Network Troubleshooting -75+ Real-World Incidents(CCNA-CCNP)

Master real-world network troubleshooting with 75+ incidents across switching and routing, diagnosing VLAN, trunk, HSRP, OSPF, and BGP issues through practical labs and structured root-cause analysis.

Diagnose a switch port misconfiguration where the interface is up but traffic fails due to VLAN assignment, align ports to the same VLAN, and enable MAC address learning.

Explain how port security on a Cisco switch restricts allowed mac addresses on an access port and triggers error disabled on violations, with sticky learning and auto recovery options.

Understand speed and duplex concepts and auto negotiation to prevent drops. A real incident shows a speed mismatch (10 vs 100 Mbps) causing a link to go down.

Demonstrates how misconfiguring an access port as a trunk disrupts inter-host communication; explains trunk and access port roles, VLAN tagging, and basic verification.

Learn how access ports carry untagged traffic for a single VLAN, how trunks carry multiple VLANs with tagging, and how misconfiguring a trunk as an access port disrupts connectivity.

Diagnose why two PCs in VLAN 10 cannot communicate when VLAN 10 is missing on the switch. Create VLAN 10, assign access ports to it, and verify connectivity with ping.

Troubleshoot VLAN communication by ensuring trunk ports allow all relevant VLANs; after permitting VLAN 20 on the trunk, PCs 20.1 and 20.2 communicate across switches.

Explore how a native VLAN mismatch on trunk links causes untagged traffic to misclassify, triggering CDP warnings and connectivity issues; learn how to fix with consistent native VLAN configuration.

Troubleshoot voice vlan misconfiguration by configuring data and voice vlans on a single switch port, diagnosing dhcp, and validating telephony settings so IP phones get addresses and extensions.

This lesson covers an end device not getting an IP caused by VLAN misconfiguration; perform post-implementation checks by configuring switch port access VLAN 10 and verifying DHCP to restore connectivity.

An L3 switch port with no switch port stops MAC learning, leaving the MAC table empty even though the gateway 192.168.1.10 is reachable.

Explore mac flapping between two ports on linked switches, causing unstable mac address tables and intermittent connectivity; resolve by removing the extra link and enabling spanning tree.

Demonstrates port security in protect mode, where only a specific mac address is allowed and the port remains up while violations silently drop other devices.

Demonstrate port security in restrict mode on interface Ethernet 0/2, showing how an unauthorized device triggers a security violation, logs, and a blocked port, contrasted with protect mode's silent drop.

Discover how BPDU guard protects access ports from STP BPDUs to prevent loops, illustrated by a port in error-disabled and recover via shutdown/no shutdown or auto-recovery.

Learn how spanning tree protocol prevents loops in Ethernet networks by electing a root bridge, blocking redundant paths, and ensuring fast convergence across VLANs, with practical insights on unmanaged switches.

Investigate a loop caused by incorrect cabling in a three-switch network, and learn how spanning tree and loop guard prevent unidirectional failures by stopping BPDUs and blocking inconsistent ports.

Investigate a vlan misassignment where a pc and server end up on different subnets due to dhcp and gateway misconfigurations, and fix the switch port vlan and dhcp gateway.

Enable ip routing on the layer 3 switch, configure switch virtual interfaces for vlan 10 and 20, and verify trunk links and gateways to restore inter-vlan communication.

Diagnose trunk encapsulation mismatch between switches causing inter-VLAN connectivity issues, explain 802.1q vs ISL, and illustrate aligning encapsulation on both ends to enable VLAN 10 traffic.

Discover how the ip default gateway on a layer 2 switch defines the router as the default next hop for management traffic to remote networks, enabling pings and ssh.

Lower the priority of switch 19 to make it the root bridge, causing blocked ports to move to learning before forwarding while STP selects the root by bridge ID.

learn how to keep a chosen switch as root by adjusting spanning tree priority when a new switch connects, observing port states and triggering topology changes.

Investigate a spanning-tree outage caused by BPDU filter suppressing BPDUs on edge ports, leaving all interfaces forwarding and causing a loop; remove BPDU filter on affected interfaces to restore STP.

