STP & Trunking confusion

exkor5000 · June 2006

as far as I understand STP is used to create a minimum-weight spanning tree from a network(or a tree with cycles) of switches. Now since by logical means you want to connect swtich to switch via trunk link, is STP working via trunk links?

in other words, does STP depend on a trunk link?

very confused about that

Thanks for your time
X

xanderuk2 · June 2006

Yes, Switches are interconnected via Trunk Links. Redundant Switched networks will have many trunk links.
Spanning Tree Protocol works by placing some of these links into the blocking state to prevent switching loops. If another link goes down, Spanning tree will re calculate the Spanning Tree topology and adjust port states where necessary.

EdTheLad · June 2006

exkor5000 wrote:

as far as I understand STP is used to create a minimum-weight spanning tree from a network(or a tree with cycles) of switches. Now since by logical means you want to connect swtich to switch via trunk link, is STP working via trunk links?

in other words, does STP depend on a trunk link?

very confused about that

Thanks for your time
X

CCNA covers the common spanning-tree not per vlan spnning-tree.Try and forget about the vlans for now.View all the ports as belonging to the same vlan so therefore you dont have to consider anything about trunks.Trunks are used to transport frames containing vlan tags.
Your description of spanning tree doesnt sound so clear, basically spanning tree picks a single point of reference in your network i.e. the root bridge,working from this reference point it disables ports so that only one path per switch to the root bridge is possible and thus removes any loops.

xanderuk2 · June 2006

It's good to know that the whole "spanning-tree instance per VLAN" isn't within the scope of CCNA

I was covering it last night....doh!

exkor5000 · June 2006

Your description of spanning tree doesnt sound so clear

STP creates a minimum-weight spanning tree.
MWST means that it takes any graph/tree, eliminates all cycles such that the sum of all link costs is minimum.

We are talking about the same thing, just that I've used mathematical definitions.

Try and forget about the vlans for now.View all the ports as belonging to the same vlan so therefore you dont have to consider anything about trunks

Actually I would love to get to the buttom of it, so i can break it down easier.

Ok, so lets say that I have 3 switches which are connected in the following way:

B
| \
A--C

A and C share the same vlan segmentation but B has something else.
If STP decides to drop/block the (A,C) link then how would A know how to forward taged packets to C (and vice versa)?

Thanks
X

EdTheLad · June 2006

When using common spanning tree with vlans,spanning tree isnt aware of the vlans and can disable ports which are in use by a vlan resulting in the lost of traffic.If using vlans its possible to configure a cisco proprietary solution pvst per vlan spanning tree which means every vlan has its own unique instance of spanning tree.So with vlans or without you just have to learn how one instance works,without taking the vlan tags into account.

exkor5000 · June 2006

Great!! thanks for the info!

X

skaeight · June 2006

So would a good way to look at both of these topics for the purposes of the CCNA be to just seperate them, pretend the other doesn't exist?

e.g. when looking at STP, VLANs don't exist and vice versa?

STP & Trunking confusion

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