LAN Design, Scaling VLANs, STP, EtherChannel and HSRP Flashcards
Learn these Qs (20 cards)
What are the things a network designer should think about when designing a company’s network for the future?
Scalability: Must support future growth and new technologies.
Redundancy: Use of backup links/devices to ensure high availability.
Hierarchical Design: Core, distribution, access layers for easier troubleshooting and scalability.
Performance: Ensure adequate bandwidth and switch forwarding rates.
Security: Incorporate features like ACLs, VLANs, port security.
Manageability: Use consistent standards, naming, and documentation
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What criteria should be used when deciding what make/model of a switch to buy?
Port density and type (e.g. 1G, 10G)
Forwarding rate (bps) and throughput
PoE capability (for powering devices like phones, cameras)
Layer 3 functionality (routing support)
Cost, reliability, stackability, and feature compatibility
What is DTP (Dynamic Trunking Protocol)? Why must care be taken with the use of the protocol?
DTP is a Cisco proprietary protocol that negotiates trunking between switches. Care must be taken because leaving DTP enabled on ports connected to unknown or untrusted devices can result in unintended trunk formation, allowing VLAN hopping or other attacks
How can an SVI (Switched Virtual Interface) be used for routing?
An SVI allows inter-VLAN routing by assigning an IP address to a VLAN interface on a Layer 3 switch. Each VLAN’s SVI acts as the default gateway for hosts in that VLAN
What are the reasons that loops are created in switched networks?
Redundant links without STP
Misconfigured switches or ports
Accidental cabling mistakes
Loops cause broadcast storms, MAC table instability, and duplicate frames
How can a network administrator configure a specific switch to be the root bridge? Why is it a good idea to do this?
Use spanning-tree vlan [id] root primary or set a lower bridge priority manually.
It’s a good idea because it ensures predictable STP topology and path selection
What is Link Aggregation and how does it add both redundancy and scalability to a network?
Combines multiple physical links into one logical link (EtherChannel). It increases bandwidth and provides redundancy—if one link fails, traffic continues on the others
What is the relationship, and what are the differences, between PAgP and LACP?
Both are used for EtherChannel negotiation.
PAgP is Cisco proprietary, while LACP is IEEE standard (802.3ad).
Modes: PAgP (auto, desirable), LACP (active, passive).
PAgP does not work with non-Cisco devices, LACP does
A network administrator is losing sleep over the VLAN configuration of the switches in his company’s growing network. Every time he makes a change to the configuration on one switch, he has to then make sure he applies the same change to every other switch in the network. Outline how VTP (VLAN Trunking Protocol) could make his life easier.
VTP allows centralized VLAN management from a VTP server.
Changes made on the server (create/delete/rename VLANs) automatically propagate to all VTP clients in the same domain.
Reduces errors and time needed to maintain VLAN consistency across multiple switches
A network administrator is troubleshooting a switch to see why it does not seem to be receiving VTP updates. The output of ‘show vtp status’ indicates that the VTP operating mode is ‘Transparent’. What does this mean?
In Transparent mode:
The switch does not process or apply incoming VTP advertisements.
It forwards VTP messages but maintains its own local VLAN database.
This mode is used when extended VLANs or local control is needed
A network administrator is trying to use DTP (Dynamic Trunking Protocol) to automatically enable trunking between switches. She has set both ends of each inter-switch link to ‘dynamic auto’, but the ports remain in access mode. What is wrong? What settings would you recommend she use, and why?
Problem:
DTP needs at least one side to initiate negotiation.
‘dynamic auto’ on both ends means neither side initiates → no trunk formed.
Recommended settings:
Set one side to dynamic desirable and the other to dynamic auto
Better yet, statically set trunk mode with:
switchport mode trunk
switchport nonegotiate
This eliminates reliance on negotiation and enhances security
What problems can disrupt inter-vlan routing, and how can they be solved?
Possible problems:
Missing or incorrect VLAN-to-subinterface mappings (router-on-a-stick).
Incorrect trunking configuration.
Disabled SVIs on Layer 3 switches.
Incorrect IP addressing or subnet masks.
Solutions:
Use show vlan, show interfaces, show ip interface, and show running-config to verify.
Ensure trunks allow all necessary VLANs.
Ensure routing is enabled (on L3 switches).
Match subinterfaces with VLAN IDs in router-on-a-stick configs
A network manager has run Wireshark on her company’s network, and is surprised to see a lot of BPDUs – maybe around 30 in a minute. What would you say to her?
This is completely normal:
BPDUs (Bridge Protocol Data Units) are sent every 2 seconds per VLAN by default.
They’re essential for STP to maintain loop-free topology.
Seeing around 30 per minute suggests active STP function – not a problem unless storm/flooding is observed
How can PVST+ be used to implement load balancing in a switched network?
PVST+ runs a separate STP instance per VLAN.
You can:
Assign different root bridges for different VLANs.
Balance traffic by manipulating STP root priorities per VLAN.
Example:
spanning-tree vlan 10 root primary
spanning-tree vlan 20 root secondary
Results in some VLANs using one uplink and others using a different one
An administrator has disconnected an unused PC from switch port and attempted to add redundancy to a network by connecting a cable from that port to a port on another switch. Why was this a risky thing to do?
This creates a Layer 2 loop, unless STP is properly configured.
Consequences include:
Broadcast storms
MAC table instability
Duplicate frames
Solution: Always check for loop protection (e.g. STP) before adding redundant links
What steps should be taken to repair a spanning tree problem?
Immediate Steps:
Disconnect redundant links temporarily.
Identify topology with show spanning-tree.
Check for misconfigurations (e.g. wrong priorities or port roles).
Permanent Fixes:
Correct bridge priorities and cost settings.
Use PortFast and BPDU Guard for end-user ports.
Reconnect links after verifying correct STP roles (Root, Designated, Blocking)
Users in a company have lost network connectivity because the router their PCs use as a default gateway has failed. What could the network administrate have done to avoid the work interruption caused by this event?
Implement a First Hop Redundancy Protocol (FHRP) like HSRP, VRRP, or GLBP.
These allow multiple routers to share a virtual IP as the default gateway.
If the active router fails, the standby takes over seamlessly for users
How can a PC automatically use a different default gateway if the one it normally uses fails?
By using HSRP or similar FHRP:
The PC only sees the virtual IP address.
The active router responds.
If it fails, the standby router assumes the role without the PC noticing
For what reasons does HSRP fail?
Misconfigurations:
Different virtual IPs, priorities, group numbers
Routers not on the same VLAN/subnet
No preemption enabled – standby won’t take over
Hardware/interface failure on both routers
End devices misconfigured with wrong default gateway
A network administrator has configured LACP between two switches, but the port channel is down. What could the problem be, and how could he fix it?
Possible causes:
Interface mismatches: speed, duplex, VLANs
One or both ends set to passive (passive-passive doesn’t form a channel)
LACP not supported on some interfaces
Fixes:
Set one side to active, other to passive or active
Ensure all bundle interfaces match in config
Use show etherchannel summary and show interfaces to troubleshoot
