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When troubleshooting duplex mismatches, which two are errors that are typically seen on the half duplex end? (Choose two.)
A. excessive collisions
B. FCS errors
D. late collisions
Correct Answer: BC Section: (none) Explanation
You are using VTP (version 2) in your network to transport VLAN information between switches. When adding a switch to the network (that has been used in the lab previously), you notice that a lot of the existing VLANs have been deleted or replaced with other names. What can you do to prevent this from happening in the future, without losing all VTP features that you are using today?
A. configure a hard-to-guess VTP domain name
B. use a hard-to-guess VTP password
C. use VTP transparent mode
D. implement VTP version 3
Correct Answer: D Section: (none)Explanation
Explanation: http://www.cisco.com/en/US/prod/collateral/switches/ps5718/ps708/solution_guide_c78_508010.ht ml
Which two combinations are valid PAgP configurations that will set up a PAgP channel? (Choose two.)
Correct Answer: DF Section: (none)Explanation
Spanning Tree Protocol IEEE 802.1 s defines the ability to deploy which of these?
A. one global STP instance for all VLANs
B. one STP instance for each VLAN
C. one STP instance per set of VLANs
D. one STP instance per set of bridges
Correct Answer: C Section: (none) Explanation
The IEEE 802.1s standard is the Multiple Spanning Tree (MST). With MST, you can group VLANs and run
one instance of Spanning Tree for a group of VLANs. Other STP types:
Common Spanning Tree (CST), which is defined with IEEE 802.1Q, defines one spanning tree instance for
Rapid Spanning Tree (RSTP), which is defined with 802.1w, is used to speed up STP convergence. Switch
ports exchange an explicit handshake when they transition to forwarding.
Which two of these are used in the selection of a root bridge in a network utilizing Spanning Tree Protocol IEEE 802.1 D? (Choose two.)
A. Designated Root Cost
B. bridge ID priority
C. max age
D. bridge ID MAC address
E. Designated Root Priority
F. forward delay
Correct Answer: BD Section: (none) Explanation
Explanation: Explanation The root bridge of the spanning tree is the bridge with the smallest (lowest) bridge ID. Each bridge has a unique identifier (ID) and a configurable priority number; the bridge ID contains both numbers. To compare two bridge IDs, the priority is compared first. If two bridges have equal priority, then the MAC addresses are compared. For example, if switches A (MAC=0200.0000.1111) and B (MAC=0200.0000.2222) both have a priority of 10, then switch A
will be selected as the root bridge. If the network administrators would like switch B to become the root bridge, they must set its priority to be less than 10.
If a port configured with STP loop guard stops receiving BPDUs, the port will be put into which state?
A. learning state
B. listening state
C. forwarding state
D. loop-inconsistent state
Correct Answer: D Section: (none)Explanation
STP Loop Guard
The STP loop guard feature provides additional protection against Layer 2 forwarding loops (STP loops).
An STP loop is created when an STP blocking port in a redundant topology erroneously transitions to the
forwarding state. This usually happens because one of the ports of a physically redundant topology (not
necessarily the STP blocking port) no longer receives STP BPDUs. In its operation, STP relies on
continuous reception or transmission of BPDUs based on the port role. The designated port transmits
BPDUs, and the non-designated port receives BPDUs.
When one of the ports in a physically redundant topology no longer receives BPDUs, the STP conceives
that the topology is loop free. Eventually, the blocking port from the alternate or backup port becomes
designated and moves to a forwarding state. This situation creates a loop. The loop guard feature makes
additional checks. If BPDUs are not received on a non-designated port, and loop guard is enabled, that
port is moved into the STP loop-inconsistent blocking state, instead of the listening / learning / forwarding
state. Without the loop guard feature, the port assumes the designated port role. The port moves to the
STP forwarding state and creates a loop. When the loop guard blocks an inconsistent port, this message is
CatOS%SPANTREE-2-LOOPGUARDBLOCK: No BPDUs were received on port 3/2 in vlan 3.
Moved to loop-inconsistent state.
Cisco IOS%SPANTREE-2-LOOPGUARD_BLOCK: Loop guard blocking port FastEthernet0/24 on
Once the BPDU is received on a port in a loop-inconsistent STP state, the port transitions into another STP
state. According to the received BPDU, this means that the recovery is automatic
and intervention is not necessary. After recovery, this message is logged: CatOS%SPANTREE-2-LOOPGUARDUNBLOCK: port 3/2 restored in vlan 3. Cisco IOS%SPANTREE-2-LOOPGUARD_UNBLOCK: Loop guard unblocking port FastEthernet0/24 on VLAN0050.
