QOS - Congestion Management and Avoidance Flashcards
What is Congestion Management?
Congestion management provides a way of managing traffic during periods of congestion and involves a combination of queuing and scheduling.
How does a queuing algorithm detect congestion?
Congestion is detected when a Layer 1 hardware queue present on physical interfaces, known as the transmit ring (Tx-ring or TxQ), is full.
What are two reasons that cause congestion to occur?
- The input interface is faster than the output interface * The output interface is receiving packets from multiple input interfaces
How do queues solve congestion?
During congestion the queues fill up, and packets can be reordered by some of the queuing algorithms so that higher-priority packets exit the output interface sooner than lower-priority ones.
What are 6 legacy queuing algorithms?
- FIFO * Round Robin * Weighted Round Robin * Custom Queuing * Priority Queuing * Weighted Fair Queuing
What are 2 present-day queuing algorithms?
- Class-based Weighted Fair Queuing (CBWFQ) * Low-latency Queuing (LLQ)
What is CBWFQ?
- Up to 256 queues * Serving up to 256 traffic classes * Each queue serviced based on bandwidth assigned to that class * Each class assigned bandwidth, weight, queue limit, and maximum packet limit
In CBWFQ, what is the bandwidth assigned to a class?
The bandwidth assigned to a class is the minimum bandwidth delivered to the class during congestion.
In CBWFQ, what is the queue limit?
The queue limit for that class is the maximum number of packets allowed to be buffered in the class queue.
What happens to packets once the queue limit has been reached?
Excess packets are dropped.
Is CBWFQ suitable for real-time traffic?
No, CBWFQ does not provide a latency guarantee.
What is Low Latency queuing?
- LLQ is an extension of CBWFQ * It is CBWFQ combined with priority queueing (PQ) and it was developed to meet the requirements of real-time traffic, such as voice.
What 4 things describe how does LLQ works?
- In addition to the CBWFQ a Low Latency strict-priority queue is created * All realtime traffic should be assigned to the LLQ * All LLQ traffic is serviced up to its assigned bandwidth before other CBWFQ queues are serviced * Provides both latency and bandwidth guarantees to high-priority real-time traffic
What must be included when LLQ is configured?
The policing rate must be specified as either a fixed amount of bandwidth or as a percentage of the interface bandwidth.
How many traffic classes does LLQ allow for?
2 Classes
What would be the reason for creating 2 traffic classes in LLQ?
So that different policing rates can be applied to different types of high-priority traffic.
What is the default Congestion Avoidance tool?
Tail drop
What is wrong with using Tail drop as a congestion avoidance technique?
- Treats all traffic equally and does not differentiate between classes of service * Should be avoided for TCP traffic because it can cause TCP global synchronization, which results in significant link underutilization
How does Tail Drop work?
When the output queue buffers are full, all packets trying to enter the queue are dropped, regardless of their priority, until congestion clears up and the queue is no longer full.
What is a better congestion avoidance tool than tail drop?
Random Early Detect (RED)
How does RED work?
RED monitors the buffer depth and performs early drops on random packets when the minimum defined queue threshold is exceeded.
Why is RED better than tail drop?
Randomly dropping packets instead of dropping them all at once, as with tail drop, avoids global synchronization of TCP streams.
What is Cisco’s implementation of RED?
Weighted RED (WRED)
What is the difference between RED and WRED?
The difference between RED and WRED is that the randomness of packet drops can be manipulated by traffic weights denoted by either IP Precedence (IPP) or DSCP.
What is an extension to WRED?
Explicit Congestion Notification (ECN)
How does the WRED extension ECN work?
In times of congestion it allows for signaling to be sent to ECN-enabled endpoints, instructing them to reduce their packet transmission rates.
Name 2 congestion avoidance techniques.
- RED * WRED
How does the legacy queuing algorithm Round Robin work?
- queues are serviced in sequence one after the other * 1 packet sent per round * No queue has priority * No mechanism to prioritize traffic
How does the legacy queuing algorithm Weighted Round Robin work?
- Same as round robin but allows a weight to be assigned to each queue * Based on that weight, each queue effectively receives a portion of the interface bandwidth that is not necessarily equal to the other queues’ portions
How does the legacy queuing mechanism Custom Queuing work?
- Cisco implementation of WRR * 16 queues with a round-robin scheduler * FIFO within each queue * Each queue customized with a portion of the link bandwidth for each selected traffic type * Other traffic types can use unused bandwidth * Caused long delays
How does the legacy queuing algorithm Priority Queuing work?
- a set of four queues (high, medium, normal, and low) are served in strict-priority order * high-priority queue is always serviced first * lower-priority queues are serviced only when all higher-priority queues are empty * Lower priority queues could be starved
How does the legacy queuing algorithm Weighted Fair Queuing work?
- automatically divides the interface bandwidth by the number of flows (weighted by IP Precedence) to allocate bandwidth fairly among all flows * provides better service for high-priority real-time flows * Can’t provide a fixed-bandwidth guarantee for any particular flow
What are the 3 tasks when configuring QOS using MQC?
- Step 1 - define a traffic class using the ‘class-map ‘ command * Step 2 - Create a traffic policy using the ‘policy-map ‘ command * Step 3 - Attach the traffic-policy to the interface with the ‘service-policy’ interface command.
When using MQC what does the policy-map do?
Associates the traffic class with one or more QoS features
