First, the essential ideas
- Interface, link and MAC address
- An interface is a network connection. A link joins neighbors without an intervening router. A MAC address is an Ethernet interface identifier used for local frame delivery. Our lab has one link and two interfaces.
- Packet and frame
- An IPv6 packet carries source and destination IPv6 addresses. An Ethernet frame wraps a packet for local delivery using MAC addresses. Finding the next neighbor is not the same task as choosing an IP route.
- Prefix and interface identifier
- A prefix supplies the leading address bits. An interface identifier, or IID, supplies the remaining interface-specific bits in this /64 exercise. A /64 leaves 64 of the 128 bits for the IID.
- Host and default gateway
- A host originates or receives traffic. Its default gateway is a local router used when no more specific route matches. Router2 acts as the address-learning endpoint here; it is Cisco software, not a model of every desktop operating system.
- SLAAC and modified EUI-64
- Stateless Address Autoconfiguration, or SLAAC, lets an interface form an address using advertised information. Modified EUI-64 is one way to derive a 64-bit IID from a 48-bit MAC address; it is not synonymous with SLAAC.
- ICMPv6 and Neighbor Discovery
- ICMPv6 carries control messages for IPv6. Neighbor Discovery, or ND, uses several of these messages to discover routers, learn local neighbors and check reachability. IPv6 does not use IPv4 ARP for these jobs.
- RS, RA, NS and NA
- Router Solicitation asks routers for information; Router Advertisement supplies it. Neighbor Solicitation asks about a particular neighbor or checks an address; Neighbor Advertisement responds in defined situations. These are different messages, not four names for a ping.
- Duplicate Address Detection
- Duplicate Address Detection, or DAD, checks a tentative address before normal use. Tentative means not yet ready for ordinary traffic. A successful check is not proof that no later configuration mistake can create a conflict.
- Flags and lifetimes
- A flag is a yes/no field. The prefix A flag permits autonomous address formation; L concerns whether a prefix is on-link. Lifetimes are timers. Address validity and default-router eligibility have separate timers.
- Console and command modes
- Console is a device text window. In GNS3, right-click the named node and choose Console before typing. Cisco prompts ending >, #, (config)# and (config-if)# indicate user, privileged, global and interface modes.
- Baseline and rollback
- A baseline records the original settings and observations. Rollback restores the intended settings after a controlled change. A retest repeats the original test and records real results rather than copied example output.
- Ping and neighbor cache
- Ping requests an ICMPv6 echo reply. A neighbor cache records local IPv6-to-link-layer information and reachability state. A cached entry is not an address assignment, a default route or proof that an application works.
An address, a route and a neighbor entry answer different questions. An address identifies this interface; a route chooses the next hop; a neighbor entry supports local delivery to that next hop. Learn these distinctions before touching the lab.
See the idea
Five topology comparisons
Learn the prefix. Form the address. Check the default router separately.
Read the prefix
Router1 advertises an eligible /64. The prefix A flag concerns address formation; it is not the same as the default-router lifetime.
What this model leaves out: This concept view omits packet timing, DNS and Internet delivery. The separate lab view contains Router1, Router2 and their only cable.
Full visual explanation and references
An address is not a gateway
Four evidence panels separate the advertised /64, locally formed IID, resulting address and learned default router. These are configuration records, not a wired topology.
This concept view omits packet timing, DNS and Internet delivery. The separate lab view contains Router1, Router2 and their only cable.
- Read the prefix
Router1 advertises an eligible /64. The prefix A flag concerns address formation; it is not the same as the default-router lifetime.
www.rfc-editor.orgwww.rfc-editor.org - Form and check an address
The receiving interface combines the prefix with its IID and performs duplicate-address checks. Modified EUI-64 is one IID method, not every modern host method.
www.rfc-editor.orgwww.rfc-editor.org - Verify the router too
Check the selected default router independently. A usable address, a neighbor entry and a peer ping cannot substitute for route evidence.
www.rfc-editor.orgwww.cisco.com
Router1 advertises. Router2 learns.
Router1 sends an advertisement over one isolated cable to Router2. Router2 tests Router1; replies return to Router2, the receiving endpoint.
