Neighbor Discovery resolves local neighbors. Router advertisements can supply the default router; a DHCPv6 lease alone does not prove one exists.

Quick reference

Item	Meaning	Example / note
Global unicast	Globally scoped address	2001:db8::/32 is documentation only
Link-local	Valid on the local link	fe80::/10; identify the interface
Loopback	This host	::1/128
Unspecified	No assigned address	::/128
Multicast	Group delivery	ff00::/8; no IPv6 broadcast
LAN prefix	Common SLAAC design	/64; not every link must be /64

What to understand first

IPv6 has 128 bits, usually written as eight hexadecimal groups. Leading zeros may be omitted; one run of zero groups can be compressed with ::. Canonical text uses lowercase and the longest eligible run.

The prefix length identifies network bits. A /48 allocation contains 65,536 /64 prefixes. 2001:db8:1200:30::/64 and 2001:db8:1200:31::/64 are separate LANs.

ICMPv6 supports Neighbor Discovery and path-MTU feedback. IPv4-style blanket ICMP blocking can break IPv6 even when addresses and routes look correct.

Duplicate Address Detection, neighbor reachability, RA flags and address lifetimes are separate checks. Dual-stack applications may choose IPv6 even while IPv4 tests pass.

Commands and interpretation

Inspection commands are read-only unless explicitly labeled otherwise. Capture commands start collection; configuration-mode commits change device state.

Cisco IOS XE

Read-only EXEC

show ipv6 interface brief
show ipv6 neighbors
show ipv6 route

Inspect: Check address state, neighbor status and default route; inspect the specific interface for RA settings.

Junos

Read-only operational

show interfaces terse
show ipv6 neighbors
show route table inet6.0

Inspect: inet6.0 is the default IPv6 table. Use the appropriate instance table for tenant traffic.

Worked example · Illustrative, not a device capture

Illustrative dual-stack failure

Client: 2001:db8:1200:30::25/64
Neighbor: fe80::1 on the client link
IPv4 application: works
IPv6 default route: absent

The neighbor can resolve locally while the remote application still has no route. Check the RA/default-router path.

These are teaching observations, not captured device output. Do not infer a default route from an assigned address.

Troubleshooting sequence

Confirm whether the failing attempt used IPv4 or IPv6; check DNS A and AAAA answers.
Check address validity and DAD status, then the intended /64 and VLAN.
Inspect the link-local neighbor on the correct interface and whether an RA supplies a usable router.
Check the destination prefix and reverse route in the correct IPv6 table.
If small traffic works but transfers stall, review ICMPv6 Packet Too Big delivery and tunnel MTU.

Common mistakes

Using multiple :: compressions in one address.
Treating a link-local address as unique without its link/interface context.
Assuming IPv4 firewall rules, DNS results or tests also validate IPv6.

Acceptance checks

Both address families pass the intended application tests.
ND/RA behavior matches design and unwanted RAs are controlled.
Path-MTU feedback works and address lifetimes are documented.

Primary references

Vendor documentation and protocol specifications support this guide. The diagrams, scenarios and troubleshooting sequences are Wolex-authored.

IPv6 addressing & neighbor discovery