Connecting a Single Interface to Multiple Locations

What we have discussed so far is good for small Frame Relay connections, but what about the needs of large organizations? If you have to connect a couple of central routers to hundreds or thousands of remote locations, you don't want to pay for multiple installations, more interface cards for the routers, more CSUs, and more serial cables. Now the subinterface comes in handy. By using subinterfaces, you can logically connect dozens of Frame Relay connections to a single physical interface.

Network Design Types

Frame Relay networks have a few popular designs. In addition to point-to-point connections, they have the full-mesh, partial-mesh, and hub and spoke (also called star) designs, as described in Table 9.2.

Table 9.2. Frame Relay Network Design Types
Type	Description
Full mesh	The full-mesh design is where each router is connected to every other router in the network. This method costs more than either of the other two methods, but when properly implemented, it results in faster data access and terrific reliability.
Partial mesh	This design combines full mesh and point-to-point. Some routers have full-mesh connections between them, whereas other routers have connections only to other specific routers. An example is a company that has several data centers with a full-mesh design between them and each data center responsible for connection to remote sites within a limited geographic area.
Hub and spoke (star)	In this design, every site is only connected to one central router or set of routers. For remote site A to talk to remote site B, it must send packets via the corporate office routers. This design is cost efficient but does not provide any method of routing around down links. If you use this method, keep backhoes away from your property.

Frame Relay and Multiple Sites

Before we jump into how subinterfaces can solve all your problems, we need to look at why you use them. Primarily, we are concerned with reachability, which involves two different connection types: point-to-point and multipoint.

With a point-to-point configuration, think of a leased line. With a leased line, you have a starting point, a single possible ending point, and nothing to be concerned about in between. A router at either end of a single serial cable is an example of a point-to-point configuration.

Figure 9.1 shows a three-router Frame Relay network where there are separate IP networks between the router on the left and each router on the right. The router on the left has a single interface that is logically divided into two. This setup is called a subinterface, which is explained later in the chapter.

Figure 9.1. A point-to-point network.

graphics/09fig01.gif

With a multipoint configuration, think of some type of shared media, such as Ethernet. You can have more than two devices connected to the same wire segment. As shown in Figure 9.2, Ethernet, Token Ring, and Fiber Distributed Data Interface (FDDI) are all examples of multipoint configurations.

Figure 9.2. A multipoint network.

graphics/09fig02.gif

Why do you care about the two different ways that you can configure Frame Relay? Think back to what you know about distance-vector routing and routing loops. One method that distance-vector routing uses to prevent loops is split horizon. Split horizon tells the router not to send information about a route out the interface that the update arrived on. In a true multipoint environment, such as Ethernet, this restriction is not a problem. All routers send either broadcasts or multicasts, and each router on that segment receives and processes the update. Because of DLCIs, Frame Relay networks do not follow this process. Without using subinterfaces, you would need to configure an interface as multipoint to connect a single router interface to multiple sites.

If a router receives a routing update through a physical interface, the router assumes that every device it can reach through that interface also received the update. With routers accessed via multiple DLCIs, that might not be the case. Split horizon was designed to prevent a router from advertising a route out the interface it learned the information from, to prevent routing loops.

Figure 9.3 shows a router connected to two others via point-to-point subinterfaces. If router B sends a routing update to router A, split horizon tells router A not to forward it out the same physical interface the update arrived on. This means that router C won't receive the update. Split horizon should be off by default on Frame Relay interfaces, but it can be turned on and is on by default for some router protocols. If your routers have trouble pinging beyond the hub in a hub and spoke network, consider that split horizon might be the problem.