Only two Cisco instances and one cable are simulated. DNS, DHCPv6 servers, Internet services and desktop privacy behavior are omitted.
- Advertise to Router2
Router1 advertises the /64 from its link-local address. The message reaches Router2 across the only cable; no external network is attached.
www.rfc-editor.orgwww.cisco.com - Inspect Router2
Router2 forms its address, checks it and learns a default router. Inspect these records separately; the animation is not a captured exchange.
www.rfc-editor.orgwww.cisco.com - Test and restore
Router2 tests Router1. After removing and restoring client autoconfiguration, repeat the same test; a reply returns to Router2, not an Internet server.
www.cisco.comwww.cisco.com
One idea at a time
Start with something familiar.
Read the definitions first. You can complete the address worksheet without installing anything. For the lab, commands go in the named GNS3 device Console, not your laptop terminal. Check the device name and prompt before every block. The expected results below are predictions for you to verify, not output recorded by QCS.
Why learn this?
A device can display an IPv6 address while lacking a usable default router. It can also have a neighbor entry without the address an application expects. Checking these facts individually prevents random configuration changes and makes your explanation useful to another engineer.
An everyday comparison
As a memory aid, imagine receiving a neighborhood name, choosing your own door label, and separately learning the exit road. This is an organizational metaphor, not a literal postal system or a description of how devices think.
What happens in a network
The advertised prefix contributes network bits, the interface forms the IID, and router discovery identifies a possible next hop. Keep these as separate lines in a worksheet. The address and gateway need not share a similar-looking ending.
Where the comparison stopsReal IPv6 has flags, timers and duplicate checks; a neighborhood story has none of those rules. Use the actual protocol fields and measured device state rather than deriving behavior from the analogy.
Follow one worked example
- Write three evidence headings before configuring.
Create columns for interface address, default router and neighbor entry. Add time and device-name columns so a later screenshot cannot be mistaken for the other router.
Why: The worksheet separates facts that a single successful ping cannot independently establish.
- Read a prefix and calculate an example IID.
On paper, combine 2001:db8:14:1::/64 with the worked IID 0211:22ff:fe33:4455. Do not configure that fictional MAC address on a real adapter.
Why: The calculation makes the method visible while avoiding a false promise that every host uses MAC-derived addressing.
- Observe the Cisco client before making a fault.
Router2 should learn the advertised prefix, form its own address and learn a default route when configured with the default keyword. Record the actual IID rather than copying the paper example.
Why: An actual baseline gives you something precise to compare with the deliberately changed state.
- Restore automatic assignment and compare again.
After the isolated removal test, restore autoconfig default on Router2, wait for the advertised information and DAD, and inspect the address and route before repeating the peer ping.
Why: Recovery requires both configuration and observed state; entering an accepted command alone is not evidence of success.
Your first small task
On paper, start with MAC 00:11:22:33:44:55. Insert ff:fe between its two three-byte halves and invert the universal/local bit in the first byte. Group the resulting eight bytes into four hexadecimal groups, then attach the /64 prefix.
Show a hint
Inverting that bit is XOR with hexadecimal 02. For first byte 00 the result is 02; it is not a rule to add two to every possible first byte.
Check the expected result
The IID is 0211:22ff:fe33:4455. The example address is 2001:db8:14:1:211:22ff:fe33:4455; leading zeros within a group may be omitted.
Pause and explain it in your own words
A host has an address but no default route. Has SLAAC necessarily failed completely?
Show a hint
Separate the prefix A flag and address lifetimes from the RA router lifetime and default-router selection.
Show the explanation
No. Address formation and default-router discovery are distinct. Check each independently. A zero router lifetime can exclude an advertising router as a default while its prefix information remains useful. A local address also does not prove Internet access.
Short answer
What should you understand today?
SLAAC uses an advertised prefix and a locally formed interface identifier to create an IPv6 address. Neighbor Discovery also supplies router information and resolves local neighbors, but those are separate jobs. Modified EUI-64 is one IID method, not a requirement for every modern host. Verify the address, selected gateway and neighbor state separately before testing traffic.
