First, the essential ideas
- CDP
- Cisco Discovery Protocol is a Layer 2 neighbor-discovery protocol. It advertises device and interface information to adjacent compatible devices; it is not a route lookup or an authentication system.
- LLDP
- Link Layer Discovery Protocol is a standards-based Layer 2 discovery protocol. Devices advertise information such as chassis ID, port ID and system name through locally scoped messages.
- Local interface and remote port ID
- The local interface is the receiving device's port. The remote port ID is the neighbor's advertised port identifier. Reversing them produces an incorrect cable record.
- Chassis ID and system name
- A chassis ID identifies an advertised device in LLDP; a system name is a human-readable advertised name. Formats differ by platform, and neither is cryptographic proof of identity.
- TLV
- A type-length-value field describes what an advertised item means and how much data it contains. Not every neighbor sends every optional field, such as a management address.
- Holdtime or TTL
- Discovery records expire if advertisements stop. CDP calls its lifetime holdtime; LLDP uses a time-to-live field. A cached record can therefore outlive a recent configuration change.
- Transmit and receive
- LLDP transmission sends local advertisements, while reception accepts a neighbor's advertisements. Disabling one direction can create asymmetric neighbor views without breaking ordinary IP traffic.
- Console and evidence record
- Console is direct access to the selected lab node. A useful evidence record includes device, local port, remote identity/port, protocol, timestamp and unresolved discrepancies.
Treat a neighbor table as an advertisement record with a lifetime. It helps check a drawing, but does not replace physical mapping, configuration inspection or an actual traffic test. The lab makes one discovery direction disappear while leaving the data link available.
See the idea
Read each cable from both ends
SW2 has two direct neighbors. SW1 and SW3 share a data path, not a direct cable.
Validate the left cable
SW1 Gi0/0 connects to SW2 Gi0/0. Read local and remote port fields from both devices.
What this model leaves out: The diagram shows the complete isolated cable chain. Discovery is link-local; the last stage shows the separate multi-switch data path, not forwarded LLDP advertisements.
Full visual explanation and references
SW1 connects to SW2, which connects to SW3. Both cable endpoints are named. No direct SW1-to-SW3 link exists.
The diagram shows the complete isolated cable chain. Discovery is link-local; the last stage shows the separate multi-switch data path, not forwarded LLDP advertisements.
- Validate the left cable
SW1 Gi0/0 connects to SW2 Gi0/0. Read local and remote port fields from both devices.
www.cisco.com - Validate the right cable
SW2 Gi0/1 connects to SW3 Gi0/0. SW3's LLDP transmit setting affects what SW2 can learn.
www.cisco.com - Keep scopes separate
SW1 can send data through SW2 to SW3, but this does not make SW3 a directly connected neighbor of SW1.
www.cisco.com
One idea at a time
Start with something familiar.
Draw SW1 Gi0/0 to SW2 Gi0/0, then SW2 Gi0/1 to SW3 Gi0/0. Make separate columns for the interface you are sitting on and the remote port reported by its neighbor.
Why learn this?
A reversed local/remote port field can send an operator to the wrong cable. A stale or missing advertisement can also make an accurate topology look wrong unless timing and protocol settings are recorded.
An everyday comparison
Two adjacent offices exchange door labels. Each office records its own connecting door and the label reported by the office on the other side, then compares that record with a building plan.
What happens in a network
The local door represents the receiving interface and the reported door represents the remote port ID. Periodic updates resemble discovery advertisements.
Where the comparison stopsAdvertisements are not verified identity documents. Labels can be stale, absent or misleading, and an advertisement does not test the application's IP path.
Follow one worked example
- Read one neighbor entry in the correct direction.
On SW2, a record received through Gi0/1 should identify SW3 and its Gi0/0 port. The two interface labels belong to different devices.
Why: A useful cable record needs both device names and both ends, not a bare port number.
- Check the reciprocal observation.
On SW3, the record should identify SW2's Gi0/1 through local Gi0/0. Compare timestamps and protocol fields.
Why: Independent views help catch transposed labels, stale documentation or disabled discovery in one direction.
- Keep missing discovery separate from a failed data test.
Disabling SW3's LLDP transmission eventually removes SW3 from SW2's LLDP table, but CDP and VLAN 10 ping can remain functional.
Why: The controlled change affects one discovery protocol direction, not the entire cable or switch.