Figure 9.3. Point-to-point Subinterfaces

graphics/09fig03.gif

Configuring Subinterfaces

A subinterface is logically a slice of a physical interface. Let's assume we have a Frame Relay link connecting to physical interface serial 1. The router we are configuring needs to be able to talk to three remote sites via the single physical link:

Identify the physical interface that the Frame Relay link is connected to. Because we know it is serial 1, we need to set Frame Relay encapsulation on interface serial 1:
```
 Router(config-if)#encapsulation frame-relay {cisco | ietf} 
```
Identify how many connections are needed via this link. For our purposes, we need three. We then create them by typing interface serial 1.x where x is a number. Here, we use serial 1.1, 1.2, and 1.3. You need to specify whether a subinterface is supposed to be point-to-point or multipoint. Typing the preceding command dynamically creates the logical interface in much the same way that typing interface loopback 0 activates the loopback 0 interface.
Make sure that no Layer 3 address exists on the physical interface configuration. This step is very important because otherwise, the router gets confused. You can use the no protocol address interface command to remove a Layer 3 address without needing to look up the specific address.
Specify on each subinterface what DLCI it is connected to by using the command Router(config-subif)#frame-relay interface-dlci DLCI.
Apply Layer 3 addresses to each subinterface.

Figure 9.4 shows a close-up of the router with the subinterfaces. Interface serial 1 is logically divided into circuits 1.1, 1.2, and 1.3.

Figure 9.4. Subinterfaces illustrated.

graphics/09fig04.gif

Table 9.3 contains an example of configuring the router.

Table 9.3. Sample Walkthrough
Command	Explanation
Router(config)#interface serial 1	This command moves the router into interface configuration mode for serial 1.
Router(config-if)#no ip address	This command removes any IP address that exists on interface serial 1.
Router(config-if)#encapsulation frame-relay	This command establishes Frame Relay encapsulation for physical interface serial 1. All subinterfaces will now use Frame Relay encapsulation.
Router(config)#interface serial 1.1 point-to-point	This command creates the serial 1.1 subinterface, establishes it as a point-to-point interface, and moves the router into subinterface configuration mode.
Router(config-subif)#ip address 10.1.1.1 255.255.255.0	This command puts an IP address in subinterface serial 1.1. Remember that the device on the other side of the Frame Relay cloud needs to have a Layer 3 address that is a member of the same network.
Router(config-subif)#frame-relay interface-dlci 100	This command tells the router that to talk to other devices on network 10.1.1.0 (one device in this case, because we specified a point-to-point connection), the router needs to send information via Frame Relay DLCI 100.
Router(config-subif)#bandwidth 256	This command changes the interface bandwidth value from the default of a T1 (1.544Mbps) to the actual value, 256Kbps in this instance.
Router(config)#interface serial 1.2 point-to-point	This command creates the serial 1.2 subinterface, establishes it as a point-to-point interface, and moves the router into subinterface configuration mode.
Router(config-subif)#ip address 10.2.2.1 255.255.255.0	This command puts an IP address in subinterface serial 1.2.
Router(config-subif)#frame-relay interface-dlci 200	This command tells the router that to talk to other devices on network 10.2.2.0 (one device in this case, because we specified a point-to-point connection), the router needs to send information via Frame Relay DLCI 200.
Router(config-subif)#bandwidth 384	This command changes the interface bandwidth value from the default of a T1 (1.544Mbps) to the actual value, 384Kbps in this instance.
Router(config)#interface serial 1.3 multipoint	This command creates the serial 1.3 subinterface, establishes it as a multipoint interface, and moves the router into subinterface configuration mode.
Router(config-subif)#ip address 10.3.3.1 255.255.255.0	This command adds an IP address to subinterface serial 1.3.
Router(config-subif)#frame-relay map ip 10.3.3.2 300 broadcast Router(config-subif)#frame-relay map ip 10.3.3.3 400 broadcast	Frame Relay map statements tell the router which DLCI to use to get to a specific IP address because interface routing can't be relied on in a multipoint environment.
Router(config-subif)#bandwidth 768	This command changes the interface bandwidth value from the default of a T1 (1.544Mbps) to the actual value, 768Kbps in this instance.

By using subinterfaces, we are able to connect a single Frame Relay line to a single physical interface and connect to multiple remote locations. Point-to-point subinterfaces allow for routing information to come in from remote location A on a subinterface and get forwarded out to remote location B via a different subinterface. This setup allows routing information to get sent out while not sacrificing protection from routing loops.