Frame Relay PIPQ
The remaining sections of this chapter focus on the Frame Relay PIPQ feature. The Frame Relay PIPQ feature works by providing a PQ scheme at the interface level. Similar to the generic PQ feature enabled at the interface level for nonserial interfaces or other encapsulation types, the Frame Relay PIPQ feature provides four queues of ascending levels of priority: low, normal, medium, and high.
NOTE
The Frame Relay PIPQ feature is supported in Cisco IOS Release 12.1(1)T and later.
PQ Versus Frame Relay PIPQ
The major difference between generic PQ and Frame Relay PIPQ is in the way the packets are classified and sent into the respective priority queues. In the generic PQ case, packet classification and prioritization are based on packet contents, such as protocol types, packet size, TCP or UDP port numbers, and the use of access lists. In the Frame Relay PIPQ case, prioritization is based solely on the destination PVC, rather than the packet contents.
With Frame Relay PIPQ, the user can designate that the Frame Relay PVC carrying high-priority traffic has absolute priority over the other Frame Relay PVCs transporting noncritical data. When a Frame Relay packet arrives at the interface level, the packet is examined for its DLCI value. After knowing which DLCI the packet belongs to, the packet is then sent to the corresponding priority queue at the interface level, based on the priority level that is mapped to that DLCI by Frame Relay PIPQ.
Because Frame Relay PIPQ determines a Frame Relay packet's priority level based on its DLCI value rather than examining the packet contents, it is extremely important for the network administrator to configure the network so that separate types of traffic are transported on separate Frame Relay PVCs. Frame Relay PIPQ does not work if there is a Frame Relay PVC carrying varying types of traffic with different QoS requirements. Frame Relay PIPQ is most advantageous if there are different PVCs for carrying different types of traffic, and the network administrator needs to ensure that delay-sensitive traffic on a Frame Relay PVC has absolute priority over the remaining classes of traffic on other PVCs. Figure 17-4 depicts the way Frame Relay PIPQ works.
Figure 17-4. Frame Relay PIPQ
As illustrated in Figure 17-4, Frame Relay PIPQ sorts traffic arriving from the Frame Relay PVCs into four interface-level priority queues, based on the packets' derived DLCI values. The figure also serves as an example of how the Frame Relay PIPQ feature is commonly used.
Before enabling the Frame Relay PIPQ feature on your network, you should ensure that your Frame Relay PVCs are configured to carry only a single type of traffic. As shown in the figure, it is imperative for users to protect delay-sensitive and network-control traffic, such as voice packets and routing updates, by transporting them on a separate Frame Relay PVC (DLCI 100) mapped to the high-priority queue.
Traffic such as voice call signaling can be transported on a dedicated Frame Relay PVC separate from the high-priority traffic. In the example shown in Figure 17-4, the voice call signaling and setup traffic carried on DLCI 200 are mapped to the medium-priority queue. It is a common practice to assign data traffic that is not mission-critical, such as users' FTP file transfers, to the low priority queue. The network manager must ensure that separate PVCs are used to service the users. All other traffic that is not classified can be transported on single designated PVC mapped to the normal priority queue.
NOTE
In principle, the Frame Relay PIPQ feature works almost exactly the same way as the PQ mechanism. As in PQ, the high-priority queue is always serviced first. As long as the high-priority queue is not empty, it is possible that the lower-priority queues can be starved of their required bandwidth to transmit. Therefore, network administrators should ensure that the network is configured with adequate call admission control.
Restrictions of Frame Relay PIPQ
This section looks at the restrictions imposed on the use of the Frame Relay PIPQ feature on supported Cisco platforms. The Frame Relay PIPQ feature is supported on all major Cisco router platforms, including the c7500 series routers in nondistributed mode.
The following list summarizes the restrictions applied to the Frame Relay PIPQ feature:
• Frame Relay PIPQ cannot be used on interfaces that do not support PQ. Examples are loopbacks or tunnel logical interfaces.
