a star of virtual circuits that are using a single subnet for all routers’ serial interfaces connected to the frame switch. You’ll usually find this implemented with the hub router in this mode and the spoke routers in physical interface (always point-to-point) or point-to-point subinterface mode.

Next, I’ll show you an example of a production router running multiple subinterfaces. In the following output, notice that the subinterface number matches the DLCI number—not a requirement, but it majorly helps you administer the interfaces:

interface Serial0
no ip address (notice there is no IP address on the physical interface!)
no ip directed-broadcast
encapsulation frame-relay
!
interface Serial0.102 point-to-point
ip address 10.1.12.1 255.255.255.0
no ip directed-broadcast
frame-relay interface-dlci 102
!
interface Serial0.103 point-to-point

ip address 10.1.13.1 255.255.255.0
no ip directed-broadcast
frame-relay interface-dlci 103
!
interface Serial0.104 point-to-point
ip address 10.1.14.1 255.255.255.0
no ip directed-broadcast
frame-relay interface-dlci 104
!
interface Serial0.105 point-to-point
ip address 10.1.15.1 255.255.255.0
no ip directed-broadcast
frame-relay interface-dlci 105
!

Notice that there’s no LMI type defined. This means that the routers are either running the Cisco default or they’re using autodetect (if running Cisco IOS version 11.2 or newer). I also want to point out that each interface maps to a single DLCI and is defined as a separate subnet. Remember—point-to-point subinterfaces solve split horizon issues as well.