2013 Latest Cisco 350-001 Exam Section 4: Data Link Layer (12 Questions)
QUESTION NO: 1
A workstation has been connected to the TestKing LAN using a Category 5e cable.
The workstation can connect to the rest of the network through the switch (i.e has
full connectivity), but is suffering from much slower than expected performance.
Looking at the interface statistics on the switch, many “runts” are being detected.
Using software to read the counters on the workstation NIC, many FCS and
alignment errors are occurring. What is the most likely cause of these errors?
A. Bad Network Interface Card on the workstation
B. Bad cable between the workstation and the switch
C. The port has incorrectly been configured as an 802.1q trunk port
D. Mismatched speed settings between the workstation and the switch
E. Mismatched duplex setting between the workstation and the switch
F. None of the above.
In half-duplex environments, it is possible for both the switch and the connected device to sense the wire and transmit at exactly the same time and result in a collision. Collisions can cause runts, FCS, and alignment errors, caused when the frame is not completely copied to the wire, which results in fragmented frames. When operating at full-duplex, FCS, cyclic redundancy checks (CRC), alignment errors, and runt counters should be minimal. If the link is operating at full-duplex, the collision counter is not active. If the FCS, CRC, alignment, or runt counters are incrementing, check for a duplex mismatch. Duplex mismatch is a situation in which the switch is operating at full-duplex and the connected device is operating at half-duplex, or the other way around. The result of a duplex mismatch is extremely slow performance, intermittent connectivity, and loss of connection. The following describes the errors and their meanings: Alignment Errors: Alignment errors are a count of the number of frames received that do not end with an even number of octets and have a bad CRC. FCS Errors: FCS error count is the number of frames that were transmitted or received with a bad checksum (CRC value) in the Ethernet frame. These frames are dropped and not propagated onto other ports. Runts: These are frames smaller than 64 bytes with a bad FCS value.
QUESTION NO: 2
Ethernet LAN’s are used throughout the TestKing network. How much data can be carried in a single standard Ethernet frame?
A. Up to 4096 bytes
B. No limit
C. Up to 1500 bytes
D. Up to 1518 bytes
E. Up to 4400 bytes
A standard Ethernet frame MTU is 1500 bytes. The MTU size or packet size refers only to Ethernet payload. Ethernet frame size refers to the whole Ethernet frame, including the header and the trailer. This question asked for the amount of data that can be carried, which is the payload. Note: Preamble is not calculated in frame size so DA (6 bytes) SA (6 bytes) Type (2 bytes) data + pad (1500 bytes) FCS (4bytes) = a total of 1518 Incorrect Answers:
D. A standard Ethernet frame MTU is 1500 bytes. This does not include the Ethernet header and Cyclic Redundancy Check (CRC) trailer, which is 18 bytes in length, to make the total Ethernet frame size of 1518. So the total size of an Ethernet packet can be as large as 1518 bytes, but the maximum payload is only 1500 bytes.
QUESTION NO: 3
Which of the following will cause a switch port to go into the err-disable state? (Choose all that apply)
A. Duplex mismatch.
B. Unidirectional Link Detection.
C. AN incorrect VTP domain name is configured on the switch.
D. Ethernet channeling is configured on the port.
E. VLANs on the trunk were not matching on both sides.
Answer: A, B
If the interface status is err-disable in the output of the “show interface status” command, refer to the common reasons below. When a port is error-disabled, the LED associated with the port on the front panel will be solid orange.
The reasons for the interface going into “err-disable” state are varied. Some of the possibilities include the following:
duplex mismatch (A is correct)
Bridge Protocol Data Unit (BPDU) Guard violation
UniDirectional Link Detection (UDLD) condition (B is correct)
Port Aggregation Protocol (PAgP) flap
Layer 2 Tunneling Protocol (L2TP) Guard
DHCP snooping rate-limiting
EtherChannel guard detects a misconfigured EtherChannel Incorrect Answers:
C. The VTP configuration relates to the switch as a whole and has no impact on individual ports.
D. Although the port could be in the err-disable state if the Ethernet channeling feature is not set correctly on both ends, simply configuring channeling will not cause the port to go into this state by itself.
E. VLAN mismatches have no bearing on port status.
QUESTION NO: 4
802.1Q trunking uses which Ethertype to identify itself?
E. None of the above
The IEEE 802.1Q specification defines the Ethertype field to be 8100 in the presence of a VLAN ID. The entire packet format is shown below:
QUESTION NO: 5
In an 802.3 LAN, PAUSE frames are used for inhibiting data transmissions for a period of time. Which MAC address does this PAUSE mechanism use in order to accomplish this?
