stp instance and bpdu

[sarahr202]

Hi everybody

How is everyone doing ?

Cisco uses one stp instance per vlan.  Let say we have two vlan1 and vlan 2   Will there not be separate BPDUs for each vlan generated by root bridges of these two vlans?

What vlan is used to send these BPDUs i.e  vlan1 bpdu and vlan2 bpdu?

Let say  we don't want vlan2 to extend beyond sw2 i.e

Vlan2------------SW2----trunk------ Rest of enterprise's switched network

If i remove vlan2  from  trunk at sw2  will it stop stp instance for vlan2 at sw2 ?

Does removing vlan2 from trunk at sw2 stops vlan2 bpdu too ?

thanks and have a great day

[nkarpysh · Accepted solution]

Hello,

First of all it will depend on the type of STP you use. If you talk about regular STP PVST then you will have BPDUs sent for each single VLAN thus when you have many VLANs and you want save sytems resources taken by those multiple BPDUs for each VLAN you can split those into grpups running MST 801.1s.

In case of regular STP each VLAn sending BPDU out of port which has that VLAN enabled. If you disable VLAn on trunk - BPDU from one switch will not be sent on it to another. But other switch will be still running BPDU on that VLAN as long as it have any other port within this VLAn.

Hope this helps,

NIk

[sarahr202]

Thanks NIk

Will stp bpdu for different vlans say for vlan 4  are sent in vlan1 over trunk?

[nkarpysh · Accepted solution]

If VLAN 4 is not on the trunk with BPDUs for that will be never sent there. Also BPDUs are only local to single VLAN in PVST - if BPDU originated in VLAN4 it will not propagate to VLAN1. So switch before sending BPDUs in case of switch just powered up and STP not converged - it verifies the ports and locates where particular VLAN is running - and then sending VLAN out f that port. Other side receiving these BPDUs only consider those in terms of originating VLAN.

Nik

[sarahr202]

Thanks Nik.

Perhaps I should have been a little bit more articulate.

This is my confusion.  

VLan1 carries a lot of traffic for other protocols e.g cdp.pagp,dtp. Does Vlan1 carry bpdu for other vlans over trunk or   bpdu related to each vlan travels in its own vlan?

[vijay.swaminathan]

Each vlan's have their own set of BPDU's. Vlan1 does not carry BPDU's of other vlan. they carry their own vlan1 Bpdu's.

-Vijay

[vijay.swaminathan · Accepted solution]

Are you talking about Cisco Switches.. if so, then by default cisco switches will run PVST+ which would run one instance for each Vlan. so members in that vlan will generate / relay BPDU's received to the other members in that VLAN.

if you want not to propogate vlan information across the trunk link, then you can filter that vlan in trunk using command swichport trunk allowed vlan <> so that the rest of the enterprise network would not see about this vlan. if required you can enable pruning as well on the swich on the enterpise network so that it will take care of it. as well.

HTH

-Vijay

[sarahr202]

Thanks Vijay

My understanding is pruning will only stop user traffic/broadcast traffic, it will not stp bpdu for pruned vlan from traversing the trunk  So STP instance will still exist for pruned vlan.

thanks