Explore how spanning tree loops create broadcast storms and how storm control, BPDU guard, and manual errors trigger or prevent outages in real-world networks.

Port fast allows an access port to move immediately to forwarding, bypassing listening and learning delays, speeding connectivity for end devices and enabling BPDU guard.

Explore how enabling PortFast on a trunk between switches causes immediate forwarding, leading to temporary loops and bpdu use; disable PortFast on inter-switch links to verify spanning tree behavior.

the lecture explains ether channel as a single logical link that increases bandwidth and redundancy through load balancing. an incident shows lacp and pagp mode mismatch preventing port channel formation.

Discover how ether channel load balancing distributes traffic across a port channel using a hashing algorithm, making each flow use a single link and sometimes causing uneven traffic.

Diagnose an ether channel VLAN mismatch by checking channel status and trunk allowed VLANs, then apply changes on the port channel to ensure consistent configuration across all member ports.

demonstrates how vlan pruning via vtp pruning stops unused vlans on trunk links, reducing broadcast traffic and improving bandwidth without disrupting trunk negotiation.

Explore HSRP as a first hop redundancy protocol that uses a virtual IP and virtual MAC for gateway redundancy, with active and standby routers and preemption during failover.

Identify HSRP active/standby flapping causes by timer mismatch, where mismatched hello and hold times trigger intermittent traffic routing issues between R1 and R2; align timers to restore stable active router.

Learn how DHCP snooping prevents rogue DHCP servers by enforcing trusted ports, blocking untrusted replies, and building a binding table to ensure clients receive correct IP configuration.

Enable IP DHCP snooping and designate the server port as trusted to allow DHCP offer and ACK messages, resolving a missing trusted DHCP snooping port issue.

Dynamic ARP inspection (DAI) validates ARP packets against the DHCP snooping binding table to prevent ARP spoofing on switches, requiring DHCP snooping and trusted or untrusted port configuration.

configure root guard on selected ports to block superior bpdu from rogue devices, preserving the intended root bridge in spanning tree and preventing root takeover.

Explore how to troubleshoot a CDP disabled issue by enabling Cisco Discovery Protocol, verifying neighbors with show CDP neighbors, and mapping one-hop switch topology.

Diagnose switch traffic blocked by access control lists, learn standard and extended ACLs, apply them to SVIs or routed ports, and follow top-down rule matching.

Diagnose how a mismatched subnet mask blocks reachability between the PC and gateway. Correct the mask to bring the PC IP into the router's subnet and restore ping success.

Identify why a pc cannot reach a server by analyzing default gateway reachability and confirming the gateway IP is configured on the device.

Explore static routing by diagnosing a missing route between networks and adding a static route with a destination network, subnet mask, and next-hop IP to restore reachability.

Troubleshoot a static route misconfigured as the wrong next-hop by setting the next-hop to 11.0.0.2 for 192.168.20.0/24 and verifying with show ip route and show ip interface brief.

Explain how RIP uses hop count and periodic updates to share routes, and review version 1 vs 2 differences and common troubleshooting for routing tables not updating.

engineers diagnose why icmp succeeds but http fails because an access-list does not permit tcp traffic to 192.168.20.10; after adding a www permit, the server becomes reachable.

Explore asymmetric routing where forward and return paths differ, how stateful devices filter traffic, and how to detect and fix misconfigured static routes using traceroute.

Diagnose a route present but traffic not flowing by validating routes and interface status, enable the lan interface that was administratively down, and observe spanning tree behavior to restore reachability.

Diagnose how an administratively down LAN interface blocked traffic between a PC and a server, and verify routing and interface status using show ip route and show ip interface brief.

Translate private addresses to public ones with NAT, including static NAT, dynamic NAT, and PAT. Explore inside and outside interfaces, translation tables, and routing, plus common NAT issues.

Diagnose how duplicate default routes with identical administrative distance cause traffic to split between two ISPs; resolve by differentiating distances to keep one active as backup.

Demonstrate route recursion in a network troubleshooting scenario, showing how a static route's next hop requires a second routing table lookup, and how correct next-hop or interface configuration fixes connectivity.