Before you begin
Learning targets
- Complete Days 12 and 13 on IPv6 notation, /64 prefixes and address scope.
- Use GNS3 with two fresh authorized Cisco IOS 15.x or IOS XE 16/17.x router instances supporting the listed commands.
- Confirm image entitlement and command support before configuration; CML or the paper alternative may be used instead.
- Prepare notes in Notepad on Windows, TextEdit on macOS or a plain-text editor on Linux.
- Separate prefix discovery, IID formation, default-router discovery and neighbor resolution.
- Calculate a modified EUI-64 IID and explain why modern hosts may use a different method.
- Distinguish RS/RA from NS/NA and explain DAD without disabling it.
- Configure and verify an advertised /64, an automatically formed address and a learned default route.
- Remove automatic assignment on the disposable client, restore it and repeat the same evidence checks.
Words you will meet
Open a word when you need its meaning. You do not need to memorize the whole list before reading.
Interface, link and MAC address
An interface is a network connection. A link joins neighbors without an intervening router. A MAC address is an Ethernet interface identifier used for local frame delivery. Our lab has one link and two interfaces.
Packet and frame
An IPv6 packet carries source and destination IPv6 addresses. An Ethernet frame wraps a packet for local delivery using MAC addresses. Finding the next neighbor is not the same task as choosing an IP route.
Prefix and interface identifier
A prefix supplies the leading address bits. An interface identifier, or IID, supplies the remaining interface-specific bits in this /64 exercise. A /64 leaves 64 of the 128 bits for the IID.
Host and default gateway
A host originates or receives traffic. Its default gateway is a local router used when no more specific route matches. Router2 acts as the address-learning endpoint here; it is Cisco software, not a model of every desktop operating system.
SLAAC and modified EUI-64
Stateless Address Autoconfiguration, or SLAAC, lets an interface form an address using advertised information. Modified EUI-64 is one way to derive a 64-bit IID from a 48-bit MAC address; it is not synonymous with SLAAC.
ICMPv6 and Neighbor Discovery
ICMPv6 carries control messages for IPv6. Neighbor Discovery, or ND, uses several of these messages to discover routers, learn local neighbors and check reachability. IPv6 does not use IPv4 ARP for these jobs.
RS, RA, NS and NA
Router Solicitation asks routers for information; Router Advertisement supplies it. Neighbor Solicitation asks about a particular neighbor or checks an address; Neighbor Advertisement responds in defined situations. These are different messages, not four names for a ping.
Duplicate Address Detection
Duplicate Address Detection, or DAD, checks a tentative address before normal use. Tentative means not yet ready for ordinary traffic. A successful check is not proof that no later configuration mistake can create a conflict.
Flags and lifetimes
A flag is a yes/no field. The prefix A flag permits autonomous address formation; L concerns whether a prefix is on-link. Lifetimes are timers. Address validity and default-router eligibility have separate timers.
Console and command modes
Console is a device text window. In GNS3, right-click the named node and choose Console before typing. Cisco prompts ending >, #, (config)# and (config-if)# indicate user, privileged, global and interface modes.
Baseline and rollback
A baseline records the original settings and observations. Rollback restores the intended settings after a controlled change. A retest repeats the original test and records real results rather than copied example output.
Ping and neighbor cache
Ping requests an ICMPv6 echo reply. A neighbor cache records local IPv6-to-link-layer information and reachability state. A cached entry is not an address assignment, a default route or proof that an application works.
One interface needs several different kinds of evidence
Stateless does not mean that a host has no state. The interface still keeps addresses and timers. The useful distinction is that SLAAC does not require a DHCPv6 server to allocate and track each address. Start with the local interface, then ask which prefix was learned, how the IID was formed, whether the address is usable and which router can carry nonlocal traffic. Do not collapse these questions into the phrase IPv6 works.
- Address formation is not the same operation as discovering a default router.
- The lab has no Internet path; even a learned default route is only configuration evidence here.
