Archive for loops

No Strings (Wires) Attached: Wireless LANs, Part II

Posted in Cisco Certification with tags , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , on July 22, 2011 by jjrinehart

Wireless = Radio!

Wireless LANs transmit signals across the air rather than across copper wires or fiber optic cable.  For those of you who can remember back far enough before cable television, you may recall seeing antennas sticking up from the back of the set (remember rabbit ears?).  Television stations transmitted one-way signals that reached the television set, were decoded, and turned back into light and sound to entertain the masses.  In the “good old days” you turned a knob on the front of the set to change the channel (frequency) that was being displayed on the screen, and only a couple were usually available, NBC, ABC, and CBS, and maybe PBS.  Understanding the basics of wireless LAN technologies actually start at this point, in getting a better grasp of how radio signals actually act and operate.

Radio signals travel through the air and require both a transmitter and receiver, which are actually separate operations, although at one time the term transceiver identified something that did both.  Just as with human speech (using sound waves), wireless technologies are analog rather than digital.  Digital signals have one of two values, namely Zero (0) or One (1), indicating on or off status of a computer circuit.  Electromagnetic radiation, including radio frequency (RF), transmit information by changing some aspect of these waves, usually termed frequency (the measure of how many waves are repeated per interval), amplitude (strength of the signal), or phase (difference between the wave and some reference point).  All wireless technologies use some form of encoding/modulation to change the signal to communicate the zeroes or ones in order to carry the digital information.  For the sake of simplicity, let’s think of frequencies the way you typically use them: channels on your television or radio.  When you want to receive a different stream of data (for example, ESPN instead of the Opera Channel), you use the remote control to change the frequency from one channel to another.  In the United States, the Federal Communications Commission (FCC) sets the rules for who can use certain frequencies, as well as power levels so that they can coexist.  Some organizations pay the “big bucks” for use of certain frequencies of operation, such as television and radio stations, and cellular telephone companies.  These are referred to as licensed frequencies because they require a valid agreement in place with the FCC in order to use them.  For our purposes, we need not worry about these RF signal families, but rather those that are part of the unlicensed frequencies group.  Since this is a much deeper topic, we will discuss this next time.

– Joe

No Strings (Wires) Attached: Wireless LANs, Part I

Posted in Cisco Certification with tags , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , on June 20, 2011 by jjrinehart

Evil Wire Monster! Run!

The evolution of networking has been rapid over the past several decades as computing moved from a single centralized mainframe to a more distributed model with server farms.  Alongside computing has been vast arrays of cable plants, originally consisting of thick coaxial cable, migrating to thin coaxial cable, to the twisted pair cables we all know and love.  However, just as a puppet without strings would be considered a marvel, a network without wires is equally impressive and desirable; this brings us to the subject of wireless LAN’s, or WLANs, as they are affectionately called.

The umbrella IEEE standard for wireless is designated as 802.11, representing a wide family of other standards, protocols, and so forth.  The establishment of the Wi-Fi Alliance (http://www.wi-fi.org/) in 1999 to promote interoperability has not only created widespread awareness of the technology, but has become synonymous with te technology itself (users frequently refer to WLAN’s as “wi-fi networks.”)

It probably sounds overly simplistic to say that the differences between wired and wireless networks are vastly different, but there is more truth to that than simply saying one medium uses wires and the other does not.  There are some similarities, however, that should not be overlooked:

  1. Layer 2 Technology: While they implement it differently, both operate at Layer 2 of the OSI stack.
  2. Communication Between Devices: Both allow for inter-device communication and data transmission.
  3. Frame Formats: While not identical, both use frame formats constructed with a similar anatomy, including headers/trailers, source/destination MAC addresses, etc.

One of the most striking differences between the 802.11 family of wireless standards and their 802.3 relatives has to do with the mechanics of data transmission; Ethernet uses Carrier Sense Multiple Access with Collision Detect (CSMA/CD), and responds to frames which collide in transit, while a WLAN uses CSMA/CA, in which is the stands for avoidance.  If you think of an intersection with cars crashing into one another as analogous of Ethernet, cars constantly swerving to get out of the way would be closer to wireless operation.  Here is the basic process a wireless devices uses to transmit data:

  1. Listen to make sure there is no traffic on the medium (in this case, the channel/air)
  2. Set a random timer and do nothing until it expires
  3. Listen again to make sure that there is no traffic
  4. Send the frame
  5. Wait for an acknowledgement
  6. If there is no acknowledgement, assume the frame was lost and start over at #1

Keep in mind that this is the process for transmitting a single frame, and you don’t have to be a rocket scientist to see the amount of overhead this takes.  The reason?  While you can control things that happen on a wire, you have no control over the air, namely transmitted signals.  Lots more to come!

