Chapter 47: Switching, VLAN, Routing, and IP Troubleshooting
This chapter follows the topics shown in the Networking chapter menu. Work through each section in order, then use the review questions to check recall and troubleshooting reasoning.
47.1 Client IP Configuration
Client IP Configuration is one of the core topics in Switching, VLAN, Routing, and IP Troubleshooting. Understand what the term represents, where it operates in the network, what information it uses, and what observable behavior confirms that it is working correctly.
Scenario: a user reports a problem related to Client IP Configuration. Start by writing the exact symptom, affected users, first known failure time, and recent changes. Then choose the smallest safe test that can prove or disprove one cause.
What to check
- Confirm the symptom and determine whether the problem affects one host, one segment, one site, or many sites.
- Compare actual configuration and measurements with the intended design, baseline, or documentation.
- Change one variable at a time, verify the result, and document both the cause and the final fix.
47.2 Wrong IP Address
Wrong IP Address is a troubleshooting condition. The useful approach is to confirm symptoms, determine scope, identify the relevant layer, compare actual values with the intended design, test one theory at a time, and verify service after the fix.
Scenario: a user reports a problem related to Wrong IP Address. Start by writing the exact symptom, affected users, first known failure time, and recent changes. Then choose the smallest safe test that can prove or disprove one cause.
What to check
- Confirm the symptom and determine whether the problem affects one host, one segment, one site, or many sites.
- Compare actual configuration and measurements with the intended design, baseline, or documentation.
- Change one variable at a time, verify the result, and document both the cause and the final fix.
Command or data example
ip addr47.3 Wrong Subnet Mask
An IPv4 subnet mask identifies which address bits belong to the network prefix and which bits are available for host addressing.
Scenario: a user reports a problem related to Wrong Subnet Mask. Start by writing the exact symptom, affected users, first known failure time, and recent changes. Then choose the smallest safe test that can prove or disprove one cause.
What to check
- Confirm the symptom and determine whether the problem affects one host, one segment, one site, or many sites.
- Compare actual configuration and measurements with the intended design, baseline, or documentation.
- Change one variable at a time, verify the result, and document both the cause and the final fix.
47.4 Wrong Gateway
Wrong Gateway is a troubleshooting condition. The useful approach is to confirm symptoms, determine scope, identify the relevant layer, compare actual values with the intended design, test one theory at a time, and verify service after the fix.
Scenario: a user reports a problem related to Wrong Gateway. Start by writing the exact symptom, affected users, first known failure time, and recent changes. Then choose the smallest safe test that can prove or disprove one cause.
What to check
- Confirm the symptom and determine whether the problem affects one host, one segment, one site, or many sites.
- Compare actual configuration and measurements with the intended design, baseline, or documentation.
- Change one variable at a time, verify the result, and document both the cause and the final fix.
47.5 Missing Gateway
Missing Gateway is one of the core topics in Switching, VLAN, Routing, and IP Troubleshooting. Understand what the term represents, where it operates in the network, what information it uses, and what observable behavior confirms that it is working correctly.
Scenario: a user reports a problem related to Missing Gateway. Start by writing the exact symptom, affected users, first known failure time, and recent changes. Then choose the smallest safe test that can prove or disprove one cause.
What to check
- Confirm the symptom and determine whether the problem affects one host, one segment, one site, or many sites.
- Compare actual configuration and measurements with the intended design, baseline, or documentation.
- Change one variable at a time, verify the result, and document both the cause and the final fix.
47.6 Duplicate IP
Duplicate IP is one of the core topics in Switching, VLAN, Routing, and IP Troubleshooting. Understand what the term represents, where it operates in the network, what information it uses, and what observable behavior confirms that it is working correctly.
Scenario: a user reports a problem related to Duplicate IP. Start by writing the exact symptom, affected users, first known failure time, and recent changes. Then choose the smallest safe test that can prove or disprove one cause.
What to check
- Confirm the symptom and determine whether the problem affects one host, one segment, one site, or many sites.
- Compare actual configuration and measurements with the intended design, baseline, or documentation.
- Change one variable at a time, verify the result, and document both the cause and the final fix.
47.7 APIPA
APIPA is the common name for IPv4 link-local self-assignment in 169.254.0.0/16 when a host cannot obtain normal DHCP configuration.