What is the purpose of the STP PortFast BPDU guard feature?
A. enforce the placement of the root bridge in the network
B. ensure that a port is transitioned to a forwarding state quickly if a BPDU is received
C. enforce the borders of an STP domain
D. ensure that any BPDUs received are forwarded into the STP domain
Correct Answer: C Section: (none)Explanation
Explanation: Explanation STP configures meshed topology into a loop-free, tree-like topology. When the link on a bridge port goes up, STP calculation occurs on that port. The result of the calculation is the transition of the port into forwarding or blocking state. The result depends on the position of the port in the network and the STP parameters. This calculation and transition period usually takes about 30 to 50 seconds. At that time, no user data pass via the port. Some user applications can time out during the period.
In order to allow immediate transition of the port into forwarding state, enable the STP PortFast feature.
PortFast immediately transitions the port into STP forwarding mode upon linkup. The port still participates in STP. So if the port is to be a part of the loop, the port eventually transitions into STP blocking mode.
As long as the port participates in STP, some device can assume the root bridge function and affect active STP topology. To assume the root bridge function, the device would be attached to the port and would run STP with a lower bridge priority than that of the current root bridge. If another device assumes the root bridge function in this way, it renders the network suboptimal. This is a simple form of a denial of service (DoS) attack on the network. The temporary introduction and subsequent removal of STP devices with low
(0) bridge priority cause a
permanent STP recalculation.
The STP PortFast BPDU guard enhancement allows network designers to enforce the STP domain
borders and keep the active topology predictable. The devices behind the ports that have STP PortFast
enabled are not able to influence the STP topology. At the reception of BPDUs, the BPDU guard operation
disables the port that has PortFast configured. The BPDU guard transitions the port into errdisable state,
and a message appears on the console. This message is an example:
2000 May 12 15:13:32 %SPANTREE-2-RX_PORTFAST:Received BPDU on PortFast enable port.
2000 May 12 15:13:32 %PAGP-5-PORTFROMSTP:Port 2/1 left bridge port 2/1
When STP UplinkFast is enabled on a switch utilizing the default bridge priority, what will the new bridge priority be changed to?
C. 49152 D. 65535
Correct Answer: C Section: (none) Explanation
Explanation: Explanation The STP UplinkFast is used to fast switchover to alternate ports when the root port fails. When STP UplinkFast is enabled on a switch utilizing the default bridge priority (32768), the new bridge priority will be changed to 49152. The reason for the priority being raised is to prevent the switch from becoming the root (recall that lower bridge priority is preferred). To enable UplinkFast feature, use the “set spantree uplinkfast enable” in privileged mode The set spantree uplinkfast enable command has the following results: Changes the bridge priority to 49152 for all VLANs (allowed VLANs). Increases the path cost and portvlancost of all ports to a value greater than 3000. On detecting the failure of a root port, an instant cutover occurs to an alternate port selected by
Spanning Tree Protocol (without using this feature, the network will need about 30 seconds to re- establish the connection.
Which of these best describes the actions taken when a VTP message is received on a switch configured with the VTP mode “transparent”?
A. VTP updates are ignored and forwarded out all ports.
B. VTP updates are ignored and forwarded out trunks only.
C. VTP updates are made to the VLAN database and are forwarded out trunks only.
D. VTP updates are ignored and are not forwarded.
Correct Answer: B Section: (none)Explanation
Explanation: Explanation You can configure a switch to operate in any one of these VTP modes: Server–In VTP server mode, you can create, modify, and delete VLANs and specify other configuration parameters, such as VTP version and VTP pruning, for the entire VTP domain. VTP servers advertise their VLAN configuration to other switches in the same VTP domain and synchronize their VLAN configuration with other switches based on advertisements received over trunk links. VTP server is the default mode.
Client–VTP clients behave the same way as VTP servers, but you cannot create, change, or delete VLANs on a VTP client.
Transparent–VTP transparent switches do not participate in VTP. A VTP transparent switch does not advertise its VLAN configuration and does not synchronize its VLAN configuration based on received advertisements, but transparent switches do forward VTP advertisements that they receive out their trunk ports in VTP Version 2. Off (configurable only in CatOS switches)–In the three described modes, VTP advertisements are received and transmitted as soon as the switch enters the management domain state. In the VTP off mode, switches behave the same as in VTP transparent mode with the exception that VTP advertisements are not forwarded.
