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New Member

spanning tree

When plug the redundant link in my switch, will it block the last port where I plug the cable in that causes the loop, and if not how would I manipulate it so it blocks the port of my choice ?

thanks

  • Other Network Infrastructure Subjects
9 REPLIES

Re: spanning tree

You can use STP port priority/cost to manipulate which port gets to forward/block. Commands vary based on the type of switch you are using.

Pls. rate all helpful posts.

HTH,

Sundar

New Member

Re: spanning tree

Hi there, Can you give me the command for my 3550 switch so that I can set that port to blcok by using the priority command, what is the default priority by the way ?

Re: spanning tree

It's not that simple unfortunately. You need to tell us what is the location of this switch to the root bridge. Different parameters may need to be tuned in different places depending on whether your switch is providing connectivity or receiving connectivity on these ports.

For instance, the port priority will almost only be useful if the two ports are connected the one to the other;-) That's unlikely to be the case.

If these two ports are uplinks going to the the root, the cost is the most simple way of selecting the root port. If these are the only uplinks, just bumping the cost of the one you want to keep blocking should be enough (spanning-tree [vlan|mst] x cost with high-value being superior to the root cost received on the other uplink for instance)

If these two ports are designated ports, the blocked port will be on the other side (revert to previous method, but configure on the remote).

There are lots of possibility, but I'm guessing you just need a cost increase on the port you which to block.

Regards,

Francois

Re: spanning tree

No problem.

Default STP port priority is 128. On the root switch, under the interface you can configure 'spanning-tree port-priority (value)'. Lower value is better.

Pls. rate all helpful posts.

HTH,

Sundar

New Member

Re: spanning tree

thanks for the reply, when you say lower priority is better, do you mean better as in if its lower it will or wont block, I thought if it was higher it will block ?

thanks

Re: spanning tree

Again, unless your two ports are connected together (directly or through a device that acts like a hub) tuning their priority will have *absolutely* no effect on the local switch!

F/

Re: spanning tree

What Francois has stated in his post is correct. Based on your post, I assume you want to influence the root port selection between two directly connected switches. You have two choices - priority or cost. Cost should always be the preferred choice. However, you can lower the priority to anything less than 128 and the downstream switch will make that port as the Root and put other port(s) between the switches in Altn.(blocking) port state.

To get a better undestanding check out the following link.

http://www.cisco.com/en/US/tech/tk389/tk621/technologies_tech_note09186a00800ae96a.shtml

Pls. rate all helpful posts.

HTH,

Sundar

Re: spanning tree

Sundar,

I actually wrote the document you mentioned to illustrate how STP based its decision on the content of the BPDUs (and to explain the weird CatOS porvlanpri command;-)) rather than to recommend the use of the priority. Tuning the priority instead of the cost is nice because it is only a local parameter and does not influence the cost your switch is going to advertise downstream. However, it is extremely unlikely to apply in the case of Carl, that is probably the selection of a blocked port on an access switch:

-1- the priority needs to be set on the bridge that sends the BPDU (the designated switch in the core) while the cost is configured on the switch that receives BPDUs (the access switch).

-2- the priority will only have an effect if the most significant parameters in the BPDU are identical (that was the purpose of the paper). This means that Root ID, Cost *AND* Sender Bridge ID have to be equal. This implies that tuning the priority only has an effect when the two links are coming from the same switch... this is what I think is unlikely in Carl's setup.

But again, we cannot tell without knowing more about his setup;-)

Regards,

Francois

Hall of Fame Super Silver

Re: spanning tree

Assuming they are on the same VLAN and you haven't disabled spanning tree, yes. The port which would introduce a loop physically will cycle:

1. From initialization to blocking

2. From blocking to listening or to disabled

3. From listening to learning or to disabled

4. From learning to forwarding or to disabled

5. From forwarding to disabled.

For a simple network, manual manipulation (via configuration commands) of spanning tree is generally not necessary.

See http://www.cisco.com/univercd/cc/td/doc/product/rtrmgmt/sw_ntman/cwsimain/cwsi2/cwsiug2/vlan2/stpapp.htm for a more detailed explanation.

Hope this helps. Please rate helpful posts.

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