Understand how ARP maps IP addresses to MAC addresses, troubleshoot ARP resolution failures, and resolve subnet mismatches with ARP requests, replies, and proxy ARP to reach the default gateway.

Investigate a proxy ARP issue where a router misrepresents the server on another subnet; disable proxy ARP and verify connectivity with arp and interface checks.

Resolve MTU mismatch issues by testing packet sizes beyond 1500, analyzing MTU, fragmentation, and DF bit; compare MTU to MSS and apply path MTU discovery in tunnels.

Examine how IP fragmentation causes drops when packets exceed MTU, and how DF bit, PMTUD, and router fragmentation affect connectivity and TCP MSS.

Explore how a VRF misconfiguration creates overlapping subnets, and learn to verify VRF tables, assign interfaces with IP VRF forwarding, and troubleshoot with show ip vrf and show ip route.

Troubleshoot OSPF dynamic routing across three routers, identify why a neighbor stays in INIT, and resolve by correcting multicast hello traffic, access lists, and DR/BDR adjacency.

duplicate router ids in ospf caused the exchange to stall in exstart; after correcting router id on router three to 3.3.3.3, ospf neighbor formation succeeded and pc1 could reach pc3.

Traces an OSPF outage caused by a prefix list block and distribute list that blocked 192.168.3.0, and restores routes after removal.

Diagnose an OSPF area mismatch between routers. Fix area IDs to match on both sides and ensure correct interface area configuration; understand virtual links if areas differ.

Troubleshoot an ospf network type mismatch by aligning interface network types (broadcast vs point-to-point), verify with show ip ospf interface and neighbor status, and restore full adjacency.

Diagnose an ospf mtu mismatch by ensuring the interface mtu matches, typically 1500, and learn to use ip ospf mtu ignore to participate in ospf despite mtu differences.

Diagnose why OSPF routes aren’t installed when Ethernet interfaces are misconfigured as passive; remove passive on Ethernet 1/1, set Ethernet 1/3 as appropriate, restore OSPF neighbor and PC connectivity.

Explore the essentials of OSPF, a link-state routing protocol with area-based design and authentication options. Learn ABR and ASBR roles, backbone area 0, LSA concepts, and adjacency troubleshooting.

Troubleshoot a router unreachable incident by configuring OSPF authentication on Ethernet 1/1 using MD5 with the same key on both sides, restoring adjacency and network reachability to 3.10.

Investigate administrative distance confusion between OSPF and static routes, and adjust the static distance to ensure OSPF preferred for the 3.0 network. Verify routing table entries and network reachability.

Trace a case where WAN link is up but PC1 cannot reach PC3 due to a statically bound ARP entry with the wrong MAC, despite OSPF and routes being fine.

Explore how IPSLA monitors reachability with synthetic traffic to trigger fast failover from a primary to a backup WAN link using floating static routes and object tracking.

Learn how improper OSPF route summarization and area border router can cause reachability issues between intra area and inter area routes, and how to correct summarized networks for accurate routing.

Investigate a bgp neighbor up but no routes by tracing route advertisements, checking show ip bgp, and advertising 192.168.10.0 for proper path selection using ebgp and ibgp concepts.

Analyze how misconfigured BGP timers caused route flapping and intermittent connectivity between network 10.x and 20.x. Learn safe keep alive and hold time values to stabilize routes and prevent outages.

Demonstrate that a BGP prefix will not be advertised unless the route is present in the routing information base, using 192.168.2.0, 10.0, and 20.0 as examples.

Bgp prefix was accepted but not installed because r3 lacks knowledge of 10.0.12; advertise the route in bgp or add a static route so r1 can reach 192.168.2.1.

Demonstrates how BGP weight influences path selection to prefer R3 over R4, including configuring weight, performing a soft reset, and verifying with ping and traceroute.

Diagnose a blackhole route misconfiguration in a static routing setup and restore connectivity. Verify interfaces, arp, and static routes to ensure reachability between 192.168.10.0 and 192.168.20.0.