VTP V2 VTP V2 is not much different than VTP V1. The major difference is that VTP V2 introduces support for Token Ring VLANs. If you use Token Ring VLANs, you must enable VTP V2. Otherwise, there
is no reason to use VTP V2. Changing the VTP version from 1 to 2 will not cause a switch to reload. VTP Password If you configure a password for VTP, you must configure the password on all switches in the VTP domain. The password must be the same password on all those switches. The VTP password that you configure is translated by algorithm into a 16-byte word (MD5 value) that is carried in all summary-advertisement VTP packets.
VTP Pruning VTP ensures that all switches in the VTP domain are aware of all VLANs. However, there are occasions when VTP can create unnecessary traffic. All unknown unicasts and broadcasts in a VLAN are flooded over the entire VLAN. All switches in the network receive all broadcasts, even in situations in which few users are connected in that VLAN. VTP pruning is a feature that you use in order to eliminate or prune this unnecessary traffic.
The classic Spanning Tree Protocol (802.1 D 1998) uses which sequence of variables to determine the best received BPDU?
A. 1) lowest root bridge id, 2) lowest sender bridge id, 3) lowest port id, 4) lowest root path cost
B. 1) lowest root path cost, 2) lowest root bridge id, 3) lowest sender bridge id, 4) lowest sender port id
C. 1) lowest root bridge id, 2) lowest sender bridge id, 3) lowest root path cost 4) lowest sender port id
D. 1) lowest root bridge id, 2) lowest root path cost, 3) lowest sender bridge id, 4) lowest sender port id
Correct Answer: D Section: (none) Explanation
Explanation: Configuration bridge protocol data units (BPDUs) are sent between switches for each port. Switches use a fourstep process to save a copy of the best BPDU seen on every port. When a port receives a better BPDU, it stops sending them. If the BPDUs stop arriving for 20 seconds (default), it begins sending them again.
Step 1 Lowest Root Bridge ID (BID) Step 2 Lowest Path cost to Root Bridge Step 3 Lowest Sender BID Step 4 Lowest Port ID
Reference Cisco General Networking Theory Quick Reference Sheets
Loop guard and UniDireclional Link Detection both protect against Layer 2 STP loops. In which two ways does loop guard differ from UDLD in loop detection and prevention? (Choose two.)
A. Loop guard can be used with root guard simultaneously on the same port on the same VLAN while UDLD cannot.
B. UDLD protects against STP failures caused by cabling problems that create one-way links.
C. Loop guard detects and protects against duplicate packets being received and transmitted on different ports.
D. UDLD protects against unidirectional cabling problems on copper and fiber media.
E. Loop guard protects against STP failures caused by problems that result in the loss of BPDUs from a designated switch port.
Correct Answer: BE Section: (none)Explanation
Answers B, D, & E are all correct. However, as UDLD is only enabled on Fiber media by default I have
selected B instead of D as you will have to manually configure UDLD if you want it to work on copper media.
The Cisco-proprietary UDLD protocol allows devices connected through fiber-optic or copper (for example, Category 5 cabling) Ethernet cables connected to LAN ports to monitor the physical configuration of the cables and detect when a unidirectional link exists. When a unidirectional link is detected, UDLD shuts down the affected LAN port and alerts the user. Unidirectional links can cause a variety of problems, including spanning tree topology loops. UDLD is a Layer 2 protocol that works with the Layer 1 protocols to determine the physical status of a link. At Layer 1, autonegotiation takes care of physical signaling and fault detection. UDLD performs tasks that autonegotiation cannot perform, such as detecting the identities of neighbors and shutting down misconnected LAN ports. When you enable both autonegotiation and UDLD, Layer 1 and Layer 2 detections work together to prevent physical and logical unidirectional connections and the malfunctioning of other protocols. Based on the various design considerations, you can choose either UDLD or the loop guard feature. In regards to STP, the most noticeable difference between the two features is the absence of protection in UDLD against STP failures caused by problems in software. As a result, the designated switch does not send BPDUs. However, this type of failure is (by an order of magnitude) more rare than failures caused by unidirectional links. In return, UDLD might be more flexible in the case of unidirectional links on EtherChannel. In this case, UDLD disables only failed links, and the channel should remain functional with the links that remain. In such a failure, the loop guard puts it into loop-inconsistent state in order to block the whole channel.