Read the advertisement before blaming the host
A host can solicit router information, and routers also send advertisements periodically. A usable Prefix Information option with autonomous configuration enabled supplies input to SLAAC. In this Ethernet exercise the prefix is /64. The A and L flags have different jobs: autonomous address formation and on-link determination. The RA source is link-local; its router lifetime governs default-router eligibility, independently of the prefix address lifetimes. The M and O flags describe DHCPv6-related information, not a command to erase all SLAAC addresses.
- Keep the A flag, prefix length and prefix lifetimes together when checking address creation.
- Do not treat a router lifetime as the expiration timer for every address.
Modified EUI-64 is a method, not all of SLAAC
For the worked 48-bit Ethernet MAC example, divide 00:11:22:33:44:55 into 00:11:22 and 33:44:55. Insert ff:fe, then invert the universal/local bit of the first byte: 00 XOR 02 becomes 02. The resulting IID is 0211:22ff:fe33:4455. This is a reversible bit manipulation, not encryption, and does not hide the original hardware identifier. If the first byte were 02, the inversion would produce 00 rather than 04.
- The Cisco autoconfig command in this lab documents EUI-64-based behavior; other operating systems can differ.
- Recognizing ff:fe in an address is a clue, not universal proof of its complete configuration history.
Neighbor Discovery is more than IPv6 ARP
RS and RA concern router information. NS and NA support local-neighbor functions, including address resolution and reachability checks; NS is also used during DAD. For ordinary Ethernet address resolution, a sender asks about the target using its solicited-node multicast group, and learns appropriate link-layer information from the response. DAD probes a tentative address before normal use, with an unspecified source in that process. Its purpose and source rules differ from an ordinary peer ping.
- Do not describe IPv6 neighbor resolution as sending an IPv4 ARP broadcast.
- Keep DAD enabled and resolve duplicate configuration rather than bypassing the check.
Troubleshoot the missing fact, not a familiar-looking address
Follow an ordered checklist: confirm the mapped interface and link state; inspect router advertising configuration; inspect the client assignment and tentative or duplicate state; inspect the learned default route; then compare the exact target and neighbor information. A missing address can reflect an unsuitable prefix or disabled autoconfiguration. A missing route can reflect router-lifetime or client-policy differences. Neither symptom licenses disabling security controls across a real network.
- SLAAC addresses can remain valid for a time after advertisements stop; absence of a new RA is not immediate revocation.
- A ping failure can involve filtering or source selection as well as addressing; record the actual failure.
Know what this exercise can and cannot demonstrate
The hands-on task uses two Cisco instances on one isolated cable. Router1 advertises, and Router2 explicitly learns an address and default route. This makes the state visible without importing your physical laptop into the lab. The planned fault removes the client automatic-address configuration, not an advertisement timer. It avoids pretending that simply suppressing RAs instantly removes a previously valid address.
- No DNS service, DHCPv6 deployment, Internet forwarding or application performance is measured.
- This chapter adds IPv6 configuration and assignment diagnosis; it does not alone prove every exam objective.
Real-world walkthrough
An address appears, but the complaint is not resolved
A hypothetical training-room computer displays an IPv6 address, yet a learner cannot reach a required service. The operator has permission to inspect the training environment, not to change the building network. The tempting response is to restart every router or disable IPv6. Instead, separate the address, route, local neighbor and application evidence. This scenario is an investigation example, not a QCS customer incident.
- Record the affected interface, address, prefix and timestamp. Confirm the address is usable rather than tentative or marked duplicate.
- Inspect the default-router information independently. An address alone does not establish a route to the requested destination.
- Check the expected local neighbor and exact test destination. Avoid self-pings, guessed interface identifiers and comparisons with a different interface.
- If the protocol state is correct, test the required service separately with authorization. ICMP replies do not prove DNS, authentication or an application transaction.
- In the isolated lesson lab, change only Router2 automatic assignment, restore it and repeat the original checks. Do not translate that test into permission for a live change.
Guided GNS3 lab
Observe, remove and restore automatic IPv6 assignment
Configure a controlled advertisement and a Cisco autoconfig client, record address, route and neighbor evidence, then remove and restore the client setting. The commands are source-reviewed instructions, not a claim that QCS executed these router images.