Scenario: a user reports a problem related to APIPA. Start by writing the exact symptom, affected users, first known failure time, and recent changes. Then choose the smallest safe test that can prove or disprove one cause.
What to check
- Confirm the symptom and determine whether the problem affects one host, one segment, one site, or many sites.
- Compare actual configuration and measurements with the intended design, baseline, or documentation.
- Change one variable at a time, verify the result, and document both the cause and the final fix.
Command or data example
GET /api/v1/interfaces
Accept: application/json47.8 DHCP Scope Exhaustion
DHCP automatically supplies IP configuration such as an address, mask or prefix, gateway, DNS servers, and lease information. IPv4 clients commonly use the Discover, Offer, Request, Acknowledge exchange.
Example: a laptop joins a LAN with no manual IP configuration. It broadcasts or multicasts the appropriate discovery traffic, receives an offer, requests the selected lease, and then applies the address, gateway, DNS settings, and lease timers.
What to check
- Confirm the symptom and determine whether the problem affects one host, one segment, one site, or many sites.
- Compare actual configuration and measurements with the intended design, baseline, or documentation.
- Change one variable at a time, verify the result, and document both the cause and the final fix.
47.9 Wrong DHCP Scope
DHCP automatically supplies IP configuration such as an address, mask or prefix, gateway, DNS servers, and lease information. IPv4 clients commonly use the Discover, Offer, Request, Acknowledge exchange.
Example: a laptop joins a LAN with no manual IP configuration. It broadcasts or multicasts the appropriate discovery traffic, receives an offer, requests the selected lease, and then applies the address, gateway, DNS settings, and lease timers.
What to check
- Confirm the symptom and determine whether the problem affects one host, one segment, one site, or many sites.
- Compare actual configuration and measurements with the intended design, baseline, or documentation.
- Change one variable at a time, verify the result, and document both the cause and the final fix.
47.10 DHCP Relay Failure
DHCP automatically supplies IP configuration such as an address, mask or prefix, gateway, DNS servers, and lease information. IPv4 clients commonly use the Discover, Offer, Request, Acknowledge exchange.
Example: a laptop joins a LAN with no manual IP configuration. It broadcasts or multicasts the appropriate discovery traffic, receives an offer, requests the selected lease, and then applies the address, gateway, DNS settings, and lease timers.
What to check
- Confirm the symptom and determine whether the problem affects one host, one segment, one site, or many sites.
- Compare actual configuration and measurements with the intended design, baseline, or documentation.
- Change one variable at a time, verify the result, and document both the cause and the final fix.
47.11 VLAN Problems
A VLAN creates a logical Layer 2 broadcast domain on switching infrastructure. Traffic between VLANs requires a Layer 3 forwarding function.
Example: users in VLAN 20 can communicate with one another through switches, but reaching VLAN 30 requires a Layer 3 gateway. If one trunk omits VLAN 20, only paths crossing that trunk fail.
What to check
- Confirm the symptom and determine whether the problem affects one host, one segment, one site, or many sites.
- Compare actual configuration and measurements with the intended design, baseline, or documentation.
- Change one variable at a time, verify the result, and document both the cause and the final fix.
47.12 Wrong Access VLAN
A VLAN creates a logical Layer 2 broadcast domain on switching infrastructure. Traffic between VLANs requires a Layer 3 forwarding function.
Example: users in VLAN 20 can communicate with one another through switches, but reaching VLAN 30 requires a Layer 3 gateway. If one trunk omits VLAN 20, only paths crossing that trunk fail.
What to check
- Confirm the symptom and determine whether the problem affects one host, one segment, one site, or many sites.
- Compare actual configuration and measurements with the intended design, baseline, or documentation.
- Change one variable at a time, verify the result, and document both the cause and the final fix.
47.13 Access Port Problems
Access Port Problems is one of the core topics in Switching, VLAN, Routing, and IP Troubleshooting. Understand what the term represents, where it operates in the network, what information it uses, and what observable behavior confirms that it is working correctly.
Scenario: a user reports a problem related to Access Port Problems. Start by writing the exact symptom, affected users, first known failure time, and recent changes. Then choose the smallest safe test that can prove or disprove one cause.
What to check
- Confirm the symptom and determine whether the problem affects one host, one segment, one site, or many sites.
