IP CONNECTIVITY (25%) — CHEAT SHEET

ROUTING TABLE ESSENTIALS

Routing Table Codes & Administrative Distance (AD)

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Longest prefix match: Most specific subnet mask wins, regardless of protocol. Equal-cost load balancing: Multiple routes with same AD + metric share traffic (default 4, max 16).

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OSPF FUNDAMENTALS

Neighbor Requirements (All 5 must match)

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OSPF Neighbor States (Progression)

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Stuck in 2-WAY? → Check for designated router (DR) on that segment — may be expected (point-to-point or no DR).

OSPF Configuration Checklist

`` router ospf network area passive-interface ! Stop hellos on this interface no passive-interface ! Enable hellos (undo passive) auto-cost reference-bandwidth ! Default 100 Mbps (fast eth = cost 1) `

Wildcard mask = inverse of subnet mask (e.g., /24 → 0.0.0.255). Network statement matches interface IP + announces that subnet into OSPF.

OSPF Cost Calculation

` Cost = Reference Bandwidth / Link Bandwidth Default: 100 Mbps / interface speed (in Mbps) Examples: • Fast Ethernet (100 Mbps) → 100/100 = 1 • Gigabit (1000 Mbps) → 100/1000 = 0 (rounds to 1, minimum) • Serial 1.544 Mbps → 100/1.544 = 64 `

Best practice: Set reference bandwidth = your fastest link (usually 10 Gbps).

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STATIC ROUTING

Static Route Types

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Next-hop choices: IP address (preferred for troubleshooting) OR outgoing interface (requires ARP on broadcast media).

IPv6 Static Route

` ipv6 route / Example: ipv6 route 2001:db8:2::/48 2001:db8:1::1 `

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FIRST-HOP REDUNDANCY (HSRP/VRRP/GLBP)

Quick Comparison

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HSRP priority range: 0–255 (default 100). Higher = preferred. Preemption: standby preempt` to reclaim active role.

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IPv4 TROUBLESHOOTING & PING

ICMP Echo Reply Codes

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Code	Protocol	AD	Decision Rule
C	Connected	0	Directly attached — always preferred
L	Local	0	Host route to interface IP itself
S	Static	1	Manual entry — preferred over dynamic
O	OSPF	110	Dynamic IGP — standard for multi-vendor
?	Unknown	—	Route not installed (AD too high or syntax error)
Parameter	Default	Mismatch = ?
Area ID	0 (backbone)	No adjacency
Hello interval	10 sec	No hello heard → stuck INIT
Dead interval	40 sec (4× hello)	Timeout → EXSTART
Network mask	Derived from config	Adjacency stalls
MTU size	1500 bytes	OSPF sync fails at DBD
State	Meaning	Action Needed
DOWN	No hellos heard	Check line protocol, passive interface, area ID
INIT	Got hello, no hello back yet	Wait or check hello/dead intervals
2-WAY	Bidirectional — router sees itself	Normal in point-to-multipoint; may be final state
EXSTART	DR/BDR negotiation starting	Check MTU, neighbor master/slave election
EXCHANGE	Swapping DBDs (database descriptions)	Allow 5–10 seconds; check MTU if stuck
LOADING	Requesting LSAs	Neighbors syncing databases
FULL	✅ Converged	Normal steady state
Type	Syntax	Use Case
Network static	ip route	Specific remote subnet
Default static	ip route 0.0.0.0 0.0.0.0	Route all unknown traffic
Host static	ip route 255.255.255.255	Single device (rarely used)
Floating static	ip route ...	Backup (AD > primary route)
Feature	HSRP	VRRP	GLBP
Virtual IP ownership	Active router	Master (VRRP)	AVG; others backup
Virtual MAC	0000.0c07.acXX	0000.5e00.01XX	0007.b400.XXYY
Preemption default	Disabled	Enabled	Disabled
Load balancing	No	No	Yes (up to 4 AVG)
Cisco/Vendor	Cisco proprietary	Standard (RFC)	Cisco proprietary
Code	Meaning	Troubleshoot
0 ✅	Reply OK	Path works end-to-end
1	Destination unreachable (host)	Host down or filtering
2	Protocol unreachable	Protocol not running
3	Port unreachable	Service not listening
11	Time exceeded (TTL=0)