In an 802.1Q-in-Q tunneling network the service provider uses an 802.1Q frame to encapsulate the 802.1Q frame from the enterprises edge switch. A unique VLAN is used to support each attached enterprise transported within the provider's network. Each provider VLAN can reach two or more sites that enable topologies such as point-to-multipoint to be implemented by the enterprises. 802.1Q-in-Q allows different enterprises to maintain their own VLAN ID assignments.

Management traffic, such as BPDUs, will not be transported through the tunnel. This creates some Spanning Tree Protocol (STP) issues. Groups of switches that exist on each end of the tunnel will elect their own root and form their own tree.

802.1Q-in-Q tunneling is a Cisco-developed solution that allows service providers the ability to add transparency between the attached enterprises.

For more information, go to the Cisco Documentation Web Site, Configuring 802.1Q and Layer 2 Protocol Tunneling, Understanding 802.1Q Tunneling.

2. Answer D is correct. Adding a switch configured in VTP server mode and with a revision number higher than the current number in the network could cause the communication problem in the scenario. If the new switch was configured in server mode and the revision number was higher than the current revision number on existing switches, it could cause the rest of the switches to update with the information contained in that new advertisement.

VTP advertisements are flooded throughout the management domain every five minutes or whenever there is a change. These advertisements originate from a switch that is in server mode and are propagated by switches that are in either client or transparent mode. Before a client or another server accepts or incorporates the information sent in the advertisement, it checks the domain name and password (if defined) against its own configuration. Next the revision number is checked to see if it is higher than the last value stored in the receiving switch. If the revision number is higher, the receiving switch will overwrite its VLAN database with the information in the advertisement.

A VTP switch in transparent switch will receive and forward VTP advertisements. It will not use the contents of the advertisement to synchronize with its own VLAN database.

For more information, go to the Cisco Documentation Web Site, Configuring Virtual LANs.

3. Answers C and D are correct. Differentiated Services (DiffServ) gives statistical preferences to certain types of traffic using queuing and congestion avoidance. Traffic is identified and classified either at the link, IP, or at the application levels. Using this identification certain queuing methods are implemented on a per-hop basis.

Integrated Services uses Resource Reservation Protocol (RSVP) and queuing to reserve resources for specific types of traffic. Each type of traffic is guaranteed a certain amount of resources.

RSVP provides an admission-control mechanism whereby a host application makes a request for a specific type of service before it sends data. The RSVP request informs the network of its traffic profile and requests a particular kind of service to fit the profile. Once the request is confirmed, the network commits to meet the requirements of the application as long as the data remains within its described traffic profile. This end-to-end application registration process allows the management of traffic on a per-flow basis. Each traffic flow must go through this admission-control process.

Best-effort Services provides basic connectivity with no guarantees. It uses first-in, first-out (FIFO) queues, which do not differentiate between flows or traffic types.

For more information, go to the Cisco Documentation Web Site, Cisco IOS Software: Quality-of-Service The Differentiated Services Model (DiffServ).

4. Answer C is correct. The standby priority command affects the preference for the router that will become the active Hot Standby Router Protocol (HSRP) router. The HSRP router with the highest priority or address will become the active router if there is no router already operating as the active router. The standby priority command sets the router priority. Standby preempt enables a router to initiate a coup attempt in order to become the active router.

You can reconfigure the standby priority of the HRSP router with the standby priority interface configuration command. The syntax for this command is shown below.

Switch(config-if)# standby   group-number   priority   priority-value

The priority value argument helps determine which router will become the forwarding router for the group. The higher the decimal value the higher the priority will be. The default is 100 and the range is 0 to 255.

The router with the highest priority will become the active router and will begin forwarding. In the event of a tie of configured priority values (e.g. if the priority values are left at the default), the highest priority tiebreaker is the highest configured IP address.

The six HSRP states are listed below:

• Initial state - All routers start in this state.
• Learn state - The router is in the learn state when it has not communicated with the active router. It does not know what the active router is and does not know the IP address of the virtual router.
• Listen state - Once the router knows the virtual IP address, it enters the listen state. It is listening for the active or standby router.
• Speak state - After a router learns the IP address of the virtual router, it enters the speak state. It participates in the active and standby router election. It sends hello messages to the active router. This is the router state if not either the active or standby router.
• Standby state - When the active router has been elected, the second router enters the standby state. This is the standby router and it will become the active router if the active router fails.
• Active state - The router is in active state when it is forwarding packets. Packets are sent to it via the virtual IP address.

For more information, go to the Cisco Documentation Web Site, Configuring HSRP, Enabling HSRP.

5. Answer D is correct. The link will not be part of an EtherChannel as negotiated using Port Aggregation Protocol (PAgP) when adding the entry of channel-group 3 mode auto is added to the switch interface configuration on both sides of a link.

The command channel-group 3 mode auto is used to set the state of the interface to one that will use PAgP and be placed in a passive negotiation state. One side of the link needs to be set into an active negotiation state such as on or desirable.

Port Aggregation Protocol (PAgP) is a Cisco proprietary protocol. Link Aggregation Control Protocol (LACP) is defined in IEEE 802.3ad. Either LACP or PAgP is used to expedite the creation of EtherChannels. An EtherChannel is a Layer 2 grouping of like physical interfaces. Up to eight interfaces with the same physical configuration (speed and duplex) can be associated together.

The PAgP mode can be set to auto, desirable, or on. The default setting is auto. For the EtherChannel to become active, the mode setting must be compatible with the other side of the link.

