VLANCCNA 200-301

VTP Transparent Mode Not Forwarding VTP Messages

Presenting Symptom

VTP clients in a VLAN domain are not receiving VTP advertisements from the VTP server, even though a VTP transparent mode switch is in the path.

Network Context

A small branch office network consists of three Cisco Catalyst switches: a VTP server (Cat3560), a VTP transparent mode switch (Cat2960), and a VTP client (Cat2950). All switches are connected via trunk links and belong to the same VTP domain 'CCNA'. The VTP client cannot see VLANs created on the server, and 'show vtp status' on the client shows 'Configuration Revision: 0' and no VLANs beyond default.

Diagnostic Steps

Check VTP status on all switches

show vtp status

VTP Version : 2
Configuration Revision : 5
Maximum VLANs supported locally : 255
Number of existing VLANs : 10
VTP Operating Mode : Server
VTP Domain Name : CCNA
VTP Pruning Mode : Disabled
VTP V2 Mode : Disabled
VTP Traps Generation : Disabled
MD5 digest : 0x12 0x34 ...
Configuration last modified by 192.168.1.1 at 3-1-25 12:00:00
Local updater ID is 192.168.1.1 on interface Vlan1

On the server, verify VTP domain name, mode, and revision number. On the transparent switch, expect 'VTP Operating Mode : Transparent'. On the client, expect 'VTP Operating Mode : Client' and a revision number that should match the server if advertisements are received. If client revision is 0, it indicates no VTP messages received.

Verify trunk links between switches

show interfaces trunk

Port        Mode         Encapsulation  Status        Native vlan
Gi0/1       on           802.1q         trunking      1

Port        Vlans allowed on trunk
Gi0/1       1-1005

Port        Vlans allowed and active in management domain
Gi0/1       1,10,20,30

Port        Vlans in spanning tree forwarding state and not pruned
Gi0/1       1,10,20,30

Ensure all links between switches are trunking and the allowed VLAN list includes the management VLAN (usually VLAN 1) used for VTP. If a trunk is down or not trunking, VTP messages cannot pass.

Check VTP pruning and version compatibility

show vtp status

VTP Version : 2
VTP Pruning Mode : Disabled
VTP V2 Mode : Disabled

All switches must run the same VTP version (1 or 2). VTP pruning can block VTP updates if enabled inconsistently. Ensure VTP version matches and pruning is either disabled or consistently configured.

Debug VTP packets on the transparent switch

debug vtp events

VTP LOG: Summary advertisement received from 192.168.1.1 on Gi0/1
VTP LOG: Processing summary advertisement
VTP LOG: Forwarding summary advertisement to Gi0/2

If the transparent switch receives VTP messages but does not forward them, you may see 'Received' but no 'Forwarding' lines. This indicates the transparent switch is not propagating VTP updates, which is expected behavior in VTP version 1 or 2 if the transparent switch is not configured to forward. However, by default, transparent switches do forward VTP advertisements. If forwarding is missing, check if VTP version is 3 (where transparent mode does not forward by default) or if the switch is in VTP client mode accidentally.

Root Cause

The VTP transparent mode switch (Cat2960) is running VTP version 3, which by default does not forward VTP advertisements from server to client. In VTP version 3, transparent mode switches do not propagate VTP messages; they only process them locally. The server and client are running VTP version 2, but the transparent switch's VTP version is set to 3, causing it to not forward VTP updates.

Resolution

Change the VTP transparent switch to VTP version 2 to ensure forwarding of VTP advertisements. Commands on the transparent switch: configure terminal vtp version 2 end Alternatively, if VTP version 3 is required, configure the transparent switch to forward VTP messages using: vtp mode transparent vtp version 3 vtp global forwarding But the simplest fix is to use VTP version 2 consistently.

Verification

On the VTP client, run: show vtp status Expected output shows 'Configuration Revision' matching the server (e.g., 5) and 'Number of existing VLANs' reflecting the new VLANs. Also run: show vlan brief Expected output includes VLANs 10, 20, 30 created on the server.

Prevention

1. Use the same VTP version across all switches in the domain. 2. Avoid using VTP version 3 unless necessary, as its transparent mode behavior differs. 3. Consider disabling VTP altogether and manually configuring VLANs for better control and troubleshooting simplicity.

CCNA Exam Relevance

On the CCNA 200-301 exam, this scenario may appear as a troubleshooting question where you must identify why VLANs are not propagating. The exam tests understanding of VTP modes (server, client, transparent) and version differences. A key fact is that VTP version 3 transparent switches do not forward advertisements by default, unlike version 1 and 2.

Exam Tips

Remember that VTP version 3 transparent mode does not forward VTP messages; this is a common trick on the exam.

Always verify VTP version with 'show vtp status' when troubleshooting VTP propagation issues.

Know that VTP pruning can also block advertisements if inconsistently configured.

Commands Used in This Scenario

show interfaces trunk

Displays trunk interface status, allowed VLANs, and pruning information for all trunk ports on a Cisco switch, used to verify trunking configuration and VLAN membership.

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