Address families (IPv4, IPv6)

1.10 Troubleshoot OSPF (v2/v3)

📘CCNP Enterprise – ENARSI (300-410)

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1. What is an Address Family in OSPF?

An Address Family (AF) is basically a type of network protocol that OSPF can handle.

• OSPFv2 → supports IPv4 only
• OSPFv3 → supports IPv6, but can also support IPv4 (with enhancements)

In other words, think of address families as “different flavors” of IP addressing that OSPF can work with. Each flavor needs slightly different configuration and troubleshooting methods.

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2. OSPF Versions and Address Families

OSPF Version	Address Family	Notes
OSPFv2	IPv4	Most widely used; classic IPv4 routing
OSPFv3	IPv6	Originally only for IPv6, now can carry IPv4 too using address-family configuration
OSPFv3	IPv4	Supports multiple address families per process (RFC 5838)

Exam Tip: You must know the difference between OSPFv2 and OSPFv3 in terms of address families.

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3. Key Differences Between IPv4 and IPv6 in OSPF

Feature	OSPFv2 (IPv4)	OSPFv3 (IPv6)
Protocol	Runs directly on IPv4	Runs on IPv6 transport
LSA Types	Types 1-5 (Router, Network, Summary, ASBR, External)	Types 1-5, plus new LSAs for IPv6 (e.g., Link-LSA, Intra-Area-Prefix-LSA)
Authentication	Uses MD5 or plain-text	Uses IPsec for authentication (security built-in)
Addressing	IPv4 only	IPv6 only (OSPFv3) or multiple AFs (OSPFv3 with IPv4 AF)

Exam Tip: Remember that OSPFv3 was redesigned to support multiple address families, while OSPFv2 only supports IPv4.

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4. How OSPF Handles Address Families

1. OSPFv2
   • Each OSPF process handles one IPv4 routing domain.
   • If a network interface is missing in the OSPFv2 process, IPv4 routes through that interface won’t be advertised.
2. OSPFv3
   • Can handle IPv6 natively.
   • Can also handle IPv4 via “address-family IPv4 unicast” configuration.
   • Allows multiple address families on the same OSPFv3 process (useful for dual-stack networks).

Example IT context:

• A data center uses OSPFv3 to handle both IPv6 (internal backbone) and IPv4 (legacy servers) on the same router. Troubleshooting must consider which address family is misbehaving.

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5. Common OSPF Address Family Issues (Troubleshooting Focus)

1. Incorrect OSPF version used
   • IPv6 networks must use OSPFv3; IPv4 networks should use OSPFv2 (or OSPFv3 with IPv4 AF).
   • Symptoms: routes not showing up, neighbors not forming.
2. Missing or misconfigured address family
   • OSPFv3 can have multiple AFs; if IPv4 AF is missing, IPv4 routes won’t propagate.
3. Neighbor adjacency issues due to AF mismatch
   • OSPFv2 neighbors must be IPv4.
   • OSPFv3 neighbors must be IPv6.
   • Misconfiguring the address family can cause 2-way stuck or dead neighbor states.
4. Interface not participating in correct AF
   • Interface must be in the right OSPF area and address family.
   • Use commands like:
     • show ip ospf neighbor → IPv4
     • show ospfv3 ipv6 neighbor → IPv6
5. LSA/route missing due to AF misconfiguration
   • IPv4 routes might be missing in routing table if IPv4 AF is not configured under OSPFv3.
   • IPv6 prefixes might not advertise if ipv6 ospf isn’t enabled on interfaces.

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6. Exam-Focused Commands for OSPF Address Families

OSPFv2 (IPv4)

show ip ospf neighbor       # Shows IPv4 OSPF neighbor status
show ip ospf interface      # Shows IPv4 OSPF interfaces and area info
show ip route ospf          # Shows IPv4 routes learned via OSPF

OSPFv3 (IPv6)

show ospfv3 ipv6 neighbor   # IPv6 neighbor info
show ospfv3 ipv6 interface  # IPv6 interface and area info
show ipv6 route ospf         # IPv6 routes learned via OSPFv3

OSPFv3 with IPv4 AF

show ospfv3 ipv4 neighbor    # IPv4 neighbors under OSPFv3 AF
show ospfv3 ipv4 route       # IPv4 routes via OSPFv3

Exam Tip: Commands differ based on AF and OSPF version. Know the difference!

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7. Key Takeaways for the Exam

1. OSPF address family = type of IP network (IPv4 or IPv6).
2. OSPFv2 → IPv4 only, OSPFv3 → IPv6 and optionally IPv4.
3. Misconfigured AF = routes not advertised, neighbors not formed, LSAs missing.
4. Troubleshooting requires checking:
   • Correct OSPF version
   • Interfaces in correct AF
   • Neighbors are forming in the correct AF
   • LSAs/routes appear in the routing table
5. Remember exam commands differ based on AF (IPv4 vs IPv6).

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✅ Simplified IT Context Summary:

• IPv4 → classic OSPFv2.
• IPv6 → OSPFv3.
• Dual-stack or IPv4 under OSPFv3 → must configure address-family IPv4.
• Troubleshooting is about checking neighbors, interface participation, and LSAs per address family.