Your first small task
On paper, interpret a fictional SW2 record: local Gi0/1, system name SW3, remote port Gi0/0. Write the complete cable pair, then state whether this gives SW1 a direct adjacency to SW3.
Show a hint
A common VLAN permits data forwarding across multiple switches but does not turn a remote switch into a direct neighbor.
Check the expected result
SW2 Gi0/1 connects to SW3 Gi0/0. SW1 is not directly adjacent to SW3 in this two-cable chain.
Pause and explain it in your own words
SW2 loses SW3's LLDP entry but can still ping it. Is that contradictory?
Show a hint
Separate LLDP advertisement transmission from IP data forwarding.
Show the explanation
No. LLDP can be disabled or unsupported while ordinary data forwarding still works. Check protocol settings and record lifetime before replacing a cable or changing VLAN configuration.
Short answer
What should you understand today?
CDP and LLDP describe advertised adjacent neighbors, not the complete routed path or verified ownership. Match each local interface with the neighbor's remote port ID, cross-check from the other end and preserve timestamps. Missing entries can mean disabled transmission, unsupported protocols or expired records as well as a failed link. Confirm data forwarding separately.
Before you begin
Learning targets
- Understand interfaces, VLAN membership and an SVI used for local IP testing.
- Use three fresh IOSvL2 nodes in an existing entitled CML 2.10 installation.
- Keep the lab disconnected and use direct Console access rather than remote discovery addresses.
- Configure CDP and LLDP globally and on selected interfaces.
- Read local port, remote port, advertised identity and lifetime fields.
- Validate both directions of a cable record.
- Diagnose an asymmetric LLDP view without assuming the cable failed.
- Restore the changed transmit setting and retain honest evidence limitations.
Words you will meet
Open a word when you need its meaning. You do not need to memorize the whole list before reading.
CDP
Cisco Discovery Protocol is a Layer 2 neighbor-discovery protocol. It advertises device and interface information to adjacent compatible devices; it is not a route lookup or an authentication system.
LLDP
Link Layer Discovery Protocol is a standards-based Layer 2 discovery protocol. Devices advertise information such as chassis ID, port ID and system name through locally scoped messages.
Local interface and remote port ID
The local interface is the receiving device's port. The remote port ID is the neighbor's advertised port identifier. Reversing them produces an incorrect cable record.
Chassis ID and system name
A chassis ID identifies an advertised device in LLDP; a system name is a human-readable advertised name. Formats differ by platform, and neither is cryptographic proof of identity.
TLV
A type-length-value field describes what an advertised item means and how much data it contains. Not every neighbor sends every optional field, such as a management address.
Holdtime or TTL
Discovery records expire if advertisements stop. CDP calls its lifetime holdtime; LLDP uses a time-to-live field. A cached record can therefore outlive a recent configuration change.
Transmit and receive
LLDP transmission sends local advertisements, while reception accepts a neighbor's advertisements. Disabling one direction can create asymmetric neighbor views without breaking ordinary IP traffic.
Console and evidence record
Console is direct access to the selected lab node. A useful evidence record includes device, local port, remote identity/port, protocol, timestamp and unresolved discrepancies.
Discovery is local evidence, not a network map by itself
CDP and LLDP advertise adjacent-device details at Layer 2. To build a wider topology, collect local observations from multiple devices and join matching endpoints. One table is not a substitute for recursively validated documentation, and a management address in an advertisement is not necessarily reachable from the observing host.
- Collect both ends rather than treating one table as global truth.
- Never use an advertised management address as authorization to log in.
Preserve the direction of the fields
Record the observing device and local interface first, then advertised neighbor identity and remote port ID. Include the protocol and timestamp so conflicting snapshots can be reconciled. CDP and LLDP may use different identity formats, so compare meaningful values rather than expecting every string to match byte-for-byte.
- A hostname and a CML canvas label are separate settings until configured.
- An absent optional management-address field is not proof that discovery failed.
Protocol configuration and aging explain gaps
CDP has global and interface controls. LLDP also distinguishes transmission from reception. Neighbor records have advertised lifetimes, so a change may not immediately remove a cached entry. Read actual timer or holdtime values and wait for expiry rather than asserting a universal exact delay.
- Do not lower timers on a shared network to make an experiment faster.
- If the image lacks LLDP, label that practical branch unsupported rather than replacing it with imagined output.
Validate a document by preserving disagreements
A reliable worksheet records expected and observed cable endpoints, evidence time and resolution status. If observations disagree with the drawing, investigate the physical mapping and both peer configurations before editing the network. Correcting a document may be the right change when the network is already correct.