Addressing plan
| Device | Interface | Address | Purpose |
|---|---|---|---|
| Router1 | Mapped cable port | 2001:db8:14:1::1/64; fe80::14:1 | Advertises the documentation prefix and provides the local test destination. |
| Router2 | Mapped cable port | Learned 2001:db8:14:1::/64 address; automatic link-local | Record the actual generated IID and learned default router; do not hard-code the worksheet IID. |
Set up the lab
- Create a new GNS3 project named Day14-SLAAC. Import or select your authorized router template according to its platform instructions; do not download unofficial Cisco images.
- Use the same supported IOS/IOS XE family on both nodes and record the exact show version output. These commands target classic IOS syntax, not IOS XR, NX-OS or VPCS. Unsupported commands require a compatible authorized platform, not guessed replacements.
- Drag two router instances into the workspace, rename them Router1 and Router2, and use Add a link to connect one Ethernet port on each. Record both exact port names, then start both nodes.
- If a fresh router asks to enter the initial configuration dialog, answer no, then press Enter for its prompt. Save a clean project snapshot before the experiment. Do not reuse a router with other addresses or services.
- Write port mappings in notes: for example Router1 cable = GigabitEthernet0/0/0, Router2 cable = GigabitEthernet0/1. Substitute each independently wherever the sample uses GigabitEthernet0/0.
- Keep the project disconnected. GNS3 does not provide Cisco images; legal entitlement must permit their use in GNS3. CML reference-platform ownership alone does not grant that permission.
- Without authorized images, use the paper worksheet: draw the two nodes and cable, calculate the IID, and predict the three verification states. Label predictions clearly; do not fabricate command output.
Record Router1 platform and port
In the isolated project, right-click Router1 and choose Console before typing. Run these read-only commands. Save output in Day14-baseline.txt using your operating system text editor; check that the cabled port exists. If absent, stop and inspect the template and cable selection.
enable
show version
show ip interface brief
show running-configWhat each command means
enable- Enter privileged mode from >; skip this line if already at #. Do not type the prompt itself.
show version- Record the exact software release and platform without sharing serial numbers or credentials.
show ip interface brief- Identify the actual Ethernet port selected in GNS3; IPv4 inspection here is for interface names and state.
show running-config- Record the clean configuration and confirm no unrelated production settings are present.
- What you should see
- Your notes identify Router1 software and its exact cable port. An administratively down fresh port will be enabled in the next configuration step.
- Why this step matters
- Port names differ by image. Mapping the real port prevents configuring an unrelated or nonexistent interface.
Record Router2 platform and port
In the isolated project, right-click Router2 and choose Console before typing. Save the same baseline for this node. Use its own port mapping, not Router1 output. The lab requires a clean client interface with no other IPv6 address assignments.
enable
show version
show ip interface brief
show running-configWhat each command means
enable- Enter privileged mode from >; skip this line if already at #. Do not type the prompt itself.
show version- Record Router2 exact release; it must support the specified autoconfig and RA-suppression syntax.
show ip interface brief- Match the cabled port to Router2 actual interface list.
show running-config- Confirm no other address assignments exist before using the removal experiment.
- What you should see
- Both nodes have distinct recorded baselines and a verified cable mapping; no shared or production configuration is involved.
- Why this step matters
- The later removal command can remove all IPv6 addresses on this interface, so a clean disposable client is mandatory.
Configure Router1 advertisement
In the isolated project, right-click Router1 and choose Console before typing. Use its mapped cable port. Type one line at a time. Stop on any rejected command; do not continue with a partial baseline. This router advertises only the disconnected documentation /64.
configure terminal
ipv6 unicast-routing
interface GigabitEthernet0/0
ipv6 address fe80::14:1 link-local
ipv6 address 2001:db8:14:1::1/64
no shutdown
end
show ipv6 interface GigabitEthernet0/0What each command means
configure terminal- Enter global configuration mode on this fresh disposable lab router.
ipv6 unicast-routing- Enable IPv6 forwarding and router advertisement behavior on this dedicated lab router.
interface GigabitEthernet0/0- Select the recorded cable port; substitute its exact name for GigabitEthernet0/0.
ipv6 address fe80::14:1 link-local- Set Router1 unique link-local address for a recognizable gateway reference.
ipv6 address 2001:db8:14:1::1/64- Assign the documentation /64, which the default Ethernet RA policy advertises.
no shutdown- Enable the mapped interface administratively; a working peer and cable are still needed.
end- Return to the # prompt to inspect or test the resulting state.
show ipv6 interface GigabitEthernet0/0- Inspect port state, link-local address, /64 assignment and RA information; substitute the mapped port.
