Prerequisites for MPLS Quality of Service
To use MPLS CoS to full advantage in your network, the following functionality must be supported:
-
Multiprotocol Label Switching (MPLS)—MPLS is the standardized label switching protocol defined by the Internet Engineering Task Force (IETF).
-
Cisco Express Forwarding—Cisco Express Forwarding is an advanced Layer 3 IP switching technology that optimizes performance and scalability in networks that handle large volumes of traffic and that exhibit dynamic traffic patterns.
-
Asynchronous Transfer Mode (ATM)—ATM signaling support is required if you are using ATM interfaces in your network.
If you are using only packet interfaces in your network, ATM functionality is not needed.
-
QoS features:
-
Weighted fair queueing (WFQ)—Used on non-GSR platforms, WFQ is a dynamic scheduling method that allocates bandwidth fairly to all network traffic.
WFQ applies priorities, or weights, to traffic to classify the traffic into flows and determine how much bandwidth to allow each flow. WFQ moves interactive traffic to the front of a queue to reduce response time and fairly shares the remaining bandwidth among high-bandwidth flows.
-
Weighted random early detection (WRED)—WRED is a congestion avoidance mechanism that extends RED functionality by allowing different RED parameters to be configured per IP precedence value.
IP precedence bits, contained in the type of service (ToS) octet in the IP packet header, are used to denote the relative importance or priority of an IP packet. WRED uses these IP precedence values to classify packets into different discard priorities or classes of service.
-
Modified deficit round robin (MDRR)—Used only on GSR platforms, MDRR is a traffic class prioritization mechanism that incorporates emission priority as a facet of quality of service. MDRR is similar in function to WFQ on non-GSR platforms.
In MDRR, IP traffic is mapped to different classes of service queues. A group of queues is assigned to each traffic destination. On the transmit side of the platform, a group of queues is defined on a per-interface basis; on the receive side of the platform, a group of queues is defined on a per-destination basis. IP packets are then mapped to these queues, based on their IP precedence value.
These queues are serviced on a round-robin basis, except for a queue that has been defined to run in either of two ways: strict priority mode or alternate priority mode.
In strict priority mode, the high priority queue is serviced whenever it is not empty; this ensures the lowest possible delay for high priority traffic. In this mode, however, the possibility exists that other traffic might not be serviced for long periods of time if the high priority queue is consuming most of the available bandwidth.
In alternate priority mode, the traffic queues are serviced in turn, alternating between the high priority queue and the remaining queues.
-
Committed access rate (CAR)—CAR is a QoS feature that limits the input or output transmission rate on an interface and classifies packets by setting the IP precedence value or the QoS group in the IP packet header.
-