22 Troubleshooting Network Issues

Startup Configuration

Stored in NVRAM and contains the commands needed to initially configure a router

Running Configuration

Actively being used by the router at that moment

VLAN Assignment/Tagging

Practice of segmenting an IT organization's network, separating users into respective network sectionsVLAN Membership policy Server (VMPS), client, server, and database

Network Performance Baseline

Defines the normal working conditions of an enterprise network infrastructure


Occurs when two hosts on the network transmit at the same time which causes the signals to combine and on the network mediumOccur in both wired and wireless networks

Collision Domain

Network segment where simultaneous data transmissions collide with one anotherUse any Layer 2 device to break apart collision domainsTurn off autonegotiationHardcode lower speedChange to half-duplex

Broadcast Storm

Occurs when a network system is overwhelmed by continuous multicast or broadcast trafficLayer 2: FF:FF:FF:FF:FF:FFLayer 3:

Broadcast Domain

A logical division of computer network where all nodes can reach each other by broadcast at the data link layerLayer 2 devices will not separate

Too large singular broadcast domain

Use a router to break up subnets into separate broadcast domains

Large volume of DHCP requests


Loops are created in the switching environment

Enable Bridge Protocol Data Units (BPDU) on managed switchesEnforce maximum number of MAC addresses per portBreak up large broadcast domains into smaller domains using routers and Layer 3 switches

Duplicate MAC Address

Used to uniquely identify a network interface card on a given network; 48-bitFirst 3 octets identify vendor, other 3 octets are a unique valueUsed in Layer 2Logical Domain Manager listens to multicast messages on a network and keeps track of the MAC addresses being usedEnable port security on switches

Duplicate IP Address/IP Address Conflict

Occurs when another computer on the same network has an identical IP to another workstation or server on the same networkStatic IP address issueDHCP server issueRogue DHCP server

Multicast Flooding

No specific host is associated with the multicast MAC address in the CAM table of the switch

Asymmetrical Routing

Network packets leave via one path and return via a different pathRouting issues cause issues with dropped packet flows

Missing Routes

When a router cannot reach a destination because there is a missing route in the routing table

Switching/Bridge Loop

Switching loops are usually an issue with how STP is configured

Routing Loop

Formed when an error occurs in the operation of the routing algorithm and creates a circular route amongst a group of network devicesCaused by logical Layer 3 circular connections that may exist in a routing tableTime to Live (TTL)

Split Horizon

Routing configuration that stops a route form being advertised back in the direction from which it came

Route Poisoning

Increasing a router's metric to an infinitely high number after detecting one of its connected routes has failed

Hold-Down Timer

Prevents bad route from being restored and passed to other routers by accident180 seconds (3 minutes)Statically-created routes are given a metric of 1 by default

Dynamic Host Allocation Protocol (DHCP)

Automatically assigns an IP address, subnet mask, default gateway, and DNS server's IP address to a client when it joins the network

Rogue DHCP Server

A DHCP server on a network which is not under the administrative control of the network administratorsDHCP snoopingPort securityIntrusion detection

DHCP Scop Exhaustion

Occurs when the DHCP server runs out of valid IPs to assign to the clients requesting access on the network

Default Lease Time

1440 seconds (1 day)7 days30 days

IP and VLAN Settings

Make sure you check your configuration and that there is proper routing setup between VLANs IP addressSubnet maskDefault gateway IPDNS server IP


Network security device that monitors and filters incoming and outgoing network traffic based upon established rule sets

Host-Based Firewall

Runs on an individual computer or device connected to the network to protect that one device

Network-Based Firewall

Deployed in line with the network traffic flow to monitor and filter incoming and outgoing network traffic based on established rule setsProvides security by blocking unauthorized users and allowing authorized users to access specific resources


Matches domain names with the corresponding IP addresses used by a serverIssue on client or larger network?Connectivity between client and DNS server?A records and CNAME records properly created?Time to live (TTL) set correctly?

DNS Latency

Time and delay that occurs whenever users request a particular domain name


Allows synchronization of system clocks between different layers of a hierarchical, semi-layered system of time sourcesEnsure network client is operating the NTP service

Network Performance Issues

High CPU usageHigh bandwidth usagePoor connectivityNetwork malfunction

DNS Problems

High CPU usage increases latency, jitter, and packet lossUse cable tester for twisted pair connections or a fiber light meter for fiber connections

Other Issues

Low optical link budgets

Optical Link Budget

A calculation that considers all the anticipated losses along the length of a fiber optic connectionReduced transmissionSlow connection speedsConnection downtime

Calculate Optical Link Budget

0.25 dB per km standard for fiber optic cablePower budget minus the loss over the fiber cable's distance

Certificate Issues

A digital certificate is used as a credential to facilitate verification of identities between users in a transaction

BYOD Challenges

Allows users to bring their own smartphones, laptops, and other devices to work and use them on the organization's networkDecrease in Capital ExpendituresIncrease in Operational Expenditures