NAtive Vlan Mismatch detected by

[WhiteHat]

Hello Friends

     Hope you are okay, While practicing on CCNA questions online from different sites i have seen a question about which protocol can detect native vlan mismatch, is it CDP, DTP or STP 

Referring to what i understood from my study it is CDP and DTP but more than one site consider it as CDP and STP which confuses me, i am seeking help on that, anyone please will be more than appreciated 

[Flavio Miranda]

Hi @WhiteHat

 It is CDP for sure. CDP carries vlan information among switches. A good lab for this would be simulate vlan mismatch and then disabled CDP on the switch. 

-If I helped you somehow, please, rate it as useful.-

[Mark Malone]

Hi
CDP detects the miss-match and STP puts it into inconsistent PVID state for that vlan until its fixed if DTP is running

[WhiteHat]

Hello Mark
Thank you for the provided information, that is correct referring o the below link
http://www.ciscopress.com/articles/article.asp?p=29803&seqNum=3
But does that means the STP detects the native vlan mismatch or affected by native vlan mismatch also i am confused with which Cisco consider as the correct answer.

Thanks my friend

[Mark Malone]

Hi the doc you posted there states it below

Cisco Discovery Protocol (CDP) version 2 passes native VLAN information between Cisco switches. If you have a native VLAN mismatch, you will see CDP error messages on the console output.

STP is the protocol that makes it inconsistent at in STP domain at layer 2 , CDP is just a discovery protocol while STP would maintain consistency at layer 2 for the network

[Peter Paluch · Accepted solution]

Friends,

Please allow me to join.

The question on what protocols can detect a native VLAN mismatch is quite frequent. Regarding the protocols that have been mentioned so far:

• DTP does not detect a native VLAN mismatch. With regards to native VLAN mismatch and its detection, DTP is irrelevant - we can safely forget about it.
• CDP detects a native VLAN mismatch both on access and trunk ports (on access ports, it detects that the access VLAN is mismatched). However, it does not take any protective action. It only logs a logging message, and that's it.
• Cisco PVST+ and Rapid-PVST+ are able to detect a native VLAN mismatch but only on trunk ports. They are unable to detect a mismatch of access VLANs between a pair of interconnected access mode ports. On trunks, PVST+ and Rapid-PVST+ will block both mismatched native VLANs until the mismatch is corrected.

Best regards,
Peter

[WhiteHat]

Thank you peter for the straight forward answer. is there a reference for me to re-study the whole concept with more clarifications.

Thanks in advance

[Peter Paluch]

Hello,

Unfortunately, I am not sure if there is a single reference that documents all protocols Cisco uses to detect native VLAN mismatches. Over time, different protocols whose operation was somehow related to VLANs and that could be affected by native VLAN mismatches got their own mechanisms of detecting them. CDP is a diagnostic protocol informing about various operational characteristics of the interface where the CDP packet is sent out, so it made perfect sense to include the native VLAN in the packet, and compare it to the receiver's native VLAN. In STP, native VLAN mismatches could cause a neverending switching loop with STP never blocking the ports, so PVST+ and RPVST+ were equipped with their own mechanism of detecting a native VLAN mismatch.

Other than this, I am actually not even aware of other protocols that would also be reporting native VLAN mismatch (it should be possible with LLDP but frankly I am not sure if this has already been implemented).

Is there any particular detail you are interested in?

Best regards,
Peter

[WhiteHat]

Thank you peter for getting back to me, Actually i am seeking understanding the mechanism to pull the answer out from, for me it make sens DTP can detect the native VLAN mismatch but referring to your answer is DTP irrelevant. i am not looking for specific particular detail but i am looking for a start can leads me to understand the behavior..
Thank you peter for your concern

[Peter Paluch]

Hello,

Perhaps the easiest way to think of detecting native VLAN mismatches is to keep things simple: Only CDP and (R)PVST+ are capable of detecting a native VLAN mismatch.

A protocol that is able to detect a native VLAN mismatch must be capable of advertising the originating VLAN in its messages. The receiving switch can then process the message and see whether the VLAN in which the message was received matches the originating VLAN in which the message was created.

CDP contains this information as one of many information elements in its packets. (R)PVST+ BPDUs also carry this information as a special TLV record that immediately follows the standard BPDU body.

What DTP does is simply advertising the mode of the sending port - whether it is an access port or a trunk, and what encapsulation is used. DTP does not carry the originating VLAN in its body - you can check it in Wireshark, for example, here:

https://wiki.wireshark.org/SampleCaptures?action=AttachFile&do=get&target=DTP.pcapng

This also means that DTP is technically unable to detect native VLAN mismatches because it does not allow the receiving switch to check what VLAN has the DTP message been originated in; such information is not carried in the packet.

Perhaps this helps a little.

Best regards,
Peter

[WhiteHat]

This is very helpful Peter.. Thank you so much for your time and that gives me the starts actually. I should go deeper with STP and CDP Messages and headers to check also what else they carry.. it is a very good start .. Thank you once again and i wish you a good day

[WhiteHat]

Hello Peter
hope my message finds you well, I have been searching, reading and trying regarding to the subject and i have found written in Todd Lammle book regarding to trunking ports
"You can try to remove VLAN 1 on a trunk link, but it will still send and receive management like CDP, DTP, and VTP" .. so that's mean DTP and VTP travels through the native vlan hence it can detect the native vlan mismatch .. i wanted to see that my own eyes and i created a lab, disabled CDP, enabled VTP and DTP is enabled by default then changed the natibe vlan ID on one of the switches and here what got:
" %SPANTREE-2-RECV_PVID_ERR: Received BPDU with inconsistent peer vlan id 5 on FastEthernet0/1 VLAN3"
"%SPANTREE-2-BLOCK_PVID_LOCAL: Blocking FastEthernet0/1 on VLAN0003. Inconsistent local vlan."
so you are totally right, Neither DTP nor VTP detected the native vlan mismatch although they travels through it.
I just wanted to confirm on your knowledge and maybe someone else can read the post and see it useful .. Thank to you