ARS exam
1. 1. Which STP port state allows a port to send and receive BPDUs but not forward user traffic?
Listening – port sends/receives BPDUs but does not forward traffic.
What is the purpose of the 'spanning-tree portfast' command on an access port?
Allows the port to transition immediately to forwarding state; speeds up host connectivity.
In a topology with multiple switches, what determines the root bridge election in STP?
Root bridge is chosen based on the lowest bridge ID (priority + MAC address).
Describe the function of BPDU Guard and explain when it should be enabled.
BPDU Guard disables a port if BPDUs are received; used on access ports to prevent loops
(Scenario) Switch SW1 connects to SW2 via Gig0/1. SW1’s priority is 4096, SW2’s is 32768. Which becomes root, and why?
SW1 becomes root because it has the lower priority (4096 vs 32768).
What are the different OSPF router types and their roles (ABR, ASBR, DR, BDR, Internal)?
Router types: Internal, ABR, ASBR, DR, BDR – each plays a role in area segmentation and route advertisement.
Which LSA types are exchanged between OSPF areas?
Type 1: Router LSAs,
Type 2: Network LSAs,
Type 3: Summary LSAs (ABR),
Type 4: ASBR summary,
Type 5: External LSAs.
(Scenario) Router R1 (Area 0) connects to R2 (Area 1). Which router generates Type 3 LSAs?
R1 (ABR) generates Type 3 LSAs between Area 0 and Area 1.
What happens if OSPF Hello and Dead timers differ between neighbors?
They will not form adjacency; Hello/Dead timers must match.
How does OSPF determine the best route when multiple equal-cost paths exist?
OSPF uses cost (based on bandwidth) and equal-cost multipath (ECMP) if costs are equal.
List the BGP neighbor adjacency states in order.
Idle → Connect → Active → OpenSent → OpenConfirm → Established.
What BGP attributes influence path selection? List at least four and describe each briefly.
Local Preference, AS_PATH, MED, Origin, Weight.
Order: Weight > Local Pref > AS Path > Origin > MED > eBGP/iBGP.
Which BGP attribute influences outbound traffic selection?
Local Preference – influences outbound traffic from an AS.
Scenario) Two routes to 10.10.10.0/24 exist with equal AS_PATH but different Local Preference. Which route is chosen?
The route with the higher Local Preference is selected.
Explain the difference between eBGP and iBGP.
eBGP exchanges routes between ASes; iBGP exchanges routes within the same AS.
Define the following: Feasible Distance (FD), Reported Distance (RD), Successor, Feasible Successor.
FD: Total metric;
RD: Neighbor’s advertised metric;
Successor: Best path;
Feasible Successor: Backup path.
What is the Feasibility Condition in EIGRP?
A route is a feasible successor if RD < FD of the current successor.
(Scenario) Router C learns route 10.1.1.0/24 via B, D, and E. FD(B)=20, RD(B)=10; FD(D)=40, RD(D)=30; FD(E)=35, RD(E)=25. Which is the successor and feasible successor?
Successor = B (lowest FD). Feasible successor = D (RD < FD(B)).
What tables does EIGRP maintain?
EIGRP maintains Neighbor, Topology, and Routing tables.
Explain how EIGRP uses DUAL to ensure loop-free paths.
DUAL ensures loop-free paths using the Feasibility Condition and recalculations.
What VXLAN control plane is commonly used in SD-Access environments?
LISP – used as control plane in SD-Access VXLAN overlay.
Describe the role of the fabric edge node in SD-Access.
Fabric Edge connects endpoints to the SD-Access fabric and maps EID-to-RLOC.
Explain the purpose of the LISP protocol within an SD-Access fabric.
LISP provides identity-to-location mapping for endpoint mobility.
What is the difference between an underlay and overlay network in SD-Access?
Underlay = physical network;
Overlay = logical VXLAN tunnels for fabric traffic.
(Scenario) A wireless client connects to a fabric-enabled AP. Explain the control and data flow from AP to WLC.
The AP sends CAPWAP control to WLC; data encapsulated in VXLAN to fabric edge.
What is the main difference between traffic policing and traffic shaping?
Policing drops excess traffic; shaping buffers excess traffic to smooth rate.
Match the following QoS mechanisms with their functions:
LLQ,
CBWFQ,
WRED,
Policing,
Shaping.
LLQ = Priority queue;
CBWFQ = Bandwidth guarantees;
WRED = Random early drop;
Policing = Drop/remark;
Shaping = Buffer.
Explain how QoS prioritizes delay-sensitive traffic such as VoIP.
QoS prioritizes voice by using LLQ and DSCP EF marking.
Describe the role of marking (DSCP) in QoS policies.
Marking (DSCP) identifies traffic classes for later QoS treatment.
(Scenario) A router applies LLQ to voice traffic but experiences drops. Suggest a possible cause and solution.
Likely cause: LLQ oversubscription. Solution: Adjust bandwidth or policing thresholds.
What is the role of the Wireless LAN Controller (WLC) in a centralized architecture?
WLC manages, authenticates, and controls lightweight APs via CAPWAP.
What are two benefits of a centralized WLAN topology?
Centralized control and uniform security policy enforcement.
Describe how CAPWAP functions between APs and WLCs.
CAPWAP tunnels control and data traffic between APs and WLCs.
Explain the difference between Local and FlexConnect modes.
Local = traffic stays local;
FlexConnect = can locally switch or tunnel to WLC.
(Scenario) A lightweight AP cannot join the WLC. List three potential causes.
Possible causes: Join failure, version mismatch, CAPWAP discovery failure.
What is a key benefit of implementing VRF on a router?
VRF enables multiple routing tables on the same device for network segmentation.
How does VRF enable overlapping IP address spaces?
Each VRF maintains its own routing table, allowing overlapping IP spaces.
Which configuration management tool uses an agentless approach?
Ansible – agentless configuration tool.
Describe a practical use case for Ansible in network automation.
Ansible automates network configurations via YAML playbooks and SSH.
Explain how REST APIs are used for managing network devices.
REST APIs use HTTP methods (GET, POST, PUT, DELETE) to manage devices programmatically.