Compatible PAgP settings:

• On - The EtherChannel will become active only if the other side of the link is set to on.
• Auto - The EtherChannel will negotiate with the other side of the link but will not initiate the negotiations. To become an EtherChannel, the other side must be set to desirable. This is the default setting.
• Desirable - The EtherChannel will negotiate with the other side of the link and will initiate the negotiations. To become an EtherChannel other side must be set to either auto or desirable.

The LACP mode can be set to on, off, passive, or active. For the EtherChannel to become active, the mode setting must be compatible with the other side of the link.

Compatible LACP settings:

• On - The EtherChannel will become active only if the other side of the link is set to on.
• Off - An EtherChannel will not be formed.
• Passive - The EtherChannel will negotiate with the other side of the link but will not initiate the negotiations. To become an EtherChannel, the other side must be set to active. This is the default setting.
• Active - The EtherChannel will negotiate with the other side of the link and will initiate the negotiations. To become an EtherChannel, the other side must be set to either passive or active.

For more information, go to the Cisco Documentation Web Site, Configuring EtherChannels, Understanding How EtherChannels Work, Understanding How EtherChannels Are Configured, EtherChannel Configuration Overview.

6. Answer B is correct. The command SwitchA(config-if)# wrr-queue cos-map 1 2 3 results in CoS values 2 and 3 being mapped to Weighted Round-Robin (WRR) queue 1.

Weighted Round-Robin (WRR) queuing is used on Catalyst Layer 3 switches egress ports. It is based on settings of IP Precedence (CoS) bits, which are used to place the packet into one of four queues. Each of the four queues has associated with it a priority that implies its delay priority. In WRR each queue has its own weight, and the higher the weight, the higher the bandwidth is of the queue. For example, if the queues have the weights 1, 2, 4, and 8, the queue with a weight of 4 is able to send four packets before servicing the next queue which is able to send eight packets, and so on.

The following graphic displays a table of the default CoS values to Queue mappings.

[Click on image for larger view.]

CoS values to queue mappings

The mappings can be modified with the following command:

SwitchA(config-if)# wrr-queue cos-map [ queue-id ] [ cos-value-1cos-value-2 ]

The weights can be modified with this command:

SwitchA(config-if)# wrr-queue bandwidth [ Q1-weight ] [ Q2-weight ] [ Q3-weight ] [ Q4-weight ]

For more information, go to the Cisco Documentation Web Site, Configuring QoS.

7. Answer A is correct. When the instance 3 vlan 7 command is issued, virtual local area network (VLAN) 7 is mapped to Multiple Spanning Tree Protocol (MST or MSTP) instance 3. MST, which is defined by the 802.1s standard, maps a distinct group of VLANs to one STP instance. Multiple STP instances can be used with MST. However, each instance must support a different group of VLANs because each VLAN can only be mapped to one instance. To map one or more VLANs to an MST instance, issue the instance   instance-ID   vlan   vlan-list command, where instance-ID is the number of the MST instance and vlan-list is the VLAN or VLANs that should be mapped to the instance. For example, the command instance 1 vlan 14-16,99 maps VLANs 14 through 16 and VLAN 99 to MST instance 1.

The instance 3 vlan 7 command will not map VLAN 7 to switch port 3. The instance vlan command cannot be used to map multiple instances to a single VLAN; each VLAN can only be mapped to one instance. When the instance 3 vlan 7 command is issued, only a single VLAN will be mapped to MST instance 3.

For more information, go to the Cisco Documentation Web Site, Configuring MSTP, Configuring MSTP Features, Specifying the MST Region Configuration and Enabling MSTP.

8. Answers A and E are correct. Cisco Express Forwarding (CEF) is a Layer 3 switching technology based on information contained in the Forwarding Information Base (FIB) and the Adjacency table.

The FIB is conceptually equivalent to a routing table in that it contains information used in the packet-forwarding decision. The adjacency table contains information about the adjacent route processors. The adjacency table contains the MAC information for the next-hop addresses for all FIB entries. A device is considered adjacent if it is reachable over a single Layer 2 connection. It is stored in DRAM.

CEF is a topology-based Layer 3 switching technology that is enabled by default on the latest Cisco products. The FIB table stores IP destination prefixes from the most-specific to the least-specific entry in the TCAM (Ternary Content Addressable Memory). The content of the FIB table is similar to the routing table and contains the forwarding information similar to what is found in the IP routing table. A single FIB entry can point to up to six adjacencies. When changes are made to the IP routing table, the FIB table is also updated.

The Layer 3 processor engine builds the FIB and adjacency tables in software. That information is distributed from the control-plane hardware to the data-plane hardware Application Specific Integrated Circuits (ASICs) at the port or line card. This enhances the Layer 3 forwarding operation by moving it from the software-based engine to the ASICs. Of course there are exception packets that are still software-processed, such as non-conforming protocols and datalink encapsulations.

9. Answer C is correct. The show interfaces command on Switch C indicates that Switch C is not aware of any ports on the suspect VLAN and VLAN 120 is being pruned from Switch C.

VTP pruning enhances network bandwidth usage by restricting unnecessary flooded traffic on trunk links. If a trunk link does not have devices in the VLAN attached, flooded traffic on that VLAN is blocked. VTP pruning can reduce broadcasts, multicasts, unknown traffic, and flooded unicast packets. VTP pruning is implemented on switches in VTP server mode.

Multicast and unicast traffic are not blocked for the VLANs that are not being pruned.

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