What Is the Symptom Pointing To?

About this interactive

The lesson's issues, by family: - **Layer 2 switching:** a loop with spanning tree off or broken, a broadcast storm, multicast flooding, duplicate MAC addresses, a port in the wrong VLAN. - **Host IP settings:** a wrong IP address or mask, a wrong default gateway (local works, remote does not, on one machine), a wrong DNS server (works by IP, not by name), a duplicate IP address. - **Routing:** a missing route, routers still converging after a change, a slower backup path. - **Firewall or port filtering:** a blocked port or address, rules in the wrong order (first match wins), the implicit deny at the end, a service on a non-standard port. - **DHCP:** a rogue DHCP server handing out the wrong settings, or a scope with fewer addresses than devices. Asymmetric routing, which the lesson also covers, is not on a card: it is a routing fault that a firewall turns into an outage, so it fits two bins.

What you'll learn

From a symptom and its evidence, name the kind of fault behind it: a Layer 2 switching fault, a host's IP settings, a routing fault, firewall or port filtering, or a DHCP fault
Recognize the lesson's Layer 2 to Layer 7 issues in practice: loops, broadcast storms, duplicate MACs, wrong VLANs, wrong gateways or DNS, missing routes, convergence, ACL order, implicit deny, non-standard ports, rogue DHCP servers and exhausted scopes

Aligned to

CompTIA Network+

5.3 Given a scenario, troubleshoot common issues with network services.

5.4 Given a scenario, troubleshoot common performance issues.

Cisco CCNA

2.5 Interpret basic operations of Rapid PVST+ Spanning Tree Protocol

Key terms

Broadcast Storm: A network condition in which excessive broadcast traffic overwhelms switch resources, preventing normal communication from occurring.
Spanning Tree Protocol: A network protocol that prevents loops in Ethernet networks by creating a loop-free logical topology.
Default Gateway: The router that a device uses to send traffic to destinations outside its local network.
DHCP Scope: The defined pool of IP addresses that a DHCP server is configured to assign to clients on a specific subnet, along with associated options such as the subnet mask, default gateway, and DNS servers. Each subnet served by a DHCP server has its own scope.
Dynamic Host Configuration Protocol: A network protocol that automatically assigns IP addresses and other configuration parameters to devices on a network.
Implicit Deny: A foundational access control principle in which any traffic or request not explicitly permitted by a rule is automatically blocked. It appears as a hidden deny-all rule at the end of every access control list.
Convergence: The state in which all routers in a network have consistent and up-to-date routing information after a topology change.

Topics

Troubleshooting Switching Loop Default Gateway Routing Firewall Dhcp Interactive Categorize

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