- Mark unknown or unobserved fields explicitly.
- Retain the original discrepancy so a later reviewer understands the correction.
Trust and scope remain operational responsibilities
These discovery advertisements are useful information, not authentication. They can reveal device details to an attached peer. Enable or restrict discovery according to the real edge's requirements; phones and other services may depend on it. Do not recommend globally disabling a protocol without understanding those dependencies.
- A discovered neighbor is not automatically a trusted device.
- Do not publish management addresses or private topology evidence in social captions.
Real-world walkthrough
The drawing is wrong, not the uplink
A hypothetical maintenance worksheet lists a switch uplink on Gi0/2. Current discovery shows the neighbor on Gi0/1, and a reciprocal check agrees. The operator could create an outage by moving a working cable to fit the old drawing. The better first step is to reconcile evidence and change history.
- Record the drawing's expected local and remote interfaces without editing it yet.
- Collect current CDP and LLDP details from both physical cable endpoints.
- Compare interface status and the actual cable map, including timestamps and device identity formats.
- Resolve whether the network or the document is incorrect before requesting any data-path change.
- Update the worksheet with the confirmed pair and note unsupported or unavailable discovery fields.
Guided GNS3 lab
A missing LLDP advertisement on an intact three-switch chain
Observe reciprocal neighbors and data reachability, remove one LLDP transmission direction, then restore it. No topology, timer or discovery output has been executed or measured by the author.
Addressing plan
| Device | Interface | Address | Purpose |
|---|---|---|---|
| SW1 | Vlan10 | 192.0.2.1/24 | Local ping control endpoint |
| SW2 | Vlan10 | 192.0.2.2/24 | Middle-switch test source |
| SW3 | Vlan10 | 192.0.2.3/24 | Far-end control endpoint |
Set up the lab
- Create three fresh entitled CML nodes and exactly the two listed cables. No additional switch, bridge or external connector is permitted.
- Record the exact IOSvL2 builds and preserve the starting lab. Check Console access and decline setup dialogs if offered.
- Prepare a private worksheet with local device/port, neighbor identity/port, protocol, timestamp, expected cable and discrepancy status.
- Use a paper cable-pair exercise if LLDP is unsupported or entitlement is unavailable; do not claim that a supplied example is observed output.
Enable discovery on SW1
In CML, select SW1, open its Console tab and connect before typing. Inspect the fresh IOSvL2 node and assign its real IOS hostname to match the lab label. Only mapped internal lab links are in scope.
enable
show version
configure terminal
hostname SW1
cdp run
lldp run
interface GigabitEthernet0/0
cdp enable
lldp transmit
lldp receive
endWhat each command means
enable- Enter privileged EXEC on the named lab node; stop if authorization is unavailable.
show version- Record the exact virtual switch build and discovery support.
configure terminal- Enter global configuration mode from privileged EXEC; changes apply as accepted.
hostname SW1- Set the advertised system name on this fresh lab device.
cdp run- Enable CDP globally for the authorized lab.
lldp run- Enable standards-based discovery globally.
interface GigabitEthernet0/0- Select only this node's mapped lab link interfaces.
cdp enable- Enable CDP on the selected interface or range.
lldp transmit- Enable local LLDP advertisement transmission.
lldp receive- Enable reception of neighbor LLDP advertisements.
end- Return to privileged EXEC without saving or reversing the configuration.
- What you should see
- The named device has both discovery protocols enabled on its lab links.
- Why this step matters
- Explicit global and interface settings prevent a missing record from being blamed on cabling before protocol state is checked.
Build SW1 data control
In CML, select SW1, open its Console tab and connect before typing. Use VLAN 10 for all chain links and a same-subnet SVI only for an independent ping control. No routing or external connectivity is required.
configure terminal
vlan 10
exit
interface GigabitEthernet0/0
switchport mode access
switchport access vlan 10
no shutdown
exit
interface Vlan10
ip address 192.0.2.1 255.255.255.0
no shutdown
endWhat each command means
configure terminal- Enter global configuration mode from privileged EXEC; changes apply as accepted.
vlan 10- Create the local data-control VLAN.
exit- Return to global configuration.
interface GigabitEthernet0/0- Select this node's mapped chain links.
switchport mode access- Use a simple untagged access link in this lab.
switchport access vlan 10- Place the chain links in the common test VLAN.
no shutdown- Enable the selected links.
exit- Return to global configuration.
interface Vlan10- Select the switch's local test IP attachment.
ip address 192.0.2.1 255.255.255.0- Assign a distinct address for the ping control.
no shutdown- Enable the SVI subject to active member forwarding.
end- Return to privileged EXEC without saving or reversing the configuration.
