Aaron's Worthless Words

We're an Avaya voice shop (for now if I have my way) and have Avaya systems of various sizes and shapes all around the Enterprise.  I was at one of our remote locations a few weeks back and helped the guys there replace a non-PoE switch so they could get the old power injector panel out of their rack.  When we moved stuff around, the phones didn't come back and had the dreaded DHCP Ack Error. Read the rest of this entry »

Aaron Conaway

I like to lean my head to the left, hit it with the palm of my right hand, and document what knowledge falls out.

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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.

1. Lowest root bridge ID (which I hope we’ll never have to use)
2. Lowest root path cost
3. Lowest bridge ID
4. Lowest port ID

In our exercise, we only have to go to the tie breakers once, but, in the wild, you may run into more.  I would guess that this is a requirement to know for the SWITCH test.

Before we get started, you can assume that a bridge is a switch since, technically, a switch is a many-ported bridge.  Let’s get started.  There are only 5 steps to take, so it shouldn’t take long.

1.  Find the path costs of each segment

This is done with a table lookup.  Each segment (link between switches) gets a cost based on the technology that it is using.  For example, GigabitEthernet has a cost of 4 while FastEthernet has a cost of 19.  There are other values, but these are the only ones that are used in the network here.  I did find a good table on STP path costs at HowStuffWorks.com if you want to take a look at more of them.  Let’s put the path costs on the diagram.

2.  Find the root bridge

After we find the path costs, we find the root bridge.  The root bridge is the switch that has the lowest bridge ID, which is the switch priority and the MAC address concatenated together.  Since all the bridges have the same priority, we can just look at the MACs to figure out which one has the lowest bridge ID.  Switch E has the lowest MAC (0000.0000.0003), so that’s the root bridge.

3.  Find the root ports of each bridge

The root port is a port on a switch that has the lowest total path cost back to the root bridge; every switch has a one and only one root port except for the root bridge itself which has none (it’s the root; it already knows where it is).  The root bridge sends out BPDUs that are passed to each switch.  When a switch receives it (not sends it), it increments the root path cost field and passes that along,  The port on which the lowest root path costs is received is the root port, and the value in the root path cost field is the root path cost for that switch.  The finger trace method works for us humans for now; there’s not that many switches involved.  Let’s add the root ports along with the root path costs to the diagram.

4.  Find the designated ports

Each segment of the network has a designated port.  This is a port on the bridge with the lowest root path cost.  Since the root bridge always has the shortest path to itself, all of its ports are designated ports.  Root ports can’t be designated ports, too, so it’s pretty easy to find the designated ports on segments with root ports.  There are two segments (A-B and B-C) that we need to calculate, though.  Let’s go to the tie breakers!

All the bridges have the same root bridge ID, so that’s a wash.  C has a lower root path cost than B, so it wins that segment.  A and B both have the same root path cost, though, so we move down the list to bridge ID.  Since A has a lower bridge ID, it wins that segment.

Let’s put all the designated ports on the diagram so you can see them.

5.  Find the blocked ports

There aren’t many ports left, are there?  There are three actually, and all those go into a blocked state.  To get rid of the loops, any port that’s not a root port or a designated port is blocked.  Here’s the network with the blocked ports marked.

And we’re done.  That wasn’t so hard.

Send me any summer home rentals questions.

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Aaron Conaway

I like to lean my head to the left, hit it with the palm of my right hand, and document what knowledge falls out.

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When I started in IT, I tried to get my gear as standardized as possible to impress everyone. I worked at it and worked at it until I realized that there were a handful of things that sound good but just won’t work. If you’re just getting started in the field, you may not agree, but come back in 5 years and see how right I am. Heh.

• Assigning switchports to VLANs in chunks just doesn’t work. This seems like a great idea. You can put client A on port 1 through 12 and client B on ports 13 through 24. Then client A winds up with 13 servers, and B only has 3, so your whole scheme is in pieces on the floor. It’s just easier to plug servers into the next available port and forget physically organizing the ports. The switches don’t care if the ports are in order by VLAN. Just keep it simple.
• Color-coding cables only works for a while. Let’s cable web servers with green cables and application boxes with blue cables and the database servers with pink and the mail servers with aubergine. I promise you, though, that you will run out of cables of one color or another and wind up having a database server in green. Then you’ll have something else wrong. It won’t be long before the color standard only applies on paper.
• Labeling switchports by name only works if you buy servers all the time. If you’re in an environment where servers change roles and names, I guarantee you that your ports are mislabeled. The only time that labeling really works is if you’re lucky enough to work for a company with enough money to buy new stuff for every project. I’ve actually resorted to labeling ports with serial numbers instead of names since those won’t change.
• Complicated naming schemes don’t work. They may sound cool, but simpler names are almost always better. Name your router “r1″ or something.  Don’t try “rtr001prod1″ or something as ludicrous. I once made up this awesome naming scheme, and it worked until the business took on other projects that didn’t fall into the standard, so I was screwed. Save yourself some problems and keep it simple.

Aaron Conaway

I like to lean my head to the left, hit it with the palm of my right hand, and document what knowledge falls out.

More Posts - Website