- Compare actual configuration and measurements with the intended design, baseline, or documentation.
- Change one variable at a time, verify the result, and document both the cause and the final fix.
47.14 Trunk Problems
Trunk Problems is one of the core topics in Switching, VLAN, Routing, and IP Troubleshooting. Understand what the term represents, where it operates in the network, what information it uses, and what observable behavior confirms that it is working correctly.
Scenario: a user reports a problem related to Trunk Problems. Start by writing the exact symptom, affected users, first known failure time, and recent changes. Then choose the smallest safe test that can prove or disprove one cause.
What to check
- Confirm the symptom and determine whether the problem affects one host, one segment, one site, or many sites.
- Compare actual configuration and measurements with the intended design, baseline, or documentation.
- Change one variable at a time, verify the result, and document both the cause and the final fix.
47.15 802.1Q Problems
IEEE 802.1Q tagging inserts VLAN information into Ethernet frames on links that carry multiple VLANs. A native VLAN may be sent untagged depending on the design.
Scenario: a user reports a problem related to 802.1Q Problems. Start by writing the exact symptom, affected users, first known failure time, and recent changes. Then choose the smallest safe test that can prove or disprove one cause.
What to check
- Confirm the symptom and determine whether the problem affects one host, one segment, one site, or many sites.
- Compare actual configuration and measurements with the intended design, baseline, or documentation.
- Change one variable at a time, verify the result, and document both the cause and the final fix.
47.16 Native VLAN Mismatch
NAT changes IP address information as traffic crosses a translation boundary. PAT is a many-to-one form that also distinguishes conversations by transport-layer port numbers.
Example: users in VLAN 20 can communicate with one another through switches, but reaching VLAN 30 requires a Layer 3 gateway. If one trunk omits VLAN 20, only paths crossing that trunk fail.
What to check
- Confirm the symptom and determine whether the problem affects one host, one segment, one site, or many sites.
- Compare actual configuration and measurements with the intended design, baseline, or documentation.
- Change one variable at a time, verify the result, and document both the cause and the final fix.
47.17 Allowed VLAN Problems
A VLAN creates a logical Layer 2 broadcast domain on switching infrastructure. Traffic between VLANs requires a Layer 3 forwarding function.
Example: users in VLAN 20 can communicate with one another through switches, but reaching VLAN 30 requires a Layer 3 gateway. If one trunk omits VLAN 20, only paths crossing that trunk fail.
What to check
- Confirm the symptom and determine whether the problem affects one host, one segment, one site, or many sites.
- Compare actual configuration and measurements with the intended design, baseline, or documentation.
- Change one variable at a time, verify the result, and document both the cause and the final fix.
47.18 Missing VLAN
A VLAN creates a logical Layer 2 broadcast domain on switching infrastructure. Traffic between VLANs requires a Layer 3 forwarding function.
Example: users in VLAN 20 can communicate with one another through switches, but reaching VLAN 30 requires a Layer 3 gateway. If one trunk omits VLAN 20, only paths crossing that trunk fail.
What to check
- Confirm the symptom and determine whether the problem affects one host, one segment, one site, or many sites.
- Compare actual configuration and measurements with the intended design, baseline, or documentation.
- Change one variable at a time, verify the result, and document both the cause and the final fix.
47.19 Inter-VLAN Routing
A VLAN creates a logical Layer 2 broadcast domain on switching infrastructure. Traffic between VLANs requires a Layer 3 forwarding function.
Example: users in VLAN 20 can communicate with one another through switches, but reaching VLAN 30 requires a Layer 3 gateway. If one trunk omits VLAN 20, only paths crossing that trunk fail.
What to check
- Confirm the symptom and determine whether the problem affects one host, one segment, one site, or many sites.
- Compare actual configuration and measurements with the intended design, baseline, or documentation.
- Change one variable at a time, verify the result, and document both the cause and the final fix.
47.20 SVI Problems
SVI Problems is one of the core topics in Switching, VLAN, Routing, and IP Troubleshooting. Understand what the term represents, where it operates in the network, what information it uses, and what observable behavior confirms that it is working correctly.
Scenario: a user reports a problem related to SVI Problems. Start by writing the exact symptom, affected users, first known failure time, and recent changes. Then choose the smallest safe test that can prove or disprove one cause.
