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BPDU and native vlan

fereshtemohebbi
Level 1
Level 1

Hello,

I am studing for the ccnp switching. I have a question regarding the BPDU and native vlan. if we change the native vlan to another vlan, and also the native vlan is tagged. what happens to stp, RSTP bpdus.

example: vlan 1 BPDU where the native vlan is 99 and native vlan is tagged

does vlan 1 tagged through vlan 1 and untagged through vlan 99?

7 Replies 7

balaji.bandi
Hall of Fame
Hall of Fame

If any topology changes, the convergence take place automatically.

 

More information for your reference :

 

https://www.cisco.com/c/en/us/support/docs/lan-switching/spanning-tree-protocol/12013-17.html

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my question is about tagging and untagging nnot convergence

IBMeR
Level 1
Level 1

Cisco send 2 types of BPDU.

1) Standart BPDU, for example STP BPDU over native vlan.

2) Proprietary BPDU, in case STP - PVST+ BPDU in all allowed vlans this trunk.

If you have trunk with 3 allowed vlan: 1,2 and 99, where 99 native vlan and tagged - STP BPDU will send tagged in vlan 99, PVST+ BPDU will send tagged in vlans 1,2 and 99.

thanks for your reply, But i am confused, base on this link: "https://www.cisco.com/c/en/us/support/docs/switches/catalyst-6500-series-switches/24330-185.html#cg5" "In PVST+, the 802.1Q IEEE BPDUs are forwarded untagged on the common Spanning Tree VLAN 1 for interoperability with other vendors, unless VLAN 1 has been cleared from the trunk. This is the case regardless of the native VLAN configuration. Cisco PVST+ BPDUs are sent and tagged for all other VLANs. See the Spanning Tree Protocol section for more details." and also "https://www.cisco.com/c/m/en_us/techdoc/dc/reference/cli/nxos/commands/l2/vlan-dot1q-tag-native.html" "

Control traffic continues to be accepted as untagged on the native VLAN on a trunked port, even when the vlan dot1q tag native command is enabled." . base on the first link standard stp is on vlan 1 and also base on the second link it is untagged. I got confused

 

I correct my answer:

STP BPDU always will be sent untagged. It's mean - without 802.1Q header. And contains STP info from STP tree for VLAN 1. If you set native vlan "always tagged" - STP BPDU still send and receive untagged.

But if you remove VLAN 1 from trunk - this port will be removed from VLAN 1 tree and STP BPDU will stop to send in this trunk (PVST+ BPDU still send, but only for allowed vlans)

I don't agree with above.

Below is the result of testing it on 2960 and 3650 switches:

- STP BPDUs use vlan 1 and they can be filtered through “sw trunk allowed vlan remove 1”

- PVST BPDUs use the corresponding vlan numbers and they can be filtered.

- MST BPDUs use vlan 1 and they CANNOT be filtered.

A general thought that everyone should know:

vlan 1 is the native vlan by default and a native vlan is usually untagged.

So, if the native vlan is changed, vlan 1 will be tagged for user data traffic and control traffic too.

 

 

Excuse me that I meddle in this matter ... but here you have a question that before I had it very clear but now I have my doubts

In which two ways are IEEE STP BPDUs forwarded if VLAN 99 is configured as native? (Choose two )
A. VLAN 1 STP BPDUs are sent tagged on VLAN 1
B. VLAN 1 STP BPDUs are sent tagged on VLAN 99
C. VLAN 1 and VLAN 99 BPDUs are sent tagged on VLAN 1
D. VLAN 1 STP BPDUs are sent untagged on VLAN 1
E. VLAN 1 STP BPDUs are sent untagged on VLAN 99

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