- What you should see
- The chain can carry VLAN 10 and this node has its own test address.
- Why this step matters
- The IP control distinguishes discovery visibility from ordinary forwarding behavior.
Enable discovery on SW2
In CML, select SW2, open its Console tab and connect before typing. Inspect the fresh IOSvL2 node and assign its real IOS hostname to match the lab label. Only mapped internal lab links are in scope.
enable
show version
configure terminal
hostname SW2
cdp run
lldp run
interface range GigabitEthernet0/0 - 1
cdp enable
lldp transmit
lldp receive
endWhat each command means
enable- Enter privileged EXEC on the named lab node; stop if authorization is unavailable.
show version- Record the exact virtual switch build and discovery support.
configure terminal- Enter global configuration mode from privileged EXEC; changes apply as accepted.
hostname SW2- Set the advertised system name on this fresh lab device.
cdp run- Enable CDP globally for the authorized lab.
lldp run- Enable standards-based discovery globally.
interface range GigabitEthernet0/0 - 1- Select only this node's mapped lab link interfaces.
cdp enable- Enable CDP on the selected interface or range.
lldp transmit- Enable local LLDP advertisement transmission.
lldp receive- Enable reception of neighbor LLDP advertisements.
end- Return to privileged EXEC without saving or reversing the configuration.
- What you should see
- The named device has both discovery protocols enabled on its lab links.
- Why this step matters
- Explicit global and interface settings prevent a missing record from being blamed on cabling before protocol state is checked.
Build SW2 data control
In CML, select SW2, open its Console tab and connect before typing. Use VLAN 10 for all chain links and a same-subnet SVI only for an independent ping control. No routing or external connectivity is required.
configure terminal
vlan 10
exit
interface range GigabitEthernet0/0 - 1
switchport mode access
switchport access vlan 10
no shutdown
exit
interface Vlan10
ip address 192.0.2.2 255.255.255.0
no shutdown
endWhat each command means
configure terminal- Enter global configuration mode from privileged EXEC; changes apply as accepted.
vlan 10- Create the local data-control VLAN.
exit- Return to global configuration.
interface range GigabitEthernet0/0 - 1- Select this node's mapped chain links.
switchport mode access- Use a simple untagged access link in this lab.
switchport access vlan 10- Place the chain links in the common test VLAN.
no shutdown- Enable the selected links.
exit- Return to global configuration.
interface Vlan10- Select the switch's local test IP attachment.
ip address 192.0.2.2 255.255.255.0- Assign a distinct address for the ping control.
no shutdown- Enable the SVI subject to active member forwarding.
end- Return to privileged EXEC without saving or reversing the configuration.
- What you should see
- The chain can carry VLAN 10 and this node has its own test address.
- Why this step matters
- The IP control distinguishes discovery visibility from ordinary forwarding behavior.
Enable discovery on SW3
In CML, select SW3, open its Console tab and connect before typing. Inspect the fresh IOSvL2 node and assign its real IOS hostname to match the lab label. Only mapped internal lab links are in scope.
enable
show version
configure terminal
hostname SW3
cdp run
lldp run
interface GigabitEthernet0/0
cdp enable
lldp transmit
lldp receive
endWhat each command means
enable- Enter privileged EXEC on the named lab node; stop if authorization is unavailable.
show version- Record the exact virtual switch build and discovery support.
configure terminal- Enter global configuration mode from privileged EXEC; changes apply as accepted.
hostname SW3- Set the advertised system name on this fresh lab device.
cdp run- Enable CDP globally for the authorized lab.
lldp run- Enable standards-based discovery globally.
interface GigabitEthernet0/0- Select only this node's mapped lab link interfaces.
cdp enable- Enable CDP on the selected interface or range.
lldp transmit- Enable local LLDP advertisement transmission.
lldp receive- Enable reception of neighbor LLDP advertisements.
end- Return to privileged EXEC without saving or reversing the configuration.
- What you should see
- The named device has both discovery protocols enabled on its lab links.
- Why this step matters
- Explicit global and interface settings prevent a missing record from being blamed on cabling before protocol state is checked.
