CCNP Switching Q&A

What is the 20/80 rule of networking?

The 20/80 rule states that 20 percent of network traffic on a LAN segment will stay

on that segment. The remaining 80 percent must go across the network core, either to

enterprise servers or to the Internet.

In which OSI layer do devices in the distribution layer typically operate?

Layer 3 devices are typically used in the distribution layer.

What are the different Ethernet technologies and their associated IEEE standards?

Ethernet (10 Mbps, IEEE 802.3), Fast Ethernet (100 Mbps, IEEE 802.3u), and Gigabit

Ethernet (1000 Mbps, IEEE 802.3z)

What benefits result with switched Ethernet over shared Ethernet?

Switched Ethernet ports receive dedicated bandwidth, have a reduced collision domain,

and have increased performance due to segmentation or fewer users per port

In a campus network, where is Fast Ethernet typically used? Where is Gigabit Ethernet

typically used?

Fast Ethernet is typically used for links between access layer and distribution layer

devices and between end users and the access layer devices. Gigabit Ethernet is typically

used between all layers—between access and distribution layer devices, between

distribution and core layer devices, and for the links between core layer devices.

What must be done to a switch before Telnet access is allowed?

An IP address must be assigned to the management interface on the switch, and the

management interface must be assigned to a VLAN.

What switch command will set the enable-mode password on an IOS-based switch? A

CLI-based switch?

For IOS-based switches, use enable password level 15 password to set the enable

password. On CLI-based switches, use the set enablepass password command.

What is a trunk link?

A trunk link is a connection between two devices that transports traffic from multiple

VLANs. Each frame is identified with its source VLAN during its trip across the trunk

link.

How many VTP management domains can a Catalyst switch participate in? How many

VTP servers can a management domain have?

A switch can be a member of only one VTP management domain. A VTP domain must

have at least one server. There can be more than one server, for redundancy, but it is

recommended to have no more than two.

Which VLAN numbers are never eligible for VTP pruning? Why?

VLAN numbers 1 and 1001–1005 are ineligible for pruning. VLAN 1 is reserved as the

management VLAN, while VLANs 1002-1005 are reserved as the default FDDI and

Token Ring function VLANs. At press time, VLAN 1001 has no special purpose, but is

reserved and cannot be pruned.

What is the difference between these two trunking methods? How many bytes are added

to trunked frames for VLAN identification in each method?

ISL uses encapsulation and adds a 26-byte header and a 4-byte trailer. 802.1Q adds a 4-

byte tag field within existing frames, without encapsulation.

Two neighboring switch trunk ports are set to auto mode with ISL trunking mode. What

will the resulting trunk mode become?

Trunking will not be established at all. Both switches are in the passive auto state and are

each waiting to be asked to start the trunking mode. Instead, the link will remain an access

link on both switches.

What are the types of VTP messages or advertisements used by Catalyst switches? What

field in these messages determines if a switch should use and record VLAN data in the

messages?

The VTP message types are Advertisement Requests, Summary Advertisements, Subset

Advertisements, and VLAN Membership Advertisements (an extension to version 1 for

VTP pruning). The Configuration Revision Number is used to determine if the VTP data

is newer and should be used.

How is traffic distributed over an EtherChannel?

Traffic is distributed according to addresses contained in frames passing through the

switch—not according to port loads or equal distribution across the individual ports in a

bundle. Switches use an XOR computation of source, destination, or both addresses of

either MAC or IP, depending on the switch capabilities.

What is PAgP used for?

PAgP is a protocol that is used to dynamically and to automatically configure an

EtherChannel over multiple physical ports.

Name two types of Spanning-Tree Protocol messages used to communicate between

bridges.

Configuration BPDUs and Topology Change Notification BPDUs. Configuration BPDUs

are used to inform bridges of global STP parameters and are used to form the Spanning

Tree topology. Topology Change Notification BPDUs are used to inform bridges that a

link state has changed potentially impacting the Spanning Tree topology.

What conditions cause a STP topology change? What effect does this have on STP and the

network?