The globally assigned 48-bit multicast address 01-80-C2-00-00-01 has been reserved for use in MAC Control PAUSE frames for inhibiting transmission of data frames from a DTE in a full duplex mode IEEE 802.3 LAN. IEEE 802.1D-conformant bridges will not forward frames sent to this multicast destination address, regardless of the state of the bridge’s ports, or whether or not the bridge implements the MAC Control sub-layer. Reference:http://www.techfest.com/networking/lan/ethernet3.htm
QUESTION NO: 6
A single end station failure can be prevented from disrupting the Spanning Tree algorithm in a LAN according to the 802.1D specification. 802.1D recommends preventing this by:
A. Clearing the Topology Change flag.
B. Re-setting the Topology Change flag to one (1).
C. Configuring the Bridge Forward Delay to less than 1/2 of the Bridge Maxage.
D. Disabling the 801.1D Change Detection Parameter.
E. Disabling the Topology Change Notifications.
The intent of the 802.1D standard is that the detectable failure of a MAC should cause the Bridge Port supported by that MAC to enter the Disabled state. A transition to the Disabled Port state causes the Bridge to initiate a topology change notification, unless, for the Port concerned, topology change detection has been explicitly disabled. Disabling this change detection will result in the prevention of the MAC failure to disrupt the Spanning Tree.
QUESTION NO: 7 What trunking protocol uses an internal tagging mechanism that inserts a 4 byte tag field in the original Ethernet frame?
802.1Q is the IEEE standard for tagging frames on a trunk and supports up to 4096 VLANs. IEEE 802.1q uses an internal tagging mechanism which inserts a 4 byte tag field in the original Ethernet frame itself between the Source Address and Type/Length fields. Since the frame is altered, the trunking device re-computes the frame check sequence (FCS) on the modified frame.
A. In ISL, a 26-byte header that contains a 10-bit VLAN ID is inserted at the beginning of
the Ethernet frame.
B, C, E. These are not trunk encapsulation options.
QUESTION NO: 8
Which of the following are true regarding Unidirectional Link Detection? (Choose all that apply.)
A. UDLD uses auto-negotiation to take care of physical signaling and fault detection.
B. Both devices on the link need to support Unidirectional Link Detection.
C. It works by exchanging protocol packets between the neighboring devices.
D. It performs tasks that autonegotiation cannot perform.
E. UDLD is a layer one protocol.
Answer: A, B, C, and D
In order to detect the unidirectional links before the forwarding loop is created, Cisco designed and implemented the UDLD protocol.
UDLD is a Layer 2 (L2) protocol that works with the Layer 1 (L1) mechanisms to determine the physical status of a link. At L1, auto-negotiation takes care of physical signaling and fault detection. UDLD performs tasks that auto-negotiation cannot perform, such as detecting the identities of neighbors and shutting down misconnected ports. When you enable both auto-negotiation and UDLD, L1 and L2 detections work together to prevent physical and logical unidirectional connections and the malfunctioning of other protocols. UDLD works by exchanging protocol packets between the neighboring devices. In order for UDLD to work, both devices on the link must support UDLD and have it enabled on respective ports. Each switch port configured for UDLD will send UDLD protocol packets containing the port’s own device/port ID, and the neighbor’s device/port IDs seen by UDLD on that port. Neighboring ports should see their own device/port ID (echo) in the packets received from the other side. If the port does not see its own device/port ID in the incoming UDLD packets for a specific duration of time, the link is considered unidirectional.
E. UDLD is a Layer 2 (L2) protocol that works with the Layer 1 (L1) mechanisms to determine the physical status of a link. Reference: http://www.cisco.com/warp/public/473/77.html
QUESTION NO: 9 On a TestKing bridge running the rapid spanning tree protocol, which port will send a BPDU with the proposal flag?
A. Designated port in forwarding state
B. Designated port in non-forwarding state or the Root port in forwarding state
C. Root port in blocking state
D. Alternate port
E. None of the above
Explanation: Proposal/Agreement Sequence
When a port has been selected by the STA to become a designated port, 802.1d still waits twice
When a designated port is in a discarding or learning state (and only in this case), it sets the proposal bit on the BPDUs it sends out. This is what happens for port p0 of the root bridge, as shown in Step 1 of the diagram above. Because Switch A receives superior information, it immediately knows that p1 is going to be its new root port. Switch A then starts a sync to ensure that all of its ports are in-sync with this new information. A port is in-sync if it meets either of the following criteria:
The port is in blocking state (which means discarding, in a stable topology).
The port is an edge port. In order to illustrate the effect of the sync mechanism on different kind of ports, suppose there exists an alternate port p2, a designated forwarding port p3, and an edge port p4 on Switch A. Notice that p2 and p4 already meet one of the criteria listed above. In order to be in sync (Step 2 of the diagram above), Switch A just needs to block port p3, assigning it the discarding state. Now that all of its ports are in sync, Switch A can now unblock its newly selected root port p1 and reply to the root by sending an agreement message (Step 3). This message is a copy of the proposal BPDU, with the agreement bit set instead of the proposal bit. This ensures that port p0 knows exactly to which proposal the agreement it receives corresponds to.
