why does STP dectect a vlan when i trunk it with another switch with the same vlan.

[dolanduck.]

i was studying for the ccna and was confused with STP and vlans.

I'm on some real equipment right now and was confused because i have 3 switches and when i made vlan 2 on all them. Spanning tree did not detect vlan 2 on any of the switches. spanning stared to work and detect vlan 2  after i made my ports trunk. or i plugged in a a router and made it a access port and assigned that vlan 2.

why does it show up ?

[Martin L]

I think it is because vlan 1 is default and Native vlan (does not need trunk) whereas vlan 2 and others do need trunks. 

Similarly if you block vlan x from crossing over the trunks, you may have several vlan x Root switches (each switch claims its Root)

Regards, ML
**Please Rate All Helpful Responses **

[dolanduck.]

does a vlan need a interface or trunk interface to be up ?

[Martin L]

Yes, I think there is restriction regarding vlan. I would have to check i think rule is for a vlan x to be active, one of ports must be assign to it or vlan must be allowed via trunk (exist).

[paul driver]

Hello
For PVST+ / RSTP each vlan is associated with a single stp instance, So if the vlan isn’t created or no activity is on that vlan (active port assigned to it or part of a active trunk) then it won’t show in stp.

However, MSTP is decoupled with vlans, So you can create and append vlans to an mst instance even without the vlans being created or having any activity on them (active port assigned to it or part of a active trunk)

[dolanduck.]

so a vlan needs a active port assigned or active trunk to be on ?

and when one of those options is met spanning spanning tree RSTP or PSTP will detect that vlan ?

[paul driver · Accepted solution]

Hello

---

 wrote:

so a vlan needs a active port assigned or active trunk to be on ?

and when one of those options is met spanning spanning tree RSTP or PSTP will detect that vlan ?

---

Yes that is correct, but not for MSTP