A topology change occurs when a port moves to the Forwarding state or from Forwarding

or Learning to the Blocking state. During a topology change, addresses are aged out in

Forward Delay seconds while active stations are not aged out of the bridging table. The

STP is not recomputed; TCN BPDUs are sent throughout the network notifying other

switches of the topology change. Only the port where the topology change is occurring is

affected, by moving through the STP states.

What is the single most important design decision to be made in a network running STP?

Root Bridge placement.

Where should the Root Bridge be located in a switched network?

It should be located as close to the center of the network as possible. For example, in a

hierarchical design, the Root Bridge should be located in the Distribution layer.

What happens to a port that is neither a Root Port nor a Designated Port?

That port is placed in the Blocking state so that no bridging loops form from it.

What mechanism is used to set the STP timer values for all switches in a network?

The timers are set on the Root Bridge, and the values are propagated to all other switches

by including them in Configuration BPDUs.

What parameters can be tuned to influence the selection of a port as a Root or Designated

Port?

Port Cost and Port Priority.

Where should the UplinkFast feature be used in a switched network?

Only on switches that are leaf-nodes in the Spanning-Tree topology, such as the Access

Layer.

How does an EtherChannel distribute broadcasts and multicasts?

Broadcasts and multicasts are sent across only one port of the bundle and are not

distributed across the EtherChannel.

What happens if one port of an EtherChannel is unplugged or goes dead? What happens

when that port is reconnected?

Traffic on the disconnected port will be moved to the next available link in the

EtherChannel bundle. When the port is reconnected, traffic will not automatically move

back to the original port of the bundle. Rather, new traffic will be learned and applied to

the restored link.

What conditions cause a STP topology change? What effect does this have on STP and the

network?

A topology change occurs when a port moves to the Forwarding state or from Forwarding

or Learning to the Blocking state. During a topology change, addresses are aged out in

Forward Delay seconds, while active stations are not aged out of the bridging table. The

STP is not recomputed; TCN BPDUs are sent throughout the network notifying other

switches of the topology change. Only the port where the topology change is occurring is

affected by moving through the STP states.

A Root Bridge has been elected in a switched network. Suppose a new switch is installed

with a lower Bridge ID than the existing Root Bridge. What will happen?

After the new switch comes up, a Root Bridge election will take place. This will occur at

the next Hello time when the new switch announces itself as root. It will become the Root

Bridge because it has the lowest Bridge ID, and the Spanning Tree topology will be

recomputed. Where switch ports change state as a result of the election and topology

change, outages will occur until the Forwarding state starts again.

What happens if the STP Hello Time is decreased to one second in an effort to speed up

STP convergence? What happens if the Hello Time is increased to ten seconds?

Setting the Hello Timer to one second doubles the amount of Configuration BPDUs that

are sent by a switch, as compared to the default 2 second timer. While this does share

BPDU information more often, it really doesn’t help the long convergence delay when a

port comes up. The significant delays come from the Forward Delay timer, which is used

to move a port through the Listening and Learning states. By default, this process takes 30

seconds and is unaffected by the Hello Timer.

What types of links can be used to interconnect switches and an external router? How

many VLANs can be carried on each?

Links can be used with one VLAN per physical link, using any supported media. Trunk

links can also be used to carry multiple VLANs over a single link, using such media as

Fast Ethernet, Gigabit Ethernet, ATM LANE, and FDDI.

What is the difference between interVLAN routing and multilayer switching (MLS)?

InterVLAN routing uses a route processor to forward data between VLANs. It also

requires that each packet crossing a VLAN boundary be processed by a routing decision

on the router. MLS also uses a route processor to forward data between VLANs, but the

routing decision is only required for the first packet exchange between two nodes. From

that point on, data is forwarded by the switching engine and does not require further

routing.

If a router is used to route between VLANs, what additional information is needed so that

traffic will actually be routed?

A default gateway is required on the end stations so that the stations can forward packets

that are destined to a different VLAN or network to the router.

What is the name of the protocol that allows a set of routers that are working together to

form one virtual-router?

Hot Standby Router Protocol or HSRP.

What is the minimum number of routers needed to perform HSRP?

The minimum number of routers needed is at least two. One functions as an active router

and one as a standby.

In a properly functioning virtual router, what happens when the active router fails?