What to check
- Confirm the symptom and determine whether the problem affects one host, one segment, one site, or many sites.
- Compare actual configuration and measurements with the intended design, baseline, or documentation.
- Change one variable at a time, verify the result, and document both the cause and the final fix.
47.21 STP Problems
STP prevents persistent Ethernet loops by placing selected redundant paths into non-forwarding states and recalculating after topology changes.
Scenario: a user reports a problem related to STP Problems. Start by writing the exact symptom, affected users, first known failure time, and recent changes. Then choose the smallest safe test that can prove or disprove one cause.
What to check
- Confirm the symptom and determine whether the problem affects one host, one segment, one site, or many sites.
- Compare actual configuration and measurements with the intended design, baseline, or documentation.
- Change one variable at a time, verify the result, and document both the cause and the final fix.
47.22 Broadcast Storms
Broadcast Storms concerns traffic delivered to every member of a Layer 2 or IP broadcast scope. Broadcast behavior matters because excessive or unintended broadcasts can consume shared capacity and reveal segmentation problems.
Scenario: a user reports a problem related to Broadcast Storms. Start by writing the exact symptom, affected users, first known failure time, and recent changes. Then choose the smallest safe test that can prove or disprove one cause.
What to check
- Confirm the symptom and determine whether the problem affects one host, one segment, one site, or many sites.
- Compare actual configuration and measurements with the intended design, baseline, or documentation.
- Change one variable at a time, verify the result, and document both the cause and the final fix.
47.23 MAC Instability
MAC Instability is a Layer 2 concept involving Ethernet hardware addressing or switch forwarding state. Switches learn source MAC addresses and use destination MAC information to choose whether to forward, flood, or filter frames.
Scenario: a user reports a problem related to MAC Instability. Start by writing the exact symptom, affected users, first known failure time, and recent changes. Then choose the smallest safe test that can prove or disprove one cause.
What to check
- Confirm the symptom and determine whether the problem affects one host, one segment, one site, or many sites.
- Compare actual configuration and measurements with the intended design, baseline, or documentation.
- Change one variable at a time, verify the result, and document both the cause and the final fix.
47.24 BPDU Guard
BPDU Guard is one of the core topics in Switching, VLAN, Routing, and IP Troubleshooting. Understand what the term represents, where it operates in the network, what information it uses, and what observable behavior confirms that it is working correctly.
Scenario: a user reports a problem related to BPDU Guard. Start by writing the exact symptom, affected users, first known failure time, and recent changes. Then choose the smallest safe test that can prove or disprove one cause.
What to check
- Confirm the symptom and determine whether the problem affects one host, one segment, one site, or many sites.
- Compare actual configuration and measurements with the intended design, baseline, or documentation.
- Change one variable at a time, verify the result, and document both the cause and the final fix.
47.25 Missing Route
Missing Route affects how Layer 3 devices choose a path toward destination networks. Correct routing depends on the destination prefix, route source, next hop or exit interface, route preference, metric, and reachability of the next step.
Example: a router receives a packet for 10.20.30.40 and has several matching routes. It selects the most specific matching prefix, then forwards toward the route's next hop or exit interface if that path is usable.
What to check
- Confirm the symptom and determine whether the problem affects one host, one segment, one site, or many sites.
- Compare actual configuration and measurements with the intended design, baseline, or documentation.
- Change one variable at a time, verify the result, and document both the cause and the final fix.
47.26 Incorrect Static Route
Incorrect Static Route affects how Layer 3 devices choose a path toward destination networks. Correct routing depends on the destination prefix, route source, next hop or exit interface, route preference, metric, and reachability of the next step.
Example: a router receives a packet for 10.20.30.40 and has several matching routes. It selects the most specific matching prefix, then forwards toward the route's next hop or exit interface if that path is usable.
What to check
- Confirm the symptom and determine whether the problem affects one host, one segment, one site, or many sites.
- Compare actual configuration and measurements with the intended design, baseline, or documentation.
- Change one variable at a time, verify the result, and document both the cause and the final fix.
47.27 Default Route
A default route is the least-specific route and is used when no more-specific destination prefix matches.
Example: a router receives a packet for 10.20.30.40 and has several matching routes. It selects the most specific matching prefix, then forwards toward the route's next hop or exit interface if that path is usable.
