Section 6 – Congestion Avoidance Methods
QUESTION 6
Which four of the following bit values are used for bits 5-7 of the DSCP field to select AF PHB? (Choose four.)
A. 000
B. 001
C. 010
D. 011
E. 100
F. 101
Answer: BCDE
Explanation:
Assured Forwarding
RFC 2597 defines the assured forwarding (AF) PHB and describes it as a means for a provider DS domain to offer different levels of forwarding assurances for IP packets received from a customer DS domain. The Assured Forwarding PHB guarantees a certain amount of bandwidth to an AF class and allows access to extra bandwidth, if available. There are four AF classes, AF1x through AF4x. Within each class, there are three drop probabilities. Depending on a given network’s policy, packets can be selected for a PHB based on required throughput, delay, jitter, loss or according to priority of access to network services. Classes 1 to 4 are referred to as AF classes. The following table illustrates the DSCP coding for specifying the AF class with the probability. Bits DS5, DS4 and DS3 define the class; bits DS2 and DS1 specify the drop probability; bit DS0 is always zero.
QUESTION 7
Which three statements regarding WRED are true? (Choose three.)
A. WRED can be IP precedence-based or DSCP-based.
B. WRED is an advanced queuing mechanism.
C. ECN is an extension of WRED.
D. WRED is used to rate-limit the incoming traffic by metering the incoming traffic rate.
E. WRED can be applied to a traffic class using CB-WRED.
F. WRED uses a shaping queue to delay excess traffic.
Answer: ACE
Explanation:
Weighted random early detection (WRED) is a queuing technique for congestion avoidance. WRED manages how packets are handled when an interface starts becoming congested. When traffic begins to exceed the interface traffic thresholds prior to any congestion, the interface starts dropping packets from selected flows. If the dropped packets are TCP, the TCP source recognizes that packets are getting dropped, and lowers its transmission rate. The lowered transmission rate then reduces the traffic to the interface, avoiding congestion. Because TCP retransmits dropped packets, no actual data loss occurs.
WRED drops packets according to the following criteria: RSVP flows are given precedence over non-RSVP flows, to ensure that time-critical packets are transmitted as required. Using IP precedence or DSCP value of the packets, packets with higher precedence are less likely to be dropped. If the default settings are preventing QoS, the precedence value can be used to control how WRED determines when and how often to drop packets. The amount of bandwidth used by the traffic flow. Flows that use the most bandwidth are more likely to have packets dropped. The weight factor defined for the interface determines how frequently packets are dropped.
WRED chooses the packets to drop after considering these factors in combination. The net result being that the highest priority and lowest bandwidth traffic is preserved. WRED differs from standard random early detection (RED) in that RED ignores IP precedence, and instead drops packets from all traffic flows, not selecting low precedence or high bandwidth flows. By selectively dropping packets before congestion occurs, WRED prevents an interface from getting flooded, necessitating a large number of dropped packets. This increases the overall bandwidth usage for the interface