Additionally, loop guard does not work on shared links or in situations where the link has been unidirectional since the link-up. In the last case, the port never receives BPDU and becomes designated. Because this behavior could be normal, this particular case is not covered by loop guard. UDLD provides protection against such a scenario.
Which standard supports multiple instances of spanning tree?
A. 802.1 D
D. 802.1 z
Correct Answer: B Section: (none) Explanation
Explanation: Multiple Spanning Tree Protocol (MSTP) was first specified in IEEE 802.1s and is standardized in IEEE 802.1Q. MSTP enables multiple VLANs to be mapped to the same spanning-tree instance, reducing the number of spanning-tree instances needed to support a large number of VLANs. MSTP provides multiple forwarding paths for data traffic and enables load balancing. It improves the fault tolerance of the network because a failure in one instance, or forwarding path, does not affect other instances
Spanning Tree Protocol calculates path cost based on which of these?
A. interface bandwidth
B. interface delay
C. interface bandwidth and delay
D. hop count
E. bridge priority
Correct Answer: A Section: (none)Explanation
STP calculates the path cost based on the media speed (bandwidth) of the links between switches and the
port cost of each port forwarding frame. Spanning tree selects the root port based on the path cost. The
port with the lowest path cost to the root bridge becomes the root port. The root port is always in the
If the speed/duplex of the port is changed, spanning tree recalculates the path cost automatically. A
change in the path cost can change the spanning tree topology.
Data rate and STP path cost
The table below shows the default cost of an interface for a given data rate.
Why does RSTP have a better convergence time than 802.1D?
A. it is newer
B. it has smaller timers
C. it has less overhead
D. it is not timer-based
Correct Answer: D Section: (none) Explanation
RSTP identifies certain links as point to point. When a point-to-point link fails, the alternate link can
transition to the forwarding state.
Although STP provides basic loop prevention functionality, it does not provide fast network convergence
when there are topology changes. STP’s process to determine network state transitions is slower than
RSTP’s because it is timer-based. A device must reinitialize every time a
topology change occurs. The device must start in the listening state and transition to the learning state and
eventually to a forwarding or blocking state.
When default values are used for the maximum age (20 seconds) and forward delay (15 seconds), it takes
50 seconds for the device to converge. RSTP converges faster because it uses a handshake mechanism
based on point-to-point links instead of the timer-based process used by STP.
An RSTP domain running switch has the following components: A root port, which is the “best path” to the root device. A designated port, indicating that the switch is the designated bridge for the other switch connecting to this port. An alternate port, which provides an alternate root port. A backup port, which provides an alternate designated port. Port assignments change through messages exchanged throughout the domain. An RSTP device generates configuration messages once every hello time interval. If an RSTP device does not receive a configuration message from its neighbor after an interval of three hello times, it determines it has lost connection with that neighbor. When a root port or a designated port fails on a device, the device generates a configuration message with the proposal bit set. Once its neighbor device receives this message, it verifies that this configuration message is better than the one saved for that port and then it starts a synchronizing operation to ensure that all of its ports are in sync with the new information.
Similar waves of proposal agreement handshake messages propagate toward the leaves of the network, restoring the connectivity very quickly after a topology change (in a well-designed network that uses RSTP, network convergence can take as little as 0.5 seconds). If a device does not receive an agreement to a proposal message it has sent, it returns to the original IEEE 802.D convention. RSTP was originally defined in the IEEE 802.1w draft specification and later incorporated into the IEEE 802.1D-2004 specification.
Under which two circumstances would an RSTP bridge flush its CAM table? (Choose two.)
A. upon a port state change
B. upon receiving a topology change notification
C. when transitioning from discarding to forwarding
D. when transitioning from forwarding to discarding
E. only when changing from listening to discarding
F. when CAM resources have been completely used up
Correct Answer: BC Section: (none)Explanation
Explanation: Explanation First, the goal of RSTP is fast re-convergence. Since ports are assumed to transition to forwarding relatively fast, simply increasing MAC address aging speed is not enough. Thus, when a topology change is detected, RSTP instructs the bridge to flush all MAC address table entries. With Ethernet, this process results in unconstrained flooding until the moment MAC addresses are re- learned. The bridge detecting a topology change sets the TC (Topology Change) bit in all outgoing BPDUs and starts sending BPDUs with the TC bit set upstream through the root port as well. This marking lasts for TCWhile=2xHelloTime seconds and allows the detecting bridge the start the flooding process.
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.
Correct Answer: C Section: (none) 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
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.
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