Build SW3 data control
In CML, select SW3, open its Console tab and connect before typing. Use VLAN 10 for all chain links and a same-subnet SVI only for an independent ping control. No routing or external connectivity is required.
configure terminal
vlan 10
exit
interface GigabitEthernet0/0
switchport mode access
switchport access vlan 10
no shutdown
exit
interface Vlan10
ip address 192.0.2.3 255.255.255.0
no shutdown
endWhat each command means
configure terminal- Enter global configuration mode from privileged EXEC; changes apply as accepted.
vlan 10- Create the local data-control VLAN.
exit- Return to global configuration.
interface GigabitEthernet0/0- Select this node's mapped chain links.
switchport mode access- Use a simple untagged access link in this lab.
switchport access vlan 10- Place the chain links in the common test VLAN.
no shutdown- Enable the selected links.
exit- Return to global configuration.
interface Vlan10- Select the switch's local test IP attachment.
ip address 192.0.2.3 255.255.255.0- Assign a distinct address for the ping control.
no shutdown- Enable the SVI subject to active member forwarding.
end- Return to privileged EXEC without saving or reversing the configuration.
- What you should see
- The chain can carry VLAN 10 and this node has its own test address.
- Why this step matters
- The IP control distinguishes discovery visibility from ordinary forwarding behavior.
Record both adjacent neighbors
In CML, select SW2, open its Console tab and connect before typing. Allow normal advertisements to arrive and member links to converge. Read complete details, including the distinction between local interface and remote port.
show cdp neighbors detail
show lldp neighbors detail
show lldp interface
ping 192.0.2.3What each command means
show cdp neighbors detail- Record both adjacent CDP identities, ports and available addresses.
show lldp neighbors detail- Record LLDP chassis/system/port fields and remaining lifetime.
show lldp interface- Inspect local LLDP transmission and reception settings.
ping 192.0.2.3- Establish a data-plane control toward SW3.
- What you should see
- Expect SW1 and SW3 as neighbors and a replying SW3 SVI.
- Why this step matters
- Two discovery protocols and a ping answer different questions; keep all three records.
Check the one-hop scope
In CML, select SW1, open its Console tab and connect before typing. Inspect SW1 separately and compare the observed neighbor with the physical chain. SW3's IP reachability does not make it directly cabled to SW1.
show cdp neighbors detail
show lldp neighbors detail
ping 192.0.2.3What each command means
show cdp neighbors detail- Identify SW2 through SW1's local Gi0/0.
show lldp neighbors detail- Cross-check the adjacent peer's remote port ID.
ping 192.0.2.3- Test data forwarding across SW2 to SW3.
- What you should see
- Expect SW2 as the direct neighbor even though SW3 can reply across the VLAN.
- Why this step matters
- The exercise distinguishes local adjacency from multi-switch data reachability.
Withdraw only LLDP transmission
In CML, select SW3, open its Console tab and connect before typing. First record SW3's reciprocal neighbor, then disable only its outbound LLDP advertisements on the mapped cable. Leave receive, CDP and forwarding enabled.
show lldp neighbors detail
configure terminal
interface GigabitEthernet0/0
no lldp transmit
end
show lldp interfaceWhat each command means
show lldp neighbors detail- Record SW2's reciprocal port information before the change.
configure terminal- Enter global configuration mode from privileged EXEC; changes apply as accepted.
interface GigabitEthernet0/0- Select the sole cable toward SW2.
no lldp transmit- Stop this interface's LLDP advertisements only.
end- Return to privileged EXEC without saving or reversing the configuration.
show lldp interface- Verify transmit is disabled while receive remains enabled.
- What you should see
- SW3 no longer sends LLDP on this cable but can still receive SW2 advertisements.
- Why this step matters
- The fault deliberately changes one discovery direction rather than the data interface.
Observe expiry beside working data
In CML, select SW2, open its Console tab and connect before typing. Wait past the previously observed remaining LLDP lifetime, then inspect again. Preserve an entry that has not yet expired rather than declaring immediate disappearance.
show lldp neighbors detail
show cdp neighbors detail
ping 192.0.2.3
show interfaces statusWhat each command means
show lldp neighbors detail- Check whether SW3's stale LLDP record has expired.
show cdp neighbors detail- Check that CDP can still advertise the same adjacent SW3.
ping 192.0.2.3- Repeat the same SW3 data-plane control.
show interfaces status- Confirm the physical cable remains connected.