• Frame Relay PIPQ cannot be used on interfaces that are already configured with queuing other than FIFO queuing or WFQ (if it is not the default interface queuing method).
NOTE
The Frame Relay PIPQ feature is supported in Cisco IOS Release 12.1(1)T and later.
PQ Versus Frame Relay PIPQ
The major difference between generic PQ and Frame Relay PIPQ is in the way the packets are classified and sent into the respective priority queues. In the generic PQ case, packet classification and prioritization are based on packet contents, such as protocol types, packet size, TCP or UDP port numbers, and the use of access lists. In the Frame Relay PIPQ case, prioritization is based solely on the destination PVC, rather than the packet contents.
With Frame Relay PIPQ, the user can designate that the Frame Relay PVC carrying high-priority traffic has absolute priority over the other Frame Relay PVCs transporting noncritical data. When a Frame Relay packet arrives at the interface level, the packet is examined for its DLCI value. After knowing which DLCI the packet belongs to, the packet is then sent to the corresponding priority queue at the interface level, based on the priority level that is mapped to that DLCI by Frame Relay PIPQ.
Because Frame Relay PIPQ determines a Frame Relay packet's priority level based on its DLCI value rather than examining the packet contents, it is extremely important for the network administrator to configure the network so that separate types of traffic are transported on separate Frame Relay PVCs. Frame Relay PIPQ does not work if there is a Frame Relay PVC carrying varying types of traffic with different QoS requirements. Frame Relay PIPQ is most advantageous if there are different PVCs for carrying different types of traffic, and the network administrator needs to ensure that delay-sensitive traffic on a Frame Relay PVC has absolute priority over the remaining classes of traffic on other PVCs. Figure 17-4 depicts the way Frame Relay PIPQ works.
Figure 17-4. Frame Relay PIPQ
As illustrated in Figure 17-4, Frame Relay PIPQ sorts traffic arriving from the Frame Relay PVCs into four interface-level priority queues, based on the packets' derived DLCI values. The figure also serves as an example of how the Frame Relay PIPQ feature is commonly used.
Before enabling the Frame Relay PIPQ feature on your network, you should ensure that your Frame Relay PVCs are configured to carry only a single type of traffic. As shown in the figure, it is imperative for users to protect delay-sensitive and network-control traffic, such as voice packets and routing updates, by transporting them on a separate Frame Relay PVC (DLCI 100) mapped to the high-priority queue.
Traffic such as voice call signaling can be transported on a dedicated Frame Relay PVC separate from the high-priority traffic. In the example shown in Figure 17-4, the voice call signaling and setup traffic carried on DLCI 200 are mapped to the medium-priority queue. It is a common practice to assign data traffic that is not mission-critical, such as users' FTP file transfers, to the low priority queue. The network manager must ensure that separate PVCs are used to service the users. All other traffic that is not classified can be transported on single designated PVC mapped to the normal priority queue.
NOTE
In principle, the Frame Relay PIPQ feature works almost exactly the same way as the PQ mechanism. As in PQ, the high-priority queue is always serviced first. As long as the high-priority queue is not empty, it is possible that the lower-priority queues can be starved of their required bandwidth to transmit. Therefore, network administrators should ensure that the network is configured with adequate call admission control.
Restrictions of Frame Relay PIPQ
This section looks at the restrictions imposed on the use of the Frame Relay PIPQ feature on supported Cisco platforms. The Frame Relay PIPQ feature is supported on all major Cisco router platforms, including the c7500 series routers in nondistributed mode.
The following list summarizes the restrictions applied to the Frame Relay PIPQ feature:
• Frame Relay PIPQ cannot be used on interfaces that do not support PQ. Examples are loopbacks or tunnel logical interfaces.
• Frame Relay PIPQ cannot be used on interfaces that are already configured with queuing other than FIFO queuing or WFQ (if it is not the default interface queuing method).