- What you should see
- Router1 shows the assigned /64 and fe80::14:1. After the peer is enabled, the link should be up/up; RAs are not deliberately suppressed.
- Why this step matters
- The router must have the intended advertised prefix; merely writing a client address would not demonstrate automatic formation.
Enable Router2 automatic assignment
In the isolated project, right-click Router2 and choose Console before typing. Use its mapped cable port. Router2 remains Cisco software but acts as the receiving endpoint. Suppress its own advertisements so Router1 is the only advertiser. Do not manually enter a documentation address on Router2.
configure terminal
ipv6 unicast-routing
interface GigabitEthernet0/0
ipv6 nd ra suppress all
ipv6 address autoconfig default
no shutdown
endWhat each command means
configure terminal- Enter global configuration mode on this fresh disposable lab router.
ipv6 unicast-routing- Enable IPv6 routing support for observing the learned route on this Cisco instance.
interface GigabitEthernet0/0- Select the recorded cable port; substitute its exact name for GigabitEthernet0/0.
ipv6 nd ra suppress all- Stop Router2 from advertising itself on this one-link exercise.
ipv6 address autoconfig default- Learn an address from eligible advertised prefixes and install a selected default-router route.
no shutdown- Enable the client cable port so discovery exchanges can take place.
end- Return to the # prompt to inspect or test the resulting state.
- What you should see
- Router2 is enabled for autoconfiguration and default-router learning. Allow discovery and DAD to complete before treating an address as ready.
- Why this step matters
- The default keyword makes route learning explicit. Suppressing client advertisements does not suppress incoming Router1 messages.
Verify Router2 address and route
In the isolated project, right-click Router2 and choose Console before typing. Run the displays separately after discovery settles. Record the actual /64 address and its IID, the default route and neighbor information. If no advertisement has arrived yet, allow up to four minutes before troubleshooting rather than declaring immediate failure.
show ipv6 interface GigabitEthernet0/0
show ipv6 route
ping ipv6 2001:db8:14:1::1
show ipv6 neighborsWhat each command means
show ipv6 interface GigabitEthernet0/0- Inspect the mapped client port and copy its actual learned address, not the fictional worksheet result.
show ipv6 route- Find ::/0 and record its next hop and cable interface; it should refer to Router1 fe80::14:1.
ping ipv6 2001:db8:14:1::1- Request echo replies from Router1 documentation address, not Router2 own address.
show ipv6 neighbors- Record the peer mapping and current state after the test; a non-REACH state is not automatically a fault.
- What you should see
- Expect a usable 2001:db8:14:1::/64 address, a default route via Router1 and peer echo replies. Record actual outcomes and any initial loss.
- Why this step matters
- The local peer ping does not exercise the default route. Its presence is verified by the routing display, not inferred from the ping.
Compare the IID with the actual MAC
In the isolated project, right-click Router2 and choose Console before typing. Read the mapped port MAC and the generated IPv6 address. On paper, apply the modified EUI-64 calculation to that actual MAC and compare the IID. Do not edit the MAC or privacy controls to force a match.
show interfaces GigabitEthernet0/0
show ipv6 interface GigabitEthernet0/0What each command means
show interfaces GigabitEthernet0/0- Record the actual Ethernet hardware address; dotted MAC notation can be regrouped into six byte pairs.
show ipv6 interface GigabitEthernet0/0- Compare the generated low 64 bits with your calculation and record platform-specific differences.
- What you should see
- For the documented Cisco autoconfig behavior, the generated IID should match the modified EUI-64 calculation. A mismatch calls for checking the actual source interface and platform documentation.