- What you should see
- Expect SW3 absent from LLDP after expiry while CDP and the ping can remain successful.
- Why this step matters
- Missing one discovery record is not equivalent to a failed cable or broken IP service.
Restore the changed advertisement
In CML, select SW3, open its Console tab and connect before typing. Restore only the transmit setting that was disabled. Do not alter timers or clear the neighbor table to manufacture a particular timeline.
configure terminal
interface GigabitEthernet0/0
lldp transmit
end
show lldp interface
show lldp neighbors detailWhat each command means
configure terminal- Enter global configuration mode from privileged EXEC; changes apply as accepted.
interface GigabitEthernet0/0- Select the same mapped cable.
lldp transmit- Re-enable the outbound advertisements from SW3.
end- Return to privileged EXEC without saving or reversing the configuration.
show lldp interface- Verify both LLDP directions are enabled again.
show lldp neighbors detail- Confirm the reciprocal SW2 observation remains available.
- What you should see
- SW3 is again configured to advertise itself on the original link.
- Why this step matters
- The exact inverse restores the discovery dependency while preserving the intact data path.
Retest and reconcile the worksheet
In CML, select SW2, open its Console tab and connect before typing. Allow the next actual advertisement, then compare both protocols and the original data test. Correct the paper Gi0/2 label to the observed Gi0/1 only after reciprocal confirmation.
show lldp neighbors detail
show cdp neighbors detail
ping 192.0.2.3What each command means
show lldp neighbors detail- Verify SW3's restored entry and remote Gi0/0 identity.
show cdp neighbors detail- Cross-check the unchanged adjacent topology.
ping 192.0.2.3- Repeat the original control destination.
- What you should see
- Expect the original neighbor relationship and successful data control to return or persist.
- Why this step matters
- Close the evidence gap by correcting the documentation, not by moving a working cable.
Verify
- Record reciprocal cable endpoints rather than a single device name.
- Verify global and per-interface protocol state and LLDP directions.
- Distinguish direct neighbor scope from the multi-switch ping path.
- Wait for actual record expiry after disabling transmit; timing is not assumed.
- Confirm restored LLDP visibility and correct only the proven documentation discrepancy.
Troubleshoot
- If neither protocol finds a peer, check the actual cable and interface state, then global and interface discovery settings on both ends.
- If only one protocol is absent, check its support and configuration without assuming an IP failure. Optional advertised fields may legitimately be missing.
- If an old entry remains, inspect holdtime/TTL and wait. A cached record does not prove the peer is still sending.
- If local and remote port labels conflict, obtain reciprocal evidence and the actual CML cable map before modifying either the network or its records.
Use an existing Cisco Modeling Labs 2.10 entitlement and its bundled reference images inside CML only. GNS3 does not provide Cisco images; use images there only when a separate applicable license permits that environment. Do not share or redistribute image files. A CML subscription alone does not authorize exporting its Cisco images to GNS3. Without entitlement, use the explicitly labeled paper exercises; they do not establish hands-on competence.
- Restore SW3 Gi0/0 LLDP transmit and retain final protocol and ping evidence.
- Stop the isolated nodes after preserving the private corrected worksheet. Do not expose advertised management details or export bundled Cisco images.
Practice set
Answer before you reveal
01Which port is local in SW2's Gi0/1-to-SW3 Gi0/0 record?
The remote Gi0/0 belongs to SW3; keeping both device names prevents a transposed cable record.
02Why might CDP and LLDP show different identity strings?
Compare meaningful system/chassis/port evidence rather than demanding byte-identical names.
03Why can a record remain after advertisements stop?
Neighbor state is cached; use timestamps and remaining lifetime rather than claiming instant removal.
04Can SW1 discover SW3 as a directly cabled peer here?
The shared data VLAN allows forwarding but does not create the missing physical adjacency.
05Does a management address authorize access?
Do not log into or scan a device merely because its discovery record exposes a reachable address.
06What restores the controlled fault?
The receive direction, CDP and VLAN data forwarding were deliberately left unchanged.
Knowledge check
Quiz: prove the reasoning
Keep these
Technical takeaways
- CDP and LLDP describe adjacent advertisements, not complete routing paths.
- Keep local and remote device/port fields in their correct direction.
- Cross-check reciprocal observations and preserve their timestamps.
- Record aging can delay the visible effect of a protocol change.
- A missing neighbor advertisement need not mean failed data forwarding.
- Discovery information is useful evidence, not authorization or identity proof.