Address families (IPv4, IPv6)

1.9 Troubleshoot EIGRP (classic and named mode; VRF and global)

📘CCNP Enterprise – ENARSI (300-410)

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EIGRP (Enhanced Interior Gateway Routing Protocol) is a dynamic routing protocol used by routers to share routing information within an autonomous system (AS). In Cisco CCNP ENARSI, you need to understand address families in EIGRP because they determine which types of routes EIGRP will manage (IPv4, IPv6, or others like VPN/VRF routes).

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1. What Are Address Families?

• An address family tells EIGRP which network layer protocol it should work with.
• EIGRP supports multiple network protocols, primarily:
  • IPv4 – the most common addressing scheme in networks.
  • IPv6 – the newer protocol with larger address space and simplified header structure.

Think of address families as different “buckets” of routing information that EIGRP keeps separate.

• When configuring EIGRP, you must specify the address family so the router knows what kind of routes to advertise and manage.

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2. Classic Mode vs Named Mode

EIGRP has two configuration modes:

Classic Mode

• Uses separate commands for IPv4 and IPv6.
• Example:
  • router eigrp 100 → only for IPv4.
  • ipv6 router eigrp 100 → only for IPv6.
• Limitations:
  • IPv4 and IPv6 are configured separately.
  • Harder to manage when multiple address families or VRFs are involved.

Named Mode

• Introduced to unify IPv4 and IPv6 under one EIGRP process.
• Command: router eigrp NAME address-family ipv4 unicast address-family ipv6 unicast
• Advantages:
  • Single EIGRP process for both IPv4 and IPv6.
  • Easier to apply VRFs or other advanced features.
  • Better control over which features apply to which address family.

Exam tip: For CCNP ENARSI, you should know the differences between classic and named mode, especially the configuration syntax and which mode is recommended for IPv6 or multiple VRFs.

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3. IPv4 Address Family

• IPv4 is the default network layer in EIGRP.
• Features:
  • Advertises IPv4 unicast routes only.
  • Supports both classic mode and named mode.
  • Can use network statements to specify which interfaces participate in EIGRP: router eigrp 100 network 192.168.1.0 0.0.0.255
• Key tasks when troubleshooting IPv4:
  • Check neighbors: show ip eigrp neighbors
  • Check routes: show ip route eigrp
  • Check topology table: show ip eigrp topology

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4. IPv6 Address Family

• IPv6 is fully supported in EIGRP, but configuration is slightly different:

Classic Mode

ipv6 router eigrp 100
  no shutdown

• IPv6 routing must be enabled on each interface.
• Neighbor relationships are formed using link-local addresses (fe80::/10).
• Troubleshooting commands:
  • show ipv6 eigrp neighbors
  • show ipv6 route eigrp
  • show ipv6 eigrp topology

Named Mode

router eigrp NETWORK1
  address-family ipv6 unicast
    af-interface GigabitEthernet0/0
      no shutdown

• Same advantages as IPv4 named mode.
• Allows combining IPv4 and IPv6 routing in one process.

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5. Why Address Families Matter in Troubleshooting

When you are troubleshooting EIGRP, address families help you pinpoint the issue:

1. Neighbor issues
   • IPv4 and IPv6 neighbors are separate.
   • A router might see IPv4 neighbors but not IPv6 neighbors if configurations differ.
2. Routing problems
   • EIGRP maintains separate topology tables for each address family.
   • If a route is missing in IPv6 but exists in IPv4, the issue may be interface configuration or IPv6 EIGRP settings.
3. VRFs (Virtual Routing and Forwarding)
   • Address families are crucial when using VRFs, because each VRF can have its own IPv4 and IPv6 EIGRP process.
   • Troubleshooting without understanding the correct address family can be misleading.

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6. Key Commands to Remember

Task	IPv4 Classic	IPv6 Classic	Named Mode
Start EIGRP	router eigrp 100	ipv6 router eigrp 100	router eigrp NAME + address-family ipv4/ipv6 unicast
Show neighbors	show ip eigrp neighbors	show ipv6 eigrp neighbors	show eigrp NAME neighbors
Show routes	show ip route eigrp	show ipv6 route eigrp	show eigrp NAME route
Show topology	show ip eigrp topology	show ipv6 eigrp topology	show eigrp NAME topology

Exam tip: You might be asked to troubleshoot missing routes, neighbors, or EIGRP connectivity for a specific address family, so always check the right family.

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7. Summary

• Address families in EIGRP separate routing information for IPv4 and IPv6.
• Classic mode is older, separate processes for IPv4 and IPv6.
• Named mode unifies IPv4 and IPv6 into one process, easier for VRFs.
• Troubleshooting requires checking:
  • Neighbor relationships
  • Routing tables
  • Topology tables
  • Correct address family and interfaces
• Understanding this ensures you can identify why EIGRP is not advertising or receiving routes for IPv4 or IPv6.

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✅ Remember for the Exam:

1. Always identify which address family you are troubleshooting.
2. Know the difference between classic and named mode.
3. Use the correct show commands depending on IPv4 or IPv6.
4. VRFs use separate address families, so misconfiguration can cause routing issues.

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