- Why this step matters
- The fictional MAC worksheet teaches the arithmetic; your router output establishes which identifier this particular instance actually uses.
Remove only the disposable client setting
In the isolated project, right-click Router2 and choose Console before typing. Confirm the mapped interface and clean baseline again. This command can remove all IPv6 addresses on that interface; use it only on this isolated client with no other assignments. Do not alter Router1 or its advertisement timers.
configure terminal
interface GigabitEthernet0/0
no ipv6 address autoconfig
end
show ipv6 interface GigabitEthernet0/0
show running-configWhat each command means
configure terminal- Enter global configuration mode on this fresh disposable lab router.
interface GigabitEthernet0/0- Select the recorded cable port; substitute its exact name for GigabitEthernet0/0.
no ipv6 address autoconfig- Remove the client autoconfiguration and its interface addresses on this disposable node only.
end- Return to the # prompt to inspect or test the resulting state.
show ipv6 interface GigabitEthernet0/0- Verify the learned documentation address is absent before interpreting a traffic test.
show running-config- Confirm the autoconfig setting is absent and no unrelated lines were changed.
- What you should see
- The automatically formed documentation address is no longer assigned. Do not assume a remaining cached route or neighbor entry means the address still exists.
- Why this step matters
- This controlled local removal avoids the misleading claim that suppressing new RAs immediately expires an already-valid SLAAC address.
Record the changed Router2 test
In the isolated project, right-click Router2 and choose Console before typing. Repeat the same peer target and the routing display. Record any error message or missing-source condition. If the old address is still assigned, stop and reconcile the command result before drawing a conclusion.
ping ipv6 2001:db8:14:1::1
show ipv6 route
show ipv6 neighborsWhat each command means
ping ipv6 2001:db8:14:1::1- Repeat the baseline destination after the automatic assignment was removed.
show ipv6 route- Record remaining or removed routes without treating cached information as a local address.
show ipv6 neighbors- Record current neighbor state; an old mapping does not restore the removed assignment.
- What you should see
- The original learned-address test is no longer a valid successful baseline. Expect failure or inability to select the former source in this clean setup; record the actual result.
- Why this step matters
- Never interpret residual control-plane entries as proof that the removed address remains usable. This change is client configuration, not a measured upstream outage.
Restore Router2 automatic assignment
In the isolated project, right-click Router2 and choose Console before typing. Return only the setting removed in this experiment. Keep Router1 configuration and both cable mappings unchanged. Wait for discovery and DAD again; compare with the baseline rather than assuming an accepted command completed recovery.
configure terminal
interface GigabitEthernet0/0
ipv6 address autoconfig default
end
show ipv6 interface GigabitEthernet0/0
show ipv6 routeWhat each command means
configure terminal- Enter global configuration mode on this fresh disposable lab router.
interface GigabitEthernet0/0- Select the recorded cable port; substitute its exact name for GigabitEthernet0/0.
ipv6 address autoconfig default- Restore automatic address formation and selected default-route installation.
end- Return to the # prompt to inspect or test the resulting state.
show ipv6 interface GigabitEthernet0/0- Check that a usable documentation /64 address reappears on the mapped interface.
show ipv6 route- Verify the default route again and record the Router1 link-local next hop.
- What you should see
- Router2 again has a usable learned address and the expected route after discovery. Preserve any timing observations rather than inventing a fixed recovery duration.
- Why this step matters
- The recovered state must match the intended network and router, not simply contain any IPv6-looking value.
Retest and save the final comparison
In the isolated project, right-click Router2 and choose Console before typing. Repeat the original peer test after the address is usable. Save before, changed and restored observations. Then stop both nodes through GNS3; preserve notes and return this disposable project to its clean snapshot only after saving your evidence.
ping ipv6 2001:db8:14:1::1
show ipv6 neighborsWhat each command means
ping ipv6 2001:db8:14:1::1- Request replies from the same Router1 destination used before the fault.
show ipv6 neighbors- Record the actual peer mapping and state after recovery.
- What you should see
- Expect peer replies again. Your three-state worksheet separates address and route evidence from ping evidence, without claiming Internet or application access.
