350-001 Exam Sample Question

Question No:1

Which of these correctly identifies a difference between the way BPDUs are handled by 802.1w and 802.1 D?

A. 802.1 D bridges do not relay

B. 802.1w bridges do not relay BPDUs

C. 802.1D bridges only relay BPDUs received from the root

D. 802.1w bridges only relay BPDUs received from the root.

Answer: C

Explanation:

A bridge sends a BPDU frame using the unique MAC address of the port itself as a source address, and a destination address of the STP multicast address 01:80:C2:00:00:00.There are three types of BPDUs:Configuration BPDU (CBPDU), used for Spanning Tree computation Topology Change Notification (TCN) BPDU, used to announce changes in the network topology Topology Change Notification Acknowledgment (TCA) BPDU are Sent Every Hello-Time

BPDU are sent every hello-time,and not simply relayed anymore.With 802.1D,a non-root bridge only generates BPDUs when it receives one on the root port. In fact,a bridge relays BPDUs more than it actually generates them. This is not the case with 802.1w. A bridge now sends a BPDU with its current information every <hello-time> seconds (2 by default), even if it does not receive

any from the root bridge.

Reference

http://www.cisco.com/en/US/tech/tk389/tk621/technologies_white_paper09186a0080094cfa.shtml#topic4

QUESTION NO: 2

In Frame Relay, FECN messages indicating congestion are sent or received by which of

following?

A. Sent by the destination

B. Received by the sender

C. Received by the destination

D. Sent by the sender

Answer: C

Explanation:

Congestion control The Frame Relay network uses a simplified protocol at each switching node. It achieves simplicity by omitting link-by-link flow-control.As a result, the offered load has largely determined the performance of Frame Relay networks.When offered load is high,due to the bursts in some services,temporary overload at some Frame Relay nodes causes a collapse in network throughput.Therefore  frame-relay networks require some effective mechanisms to control the Congestion.Congestion control in frame-relay networks includes the following elements:Admission Control provides the principal mechanism used in Frame Relay to ensure the guarantee of resource requirement once accepted.It also serves generally to achieve high network performance.The network decides whether to accept a new connection request, based on the relation of the requested traffic descriptor and the network's residual capacity. The traffic descriptor consists of a set of parameters communicated to the switching nodes at call set-up time or at service-subscription time,and which characterizes the connection's statistical properties.The traffic descriptor consists of three elements:Committed Information Rate (CIR) -The average rate (in bit/s) at which the network guarantees to transfer information units over a measurement interval T.This T interval is defined as: T = Bc/CIR.Committed Burst Size (BC) - The maximum number of information units transmittable during the interval T. Excess Burst Size (BE) -The maximum number of uncommitted information units (inbits) that the network will attempt to carry during the interval.Once the network has established a connection,the edge node of the Frame Relay network must monitor the connection's traffic flow to ensure that the actual usage of network resources does not exceed this specification. Frame Relay defines some restrictions on the user's information rate.It allows the network to enforce the end user's information rate and discard information when the subscribed access rate is exceeded.Explicit congestion notification is proposed as the congestion avoidance policy.It tries to keep the network operating at its desired equilibrium point so that a certain Quality of Service (QoS) for the network can be met.To do so,special congestion control bits have been incorporated into the address field of the Frame Relay :FECN and BECN.The basic idea is to avoid data accumulation inside the network. FECN means Forward Explicit Congestion Notification.The FECN bit can be set to 1 to indicate that congestion was experienced in the direction of the frame transmission, so it informs the destination that congestion has occurred.BECN means Backwards Explicit Congestion Notification.The BECN bit can be set to 1 to indicate that congestion was experienced in the network in the direction opposite of the frame transmission,so it informs the sender that congestion has occurred.