Once p0 receives that agreement, it can immediately transition to forwarding. This is Step 4 of the figure above. Notice that port p3 was left in a designated discarding state after the sync. In Step 4, that port is in the exact same situation as was port p0 during Step 1. It then starts proposing to its neighbor, attempting to quickly transition to forwarding.
The proposal agreement mechanism is very fast, as it does not rely on any timers. This wave of handshakes propagates quickly towards the edge of the network, and quickly restores connectivity after a change in the topology.
If a designated discarding port does not receive an agreement after having sent a proposal, it slowly transitions to the forwarding state, falling back to the traditional 802.1d listening-learning sequence. This could happen for instance if the remote bridge doesn’t understand RSTP BPDUs, or if the remote bridge’s port is blocking.
Cisco introduced an enhancement to the sync mechanism that allows a bridge to put only its former root port in the discarding state when syncing. Detailing the way this mechanism works is beyond the scope of this document. However, one can safely assume that it will be invoked in most common reconvergence cases. The scenario described in the Convergence with 802.1w section of this document now becomes extremely efficient, as only the ports on the path to the final blocked port are temporarily confused.
QUESTION NO: 10 A new TestKing switch is running the rapid spanning tree protocol (RSTP). Upon a topology change, what happens to dynamic entries in the L2 forwarding table?
A. All entries are removed (purged)
B. Aging timer it set to 15 seconds, so idle entries age out
C. Only entries behind port where TC was received are removed
D. All entries are removed except for entries behind edge ports
E. All entries are removed except for those behind edge ports and the port where the TC notification was received
F. None of the above
Explanation: Topology Change Detection
In RSTP, only non-edge ports moving to the forwarding state cause a topology change. This means that a loss of connectivity is not considered as a topology change any more, contrarily to 802.1d (that is, a port that moves to blocking no longer generates a TC). When a RSTP bridge detects a topology change, the following happens:
It starts the TC While timer with a value equal to twice the hello time for all its non-edge designated ports and its root port if necessary.
It flushes the MAC addresses associated with all these ports.
Topology Change Propagation
When a bridge receives a BPDU with the TC bit set from a neighbor, the following happens:
It clears the MAC addresses learnt on all its ports except the one that received the topology change.
It starts the TC While timer and sends BPDUs with TC set on all its designated ports and root port (RSTP no longer uses the specific TCN BPDU, unless a legacy bridge needs to be notified).
QUESTION NO: 11 All of the TestKing LAN switches are running the Rapid Spanning Tree Protocol (RSTP). On a bridge running RSPT, BPDU information on the port will be aged:
A. After MaxAge time
B. 15 seconds
C. RSTP does not age out BPDU information on ports
D. After BPDU Age will reach MaxAge or after 3 hello times -which ever occurs first
E. After 6 seconds
On a given port, if hellos are not received for three consecutive times, protocol information can be immediately aged out (or if max_age expires). Because of the previously mentioned protocol modification, BPDUs are now used as a keep-alive mechanism between bridges. A bridge considers that it has lost connectivity to its direct neighboring root or designated bridge if it misses three BPDUs in a row. This fast aging of the information allows quick failure detection. If a bridge fails to receive BPDUs from a neighbor, it is certain that the connection to that neighbor has been lost, as opposed to 802.1d where the problem could have been anywhere on the path to the root.
QUESTION NO: 12 In RSTP (Rapid Spanning Tree Protocol) what is the port that provides an alternative path to the leaves of the Spanning Tree and what state is it in when it is not in the active topology?
A. Root port and listening
B. Designated port and learning
C. Backup port and discarding
D. Alternate port and forwarding
E. Alternate port and learning
The RSTP provides rapid convergence of the spanning tree by assigning port roles and by determining the active topology. The RSTP builds upon the IEEE 802.1D STP to select the switch with the highest switch priority (lowest numerical priority value) as the root switch. Then the RSTP assigns one of these port roles to individual ports:
Root port-provides the best path (lowest cost) when the switch forwards packets to the root switch.
Designated port-connects to the designated switch, which incurs the lowest path cost when forwarding packets from that LAN to the root switch. The port through which the designated switch is attached to the LAN is called the designated port.
Alternate port-offers an alternate path toward the root switch to that provided by the current root port.
Backup port-acts as a backup for the path provided by a designated port toward the leaves of the spanning tree. A backup port can exist only when two ports are connected together in a loopback by a point-to-point link or when a switch has two or more connections to a shared LAN segment.
Disabled port-has no role within the operation of the spanning tree. A port with the root or a designated port role is included in the active topology. A port with the alternate or backup port role is excluded from the active topology. In a stable topology with consistent port roles throughout the network, the RSTP ensures that every root port and designated port immediately transition to the forwarding state while all alternate and backup ports are always in the discarding state (equivalent to blocking in 802.1D).
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