- Why this step matters
- Restoration plus the same successful test demonstrates recovery of this limited exercise. All changes and snapshot operations remain confined to the isolated lab.
Verify
- Record both exact software releases and the two cable-port mappings before configuration.
- Verify the actual learned /64 address rather than copying the fictional EUI-64 worksheet result.
- Record ::/0 next hop independently; the same-link ping does not prove default-route forwarding.
- Compare the same target before removal, while the learned assignment is absent, and after restoration.
- Keep DAD enabled; do not treat a tentative or duplicate address as ready for normal use.
- No router emulator was executed during authoring. These are source-reviewed instructions; learners must record actual runtime observations.
Troubleshoot
- If the expected port is missing, stop and match the GNS3 template and cable to show ip interface brief. If administratively down, recheck the mapped interface and its no shutdown step; if up/down, inspect peer state and cabling before changing addressing.
- If no learned address appears, check Router1 prefix, Router2 autoconfig setting, and RA configuration. An unsuitable prefix length or A flag policy can prevent formation. Do not disable DAD, privacy or security controls as a shortcut.
- If an address exists but ::/0 is missing, inspect the default keyword and router-lifetime policy separately. A local peer reply does not exercise that route; there is deliberately no off-link destination in this lab.
- If a ping misses replies, check the precise destination and actual source/address state. Initial discovery, filtering, rate limits and platform behavior can also affect replies. One lost probe does not diagnose an entire network layer.
- If a removal leaves unexpected addresses or a test still succeeds, stop. Recheck the fresh-client prerequisite, selected interface and actual source instead of forcing the expected answer. Restore the baseline only in this isolated project.
GNS3 does not provide Cisco images. Use them only when the applicable license or entitlement permits your selected environment. Do not share or redistribute image files. Cisco Modeling Labs is the official alternative; its reference images are not licensed for export into GNS3 merely because you have CML. Without an authorized image, use the labeled paper exercise and do not claim device execution.
- Restore Router2 autoconfig default and confirm the final checks before stopping the experiment.
- Save the evidence, stop Router1 and Router2, and use the recorded clean snapshot for a complete disposable-project reset if required.
- Do not erase real routers, change shared interfaces or export Cisco images. Perform these tests only in the isolated lab, never on production or shared-use networks.
Practice set
Answer before you reveal
01Does SLAAC require a DHCPv6 server to allocate each address?
Stateless does not mean no host state. Addresses and timers still exist, and a separate mechanism may supply other configuration. This lab does not deploy DHCPv6.
02How does 00:11:22:33:44:55 become a modified EUI-64 IID?
The operation on the first byte is XOR 02, not unconditional addition. Apply the steps to the actual lab interface MAC when comparing device output.
03Why might a modern laptop IID not contain ff:fe?
Stable privacy-preserving methods and temporary addresses differ from the worked EUI-64 example. Do not disable privacy merely to reproduce an old textbook pattern.
04Can an RA with router lifetime zero still contain useful prefix information?
Do not use that advertisement as a default-router offer. Eligible prefix information may still support an address, subject to its own flags and lifetimes.
05Why is removing RAs not an immediate address-revocation test?
Our controlled fault explicitly removes client autoconfiguration. It does not claim that simply stopping new advertisements instantly removes existing state.
06What does a STALE neighbor entry prove about a broken cable?
A cache state is not an application verdict. Compare actual interface state, selected destination and test results before diagnosing a cable or disabling controls.
07Which evidence proves the lab default route was installed?
The peer destination is on the local link. Its ping can succeed without exercising the default route, so the route must be inspected separately.
Knowledge check
Quiz: prove the reasoning
Keep these
Technical takeaways
- Keep address, default-route and neighbor evidence separate.
- SLAAC is not synonymous with MAC-derived modified EUI-64.
- RA prefix flags and address lifetimes differ from default-router lifetime.
- RS/RA and NS/NA support different Neighbor Discovery jobs.
- A same-link ping cannot prove default-route forwarding or application health.
- Restore the isolated baseline and record actual results; never claim a paper worksheet or static review is device execution.