What to check
- Confirm the symptom and determine whether the problem affects one host, one segment, one site, or many sites.
- Compare actual configuration and measurements with the intended design, baseline, or documentation.
- Change one variable at a time, verify the result, and document both the cause and the final fix.
47.28 Longest Prefix Match
Longest-prefix match chooses the route whose prefix contains the destination and has the greatest number of matching leading bits.
Scenario: a user reports a problem related to Longest Prefix Match. Start by writing the exact symptom, affected users, first known failure time, and recent changes. Then choose the smallest safe test that can prove or disprove one cause.
What to check
- Confirm the symptom and determine whether the problem affects one host, one segment, one site, or many sites.
- Compare actual configuration and measurements with the intended design, baseline, or documentation.
- Change one variable at a time, verify the result, and document both the cause and the final fix.
47.29 Routing Loops
Routing Loops affects how Layer 3 devices choose a path toward destination networks. Correct routing depends on the destination prefix, route source, next hop or exit interface, route preference, metric, and reachability of the next step.
Example: a router receives a packet for 10.20.30.40 and has several matching routes. It selects the most specific matching prefix, then forwards toward the route's next hop or exit interface if that path is usable.
What to check
- Confirm the symptom and determine whether the problem affects one host, one segment, one site, or many sites.
- Compare actual configuration and measurements with the intended design, baseline, or documentation.
- Change one variable at a time, verify the result, and document both the cause and the final fix.
47.30 Black Holes
Black Holes is one of the core topics in Switching, VLAN, Routing, and IP Troubleshooting. Understand what the term represents, where it operates in the network, what information it uses, and what observable behavior confirms that it is working correctly.
Scenario: a user reports a problem related to Black Holes. Start by writing the exact symptom, affected users, first known failure time, and recent changes. Then choose the smallest safe test that can prove or disprove one cause.
What to check
- Confirm the symptom and determine whether the problem affects one host, one segment, one site, or many sites.
- Compare actual configuration and measurements with the intended design, baseline, or documentation.
- Change one variable at a time, verify the result, and document both the cause and the final fix.
47.31 OSPF Neighbor Failure
OSPF is a link-state interior routing protocol. Routers exchange topology information, calculate shortest paths, and converge when the network changes.
Scenario: a user reports a problem related to OSPF Neighbor Failure. Start by writing the exact symptom, affected users, first known failure time, and recent changes. Then choose the smallest safe test that can prove or disprove one cause.
What to check
- Confirm the symptom and determine whether the problem affects one host, one segment, one site, or many sites.
- Compare actual configuration and measurements with the intended design, baseline, or documentation.
- Change one variable at a time, verify the result, and document both the cause and the final fix.
47.32 ACL Problems
ACL Problems is one of the core topics in Switching, VLAN, Routing, and IP Troubleshooting. Understand what the term represents, where it operates in the network, what information it uses, and what observable behavior confirms that it is working correctly.
Scenario: a user reports a problem related to ACL Problems. Start by writing the exact symptom, affected users, first known failure time, and recent changes. Then choose the smallest safe test that can prove or disprove one cause.
What to check
- Confirm the symptom and determine whether the problem affects one host, one segment, one site, or many sites.
- Compare actual configuration and measurements with the intended design, baseline, or documentation.
- Change one variable at a time, verify the result, and document both the cause and the final fix.
47.33 NAT Problems
NAT changes IP address information as traffic crosses a translation boundary. PAT is a many-to-one form that also distinguishes conversations by transport-layer port numbers.
Scenario: a user reports a problem related to NAT Problems. Start by writing the exact symptom, affected users, first known failure time, and recent changes. Then choose the smallest safe test that can prove or disprove one cause.
What to check
- Confirm the symptom and determine whether the problem affects one host, one segment, one site, or many sites.
- Compare actual configuration and measurements with the intended design, baseline, or documentation.
- Change one variable at a time, verify the result, and document both the cause and the final fix.
47.34 MTU Problems
MTU Problems is one of the core topics in Switching, VLAN, Routing, and IP Troubleshooting. Understand what the term represents, where it operates in the network, what information it uses, and what observable behavior confirms that it is working correctly.
Scenario: a user reports a problem related to MTU Problems. Start by writing the exact symptom, affected users, first known failure time, and recent changes. Then choose the smallest safe test that can prove or disprove one cause.
