Where to Shape Interfaces Subinterfaces and VCs

Shaping can be applied to the physical interface, a subinterface, or in some cases, to an individual VC. Depending on the choice, the configuration causes traffic shaping to occur separately for each VC, or it shapes several VCs together. In most cases, engineers want to shape each VC individually.

When shaping is applied to an interface for which VCs do not exist, shaping is applied to the main interface, because there are no subinterfaces or VCs on those interfaces. On Frame Relay and ATM interfaces, however, some sites have multiple VCs terminating in them, which means that subinterfaces will most likely be used. In some cases, more than one VC is associated with a single multipoint subinterface; in other cases, point-to-point subinterfaces are used, with a single VC associated with the subinterface. The question becomes this: To shape per VC, where do you enable traffic shaping?

First, consider a typical branch office, such as R24 in Figure 5-10.

Figure 5-10 PB Tents Network: Shaping on Subinterfaces and VCs

Figure 5-10 PB Tents Network: Shaping on Subinterfaces and VCs

R24 has a single VC to the Main site at PB Tents. Because R24 only has the single VC, the configuration on R24 may not even use subinterfaces at all. If the configuration does not use subinterfaces on R24's serial link, traffic shaping can be configured on the physical interface. If the configuration includes a subinterface, you can enable traffic shaping on the physical interface, or on the subinterface. Because there is only one VC, it does not really matter whether shaping is enabled on the physical interface, or the subinterface—the behavior is the same.

Now consider the Main router. It has a VC to each remote site. (Also notice that a VC has been added between R1 and R2, just to make things interesting.) So, on the main router, point-to-point subinterfaces are used for the VCs to branches 3 through 24, and a multipoint subinterface is used for the two VCs to R1 and R2. To shape each VC to branches 3 through 24 separately, shaping can be configured on the subinterface. However, shaping applied to a multipoint subinterface shapes all the traffic on all VCs associated with the subinterface. To perform shaping on each VC, you need to enable shaping on each individual data-link connection identifier (DLCI).

In summary, most QoS policies call for shaping on each VC. The configuration commands used to enable shaping differ slightly based on the number of VCs, and how they are configured. Table 5-4 summarizes the options.

Table 5-4 Options of How to Enable Shaping forper-VC Shaping

Location

Requirements for Shaping per VC

No VCs, for example, point-to-point links

Shape on the main interface. Shaping occurs for all traffic on interface.

Physical interface, 1 VC, no subinterfaces

Shaping shapes the individual VC associated with this interface. Shaping can be enabled on the physical interface.

Physical interface, 1 VC, 1 subinterface

Shaping shapes the individual VC associated with this interface. Shaping can be enabled on the physical interface, the subinterface, or the VC (DLCI).

Multiple VCs on 1 interface, point-to-point subinterfaces only

Shaping can be enabled on the subinterface, or per DLCI. Both methods work identically.

Multiple VCs on 1 interface, some multipoint subinterfaces with > 1 VC per subinterface

Must enable shaping on each DLCI to shape per VC.

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