In a properly functioning HSRP environment, packets will still be routed in the event of a

failed router.

How many standby groups can exist on any one LAN?

In any one LAN, up to 255 standby groups can exist.

Name the six states that an HSRP configured router can be in.

Initial, Learn, Listen, Speak, Standby, and Active.

When configuring HSRP on a particular router interface, if the standby group is not

explicitly configured, what standby group does the interface fall into by default?

Standby group 0 is the default setting when configuring an interface for HSRP. You may,

however, override this by configuring a setting of your own choosing.

What command is used to display the HSRP virtual router IP and MAC address?

The Cisco command show standby will display the HSRP virtual router IP and MAC

address.

Which router in an HSRP group becomes the forwarding router and how is it determined?

The router that becomes the forwarding router in an HSRP group is the one with the

highest priority. The priority is determined by what has been configured. The default value

is 100, but the priority can be any number between 0 and 255. If the priorities are equal,

then the highest IP address takes priority.

In the command standby 35 priority 90, what does the “35” stand for?

The “35” indicates the standby group number.

An HSRP router exchanges Hello messages with other HSRP routers. What is contained

in the hello message?

The HSRP Hello message contains the hellotime and the holdtime values, in addition to

the priority, group number, password, and virtual router.

What does the term tracking imply in an HSRP environment?

Interface tracking enables the priority of a standby group router to be automatically

adjusted based on availability

becomes unavailable, the HSRP priority of the router is decreased. The HSRP tracking

feature reduces the likelihood that a router with an unavailable key interface will remain

the active router.

What does the preempt command do in the HSRP environment?

The preempt command is used to ensure that after an active router has failed, it will

resume its active router role once the router has recovered. That is assuming that the router

has higher priority AND preempt is configured. Without this feature, the “new” active

router will remain the active router indefinitely.

What problem makes HSRP necessary?

The fact that there isn’t a dynamic protocol to discover new default gateways for hosts in

the event of failure.

What constitutes an HSRP group?

An HSRP group consists of an active router, a standby router, and the virtual router.

Assume you are using five VLANs within your network and want to implement HSRP. How

many HSRP groups would you need to create?

Five; when using multiple VLANs in an HSRP implementation, a separate HSRP group

is created for each VLAN.

What is the name of the protocol used to report their multicast group membership with

neighboring multicast routers?

Internet Group Management Protocol (IGMP).

What does a host send to the multicast group address to join a group?

The host can send a Host Membership Report to join a multicast group.

Name the two types of sparse mode routing protocols.

Core-Based Trees (CBT) and Protocol Independent Multicast Sparse Mode (PIM SM).

What is the algorithm used in a source specific distribution tree?

Reverse Path Forwarding (RPF).

What is used to manage the scope of multicast delivery?

Time-to-live (TTL).

What VLAN is the default VLAN for a Catalyst switch and why is it a good idea to change

this?

The default VLAN is VLAN 1. The reason it’s a good idea to change this is that anyone

plugging into a port will automatically be in VLAN 1 without further configuration.

Because VLAN 1 is commonly used as the management VLAN, this represents a risk of

the wrong person getting access to this switch or others within the network.

What does port security do on a Catalyst series switch?

Port security is the feature that can be used to limit access to only specified MAC

addresses. All others will be denied access.

In general, what type of policies should be implemented in the core layer?

Only Quality of Service (QoS) policies should be implemented at the core layer.

When implementing route filtering, what type of access list is used—a standard or an

extended access list?

Only standard access lists are used when filtering routes or routing update traffic.

What does the access-class command do when applied to a virtual terminal

configuration?

The access-class command is used as a means of allowing only certain hosts access to the

virtual terminal lines.

What does the Cisco command login local do on a router?

The login local command requires that you have preconfigured a username/password pair

using the command username xxxx password xxxx. This is done in lieu of using

authentication servers such as TACACS or RADIUS.

What is the main method of out-of-band management for Cisco switches?

The main method of out-of-band management is the console connection.

What is the command to verify that RMON is enabled on the switch?

show snmp

What is the default value for the read-write community string?

Private

How many simultaneous Telnet sessions are supported on a Cisco switch?

Eight.