What to check
- Confirm the symptom and determine whether the problem affects one host, one segment, one site, or many sites.
- Compare actual configuration and measurements with the intended design, baseline, or documentation.
- Change one variable at a time, verify the result, and document both the cause and the final fix.
47.35 Jumbo Frames
Jumbo Frames is one of the core topics in Switching, VLAN, Routing, and IP Troubleshooting. Understand what the term represents, where it operates in the network, what information it uses, and what observable behavior confirms that it is working correctly.
Scenario: a user reports a problem related to Jumbo Frames. Start by writing the exact symptom, affected users, first known failure time, and recent changes. Then choose the smallest safe test that can prove or disprove one cause.
What to check
- Confirm the symptom and determine whether the problem affects one host, one segment, one site, or many sites.
- Compare actual configuration and measurements with the intended design, baseline, or documentation.
- Change one variable at a time, verify the result, and document both the cause and the final fix.
47.36 DNS vs Routing
DNS translates names into resource records such as IP addresses, aliases, mail-routing information, and service data. Client caching and TTL values affect how quickly changes become visible.
Example: a user can reach 203.0.113.20 but cannot reach server.example by name. That difference points toward name resolution, DNS reachability, record content, cache state, or search-suffix behavior rather than basic IP routing.
What to check
- Confirm the symptom and determine whether the problem affects one host, one segment, one site, or many sites.
- Compare actual configuration and measurements with the intended design, baseline, or documentation.
- Change one variable at a time, verify the result, and document both the cause and the final fix.
47.37 Asymmetric Routing
Asymmetric Routing affects how Layer 3 devices choose a path toward destination networks. Correct routing depends on the destination prefix, route source, next hop or exit interface, route preference, metric, and reachability of the next step.
Example: a router receives a packet for 10.20.30.40 and has several matching routes. It selects the most specific matching prefix, then forwards toward the route's next hop or exit interface if that path is usable.
What to check
- Confirm the symptom and determine whether the problem affects one host, one segment, one site, or many sites.
- Compare actual configuration and measurements with the intended design, baseline, or documentation.
- Change one variable at a time, verify the result, and document both the cause and the final fix.
47.38 Gateway Redundancy
Gateway Redundancy is one of the core topics in Switching, VLAN, Routing, and IP Troubleshooting. Understand what the term represents, where it operates in the network, what information it uses, and what observable behavior confirms that it is working correctly.
Scenario: a user reports a problem related to Gateway Redundancy. Start by writing the exact symptom, affected users, first known failure time, and recent changes. Then choose the smallest safe test that can prove or disprove one cause.
What to check
- Confirm the symptom and determine whether the problem affects one host, one segment, one site, or many sites.
- Compare actual configuration and measurements with the intended design, baseline, or documentation.
- Change one variable at a time, verify the result, and document both the cause and the final fix.
47.39 Scope-Based Troubleshooting
Scope-Based Troubleshooting is a troubleshooting condition. The useful approach is to confirm symptoms, determine scope, identify the relevant layer, compare actual values with the intended design, test one theory at a time, and verify service after the fix.
Scenario: a user reports a problem related to Scope-Based Troubleshooting. Start by writing the exact symptom, affected users, first known failure time, and recent changes. Then choose the smallest safe test that can prove or disprove one cause.
What to check
- Confirm the symptom and determine whether the problem affects one host, one segment, one site, or many sites.
- Compare actual configuration and measurements with the intended design, baseline, or documentation.
- Change one variable at a time, verify the result, and document both the cause and the final fix.
47.40 End-to-End Troubleshooting
End-to-End Troubleshooting is a troubleshooting condition. The useful approach is to confirm symptoms, determine scope, identify the relevant layer, compare actual values with the intended design, test one theory at a time, and verify service after the fix.
Scenario: a user reports a problem related to End-to-End Troubleshooting. Start by writing the exact symptom, affected users, first known failure time, and recent changes. Then choose the smallest safe test that can prove or disprove one cause.
What to check
- Confirm the symptom and determine whether the problem affects one host, one segment, one site, or many sites.
- Compare actual configuration and measurements with the intended design, baseline, or documentation.
- Change one variable at a time, verify the result, and document both the cause and the final fix.