Chapter 5 - section 5.3 Given a scenario, troubleshoot common issues with network services

section 5.3

Objective – Switching and IP Configuration Issues

• Switching issues
▸ Spanning Tree Protocol (STP)
▸ Network loops
▸ Root bridge selection
▸ Port roles
▸ Port states
▸ Incorrect VLAN assignment
▸ Access control lists (ACLs)
• Route selection
▸ Routing table
▸ Default routes
• Address pool exhaustion
• Incorrect default gateway
• Incorrect IP address
▸ Duplicate IP address
• Incorrect subnet mask

Switching Issues – Spanning Tree Protocol (STP)

• Prevents Layer 2 network loops
• Blocks redundant paths
• Elects a root bridge
• Critical for switched networks (exam)

STP – Network Loops

• Occur when redundant paths exist
• Cause broadcast storms
• Lead to MAC table instability
• Prevented by STP

STP – Root Bridge Selection

• Switch with lowest bridge ID becomes root
• Bridge ID = priority + MAC address
• Root bridge controls STP topology
• Lower priority preferred (exam tip)

STP – Port Roles

• Root port: best path to root bridge
• Designated port: forwards traffic for segment
• Blocking port: prevents loops
• Role based on STP calculations

STP – Port States

• Blocking: no frame forwarding
• Listening: prepares to forward
• Learning: builds MAC table
• Forwarding: passes traffic
• Disabled: administratively down

Switching Issues – Incorrect VLAN Assignment

• Device placed in wrong VLAN
• Causes loss of network access
• Common configuration mistake
• Verify access vs. trunk ports

Switching Issues – ACLs

• Filters traffic based on rules
• Can block valid communication
• Order of rules matters
• Implicit deny at end (exam)

Route Selection – Routing Table

• Stores known network paths
• Used to forward packets
• Longest prefix match preferred
• Dynamic or static entries

Route Selection – Default Routes

• Used when no specific route exists
• Represented as 0.0.0.0/0
• Often points to ISP or gateway
• Misconfiguration causes loss of connectivity

Address Pool Exhaustion

• DHCP scope has no available IPs
• New devices fail to obtain address
• Caused by small scope or long leases
• Results in APIPA assignment

Incorrect Default Gateway

• Gateway directs traffic outside subnet
• Wrong gateway prevents internet access
• Local communication may still work
• Common end-user issue

Incorrect IP Address

• IP does not match network range
• Prevents proper communication
• Often caused by manual configuration errors

Incorrect IP – Duplicate IP Address

• Two devices share same IP
• Causes intermittent connectivity
• Detected via ARP conflicts
• DHCP reservations help prevent this

Incorrect Subnet Mask

• Defines network and host portions
• Wrong mask breaks routing
• Devices think they are on wrong network
• Common subnetting error (exam)

N10-009 Sample Questions – Switching & IP Issues

• Q: What protocol prevents Layer 2 loops?

▸ A: Spanning Tree Protocol (STP)

• Q: Which STP port forwards traffic toward the root bridge?

▸ A: Root port

• Q: What happens when a DHCP scope runs out of IPs?

▸ A: Address pool exhaustion

• Q: What routing entry is used when no specific route matches?

▸ A: Default route