I’ve got a few switches daisy chained together with single links and have enabled UplinkFast on them.  This switch is not the root bridge; F0/24 is the root port and F0/23 is a blocked alternate port. I’ve got debug spanning-tree uplinkfast on to help out.

SW3#sh span | incl 0/2[34]
Fa0/23           Altn BLK 3019      128.23   P2p
Fa0/24           Root FWD 3019      128.24   P2p

Now let’s unplug F0/24 and see what happens.

I was doing some STP labs tonight and found something that caught me off guard a bit.  I had been meddling with some EtherChannels between a pair of 3750s earlier today, and I forgot to reset the configs before starting on the STP stuff.  One my secondary root switch, I ran a show spanning-tree vlan 1 to see what status the ports were in, and I noticed the root path cost.

Did you guys have any trouble with the solution to the STP exercise?  Let’s work through it and see what happens.  I got a few responses to the solution, and everyone seems to get the same answer, so I assume we’re all right.

Before we get started, I wanted to mention the tie breakers since there can be ties in STP.  If there is a tie in any calculation, the same tie breakers are used, so I’ll list them here to use as we move through the calculations.

Here’s an STP exercise for you.  Given the bridge priorities, MAC addresses, and interface types in the diagram, calculate the root bridge, root ports, designated ports, and blocked ports.  You can click on the image to enlarge it.  I’ll post a solution in the next few days.  As always, feel free to comment and ridicule my utter idiocy.  Be gentle, though; I don’t usually post exercises like this.

Send any configuration BPDUs questions my way.

I’ve decided to take on the CCNP certification, so I’m going to wind up with a few posts will be more my own notes than anything.  :)

A switch port on a 2960 comes up with a default configuration on VLAN 1.  What happens from the perspective of spanning-tree?

• First, the port comes up on blocking mode.  This is to make sure that loops aren’t created without first listening to the network to see what’s going on.
• Next, if the port may be a root or designated port, the port is moved to the listening state.  In this state, the port can send and receives BPDUs only.  It can’t send traffic, but it can discover the other switches participating in STP.
• After the forwarding delay, the port goes into the learning state.   In this state, the port can send and receive BPDUs as in listening, but it can now receive traffic.  It can’t yet send any.
• After the forwarding delay again, the port goes into the forwarding state.  The port can now send and receive data.

If the port is configured with spanning-tree portfast, the mode goes from blocking directly to forwarding without going through these steps.  Obviously you don’t want a switch plugged into a port configured for portfast since you may wind up with a loop.

We just an HP C-class blade chassis which included two GbE2c network modules.  These modules are Nortel switches running AlteonOS that connect the blades to the rest of your network.  When I turned these guys up the other day, every VLAN stopped working, so I ran down to the data center and unplugged the uplink.  I called HP and soon found out that the GbE2c doesn’t play nice with Cisco switches out-of-the-box.  Since we have a Cisco network (not now, I guess), we can into some problems.