Network Services Issues — N10-009 Study Guide

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Diagnose by reachability: what works tells you the layer
Switching faults: wrong VLAN, spanning tree, and ACLs
Routing tables, default routes, and DHCP exhaustion
Exam-pattern recognition: match the symptom to the one fix

Network-services symptom to most likely cause to confirming check to fix

Symptom	Most likely cause	Confirming check	Fix
Host has a 169.254.x.x address	DHCP scope exhausted or server/relay down	Inspect DHCP scope utilization and reachability	Reclaim/widen scope, restore server, add IP helper
Reaches local subnet, no remote networks	Wrong or missing default gateway	Verify gateway address and that it answers	Set the correct default gateway
Partial, inconsistent remote reachability	Wrong subnet mask (bad local/remote split)	Compare host mask to the subnet's correct mask	Correct the subnet mask
Up port, link light, reaches nothing	Wrong VLAN assignment on the access port	Check the port's VLAN vs intended subnet	Reassign the port to the correct VLAN
Segment slow, broadcast storm	Switching loop / missing or broken STP	Confirm STP enabled, find the looped link	Enable STP, remove the loop
Legit traffic dropped, good IP and cable	ACL rule too broad or out of order	Review ACL order and the implicit deny	Reorder or scope the ACL rule
Remote network unreachable from a router	Missing route or wrong/absent default route	Trace where the path stops, read the table	Add or correct the route / default route
Intermittent drops, IP-conflict alerts	Duplicate IP address	Find the second host with the same address	Re-address one host

Decision tree

Cheat sheet

Diagnose service faults by the reachability pattern, not by guessing the device
A 169.254.x.x address means DHCP failed, so chase the DHCP path
DHCP scope exhaustion produces silence, never a DHCPNAK
A missing DHCP relay sends a whole remote subnet to APIPA
An APIPA host can ping its own subnet but nothing routed
A wrong VLAN isolates a host that passes every physical test
STP elects the lowest bridge ID as root; a change can leave a port blocking
A switching loop with no STP becomes a broadcast storm that saturates the segment
An ACL is evaluated top-down, first match wins, with an implicit deny at the end
A missing route drops only the destinations it lacks an entry for
A missing default route breaks reach to every network not explicitly known
A wrong next hop drops packets even though the route exists
Asymmetric routing can break stateful firewalls even when both paths work
A wrong default gateway loses all off-subnet traffic but keeps the local subnet
A wrong subnet mask gives partial, inconsistent reachability
A duplicate IP causes intermittent connectivity and conflict warnings
Keep static IPs out of the DHCP scope to avoid duplicate-address conflicts
STP topology changes flush MAC tables by shortening the aging timer
A native VLAN mismatch lands untagged frames in the wrong VLAN
An access port drops tagged frames, so multi-VLAN links need a trunk
A VLAN missing from the trunk's allowed list is pruned across switches
Inter-VLAN routing requires a Layer 3 interface per VLAN
PortFast on a switch-to-switch link can create a Layer 2 loop
RSTP collapses disabled, blocking, and listening into discarding
RSTP root, alternate, and backup ports have distinct redundancy roles
Classic STP listening and learning add about 30 seconds before forwarding

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References

IEEE 802.1D — Media Access Control (MAC) Bridges (Spanning Tree Protocol) Whitepaper
RFC 2131 — Dynamic Host Configuration Protocol Whitepaper
RFC 3927 — Dynamic Configuration of IPv4 Link-Local Addresses (APIPA) Whitepaper