Troubleshooting Network Issues Flashcards
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 sections
▪ VLAN Membership Policy Server (VMPS), client, server, and database
Network Performance Baseline
▪ Defines the normal working conditions of an enterprise network
infrastructure
Collision
▪ Occurs when two hosts on the network transmit at the same time which
causes the signals to combine on the network medium
▪ Collisions occur in both wired and wireless networks
Collision Domain
▪ Network segment where simultaneous data transmissions collide with
one another
▪ Use any Layer 2 device to break apart collision domains
● Turn off autonegotiation
● Hardcode lower speed
● Change to half-duplex
Broadcast Storm
▪ Occurs when a network system is overwhelmed by continuous multicast
or broadcast traffic
Layer 2
o FF:FF:FF:FF:FF:FF
Layer 3
o 255.255.255.255
Broadcast Domain
▪ A logical division of computer network where all nodes can reach each
other by broadcast at the data link layer
▪ Layer 2 devices will not break up a broadcast domain
Too large singular broadcast domain
o Use a router to break up subnets into separate broadcast
domains
Large volume of DCHP requests
o Discover
o Offer
o Request
o Acknowledge
Loops are created in the switching environment
o Enable Bridge Protocol Data Units (BPDU) on managed
switches
o Enforce a maximum number of MAC addresses per port
o Break up large broadcast domains into smaller domains
using routers and Layer 3 switches
MAC Address
▪ Used to uniquely identify a network interface card on a given network
● D2:51:F1:3A:34:65
o Vendor code: D2:51:F1
o Unique Value: 3A:34:65
▪ MAC addresses are only used in your Layer 2 networks
▪ Logical Domain Manager
▪ Listens to multicast messages on a network and keeps track of the MAC
addresses being used
▪ Enable port security on your 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 network
● Static IP address issue
● DHCP server issue
● Rogue 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 path
▪ Routing 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 devices
▪ Routing loops are caused by logical Layer 3 circular connections that may
exist in a routing table
● Time to Live (TTL)
Split Horizon
▪ Routing configuration that stops a route from being advertised back in
the direction from which it came
● ip split-horizon
● no ip split-horizon
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 routes from being restored and passed to other routers by
accident
● Hold-down period
● 180 seconds (3 minutes)
▪ Statically-created routes are given a metric of 1 by default
Dynamic Host Configuration Protocol (DHCP)
▪ Automatically assigns an IP address, subnet mask, default gateway, and
DNS server’s IP address to a client when it joins a network
Rogue DHCP Server
▪ A DHCP server on a network which is not under the administrative control of the network administrators ● DHCP snooping ● Port security ● Intrusion detection ● DHCP Scope Exhaustion
o Occurs when the DHCP server runs out of valid IPs to
assign to the clients requesting access on the network
▪ 192.168.1.0/24
▪ 192.168.1.1
▪ 192.168.1.255
● Default Lease Time
o 1440 seconds (1 day)
o 7 days
o 30 days
● Default Lease Time
o 1440 seconds (1 day)
o 7 days
o 30 days
● Default Lease Time
o 1440 seconds (1 day)
o 7 days
o 30 days
Rogue DHCP Server
▪ A DHCP server on a network which is not under the administrative control of the network administrators ● DHCP snooping ● Port security ● Intrusion detection ● DHCP Scope Exhaustion
o Occurs when the DHCP server runs out of valid IPs to
assign to the clients requesting access on the network
▪ 192.168.1.0/24
▪ 192.168.1.1
▪ 192.168.1.255
● Default Lease Time
o 1440 seconds (1 day)
o 7 days
o 30 days
● Default Lease Time
o 1440 seconds (1 day)
o 7 days
o 30 days
● Default Lease Time
o 1440 seconds (1 day)
o 7 days
o 30 days
IP and VLAN Settings
o Make sure you check your configuration and that there is proper routing setup
between the VLANs, because this is the number one cause of issues when you’re
dealing with VLANs that won’t communicate
IP address
● Occur when you have an incorrect IP address, subnet mask,
gateway, or DNS server IP address assigned to a client
▪ Subnet mask
▪ Default gateway IP
▪ DNS server IP
● Make sure you have a working DNS server and the IP is properly
entered on the client
IP address
● Occur when you have an incorrect IP address, subnet mask,
gateway, or DNS server IP address assigned to a client
▪ Subnet mask
▪ Default gateway IP
▪ DNS server IP
● Make sure you have a working DNS server and the IP is properly
entered on the client
Firewall
▪ 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 sets
● Access to protected resources from unprotected networks is not
working
● Access to unprotected resources from protected networks is not
working
● Access to the firewall and its configurations is not working
● Access Control List (ACL)
▪ Provides security by blocking unauthorized users and allowing authorized
users to access specific resources
DNS
▪ Matches domain names with the corresponding IP addresses used by a
server
● Issue on a single network client or on a larger network?
● Connectivity between client and DNS server?
● A records and CNAME records properly created?
o nslookup
▪ Time to live (TTL) set correctly?
DNS Latency
▪ Time and delay that occurs whenever users request a particular domain
name
NTP
▪ Allows synchronization of system clocks between different layers of a
hierarchical, semi-layered system of time sources
● Not received
● Not processed
● Errors or packet loss
▪ Ensure network client is operating the NTP service
● Network saturation
● Network connectivity
Network Performance Issues
o High CPU usage
o High bandwidth usage
o Poor connectivity
o Network malfunction
DNS problems
o High CPU usage increases latency, jitter, and packet loss
o Use a cable tester for twisted pair connections or a fiber light meter for fiber
optic connections
Other Issues (Certificate, Licensed, BYOD, and Hardware failures are all “other issues”)
o Low optical link budgets
▪ Optical Link Budget is a calculation that considers all the anticipated
losses along the length of a fiber optic connection
● Reduced transmission
● Slow connection speeds
● Connection downtime
o 0.25 dB per km for a standard fiber optic cable
o Calculate optical link budget by using power 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
▪ Bring Your Own Device (BYOD) policies allows a user to bring their own
smartphones, laptops, and other devices to work and use them on the
organization’s network
● Decrease in Capital Expenditures
● Increase in Operational Expenditures