– Joe

The (Necessary) Evils of Spanning Tree, Part III

Posted in Cisco Certification with tags , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , on June 16, 2011 by jjrinehart

Port States, Get it?

To pick up where we left off, I wanted to take a minute to talk about 802.d port states, not to be confused with US states on the eastern/western seaboard (insert groan here at the bad joke).  In traditional spanning tree, remember that a loop-free path through te network is essential, and the mechanics of that is a little bit of paranoia.  The protocol assumes that loops can creep in undetected at various stages of the game (so to speak) and one of the mechanisms to prevent that is port states…a series of stages that a switch port must go through in order to pass traffic.  Think of each as a “check point” on a tightly guarded road, at which the vehicle is allowed through to the next point.  Here are the port states, in order:

  1. Blocking (does not pass traffic, forward frames, or learn MAC addresses)
  2. Listening (does not pass traffic, forward frames, or learn MAC addresses)
  3. Learning (does not pass traffic, forward frames, but does learn MAC addresses)
  4. Forwarding (passes traffic, forwards frames, and learns MAC addresses)
  5. Disabled (shutdown , does not pass traffic, forward frames, or learn MAC addresses)

Another important part of the spanning-tree process is how the path through the network is determined, and the short answer is cost.  Every functioning interface has a cost associated with it that is based on the bandwidth of the port, which is selected by default but can be changed through manual configuration.  At the grocery store, for example, you are far more likely to buy an item on sale at a discount than you are to pay full price, given a choice.  Spanning-tree does the same thing by choosing lower-cost ports (and as a result, paths) to find the best way back to the root switch.  Keep in mind that cost is cumulative from a given switch back to the root, but the principle is pretty straightforward.

What if could get all the strengths of spanning tree (loop-free path calculation, the enhancements explained in the last blog, etc.) without its weaknesses (such as a 50-second convergence time)?  That was the essential question that sparked a more improved/updated version of spanning-tree, known as Rapid Spanning-Tree (RSTP, 802.1w), which incorporated all of the Cisco-based improvements such as portfast, uplinkfast, and backbonefast and made them standard features.  In addition, the port states discussed earlier collapse from five to just three, namely discarding (replaces disabled, blocking and listening), learning, and forwarding.  As a sort of “icing on the cake,” the Maxage timer is dropped from 20 seconds to 5 and forward-delay is eliminated altogether, resulting in a convergence time of less than 10 seconds.  Take that, 802.1d!

– Joe

The (Necessary) Evils of Spanning Tree, Part II

Posted in Cisco Certification with tags , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , on June 13, 2011 by jjrinehart

Timing Is Everything

One of the more important aspects of 802.1d Spanning Tree Protocol operation has to do with the use of specific timers, three of them specifically.  These critical timers are as follows:

  1. Hello Timer: As the name suggests, this timer is concerned with the frequency at which hello messages are sent from the root switch and subsequently propagated throughout the switched network.  By default, the interval is set at two (2) seconds, and the root switch sends out the Hello BPDU out all of its actively functioning interfaces.
  2. Max Age Timer: The Max Age timer is not the point at which a worker should retire, but rather the length of time switches should wait before triggering changes in the spanning-tree topology.  This happens in response to events in which the hello messages are failing to appear.  Rather than being dependent on a “Max Age” setting, this timer is derived by multiplying the hello interval by 10, yielding a default setting of twenty (20).
  3. Forward Delay: Ever paranoid about loops, spanning-tree sets yet another timer as a blocked port moves to a full forwarding state (more about port states later).  There are two steps in this process, each of which are allocated fifteen (15) seconds each.

For spanning-tree to converge, the Max Age (20 seconds and Forward Delay (15+15=30 seconds) timers have to expire, which ensures that no accidental path loops will be introduced during a topology change (50 seconds total).  I would imagine you cannot think of anyone that would be willing to wait as long as a full minute for network resources to become available again, right?  To get around this issue, Cisco created some new feature to trim or eliminate the ridiculously long convergence time, as follows:

  1. Etherchannel: Since spanning-tree blocks multiple links, why not bond ports to create a single, larger logical connection?  This is exactly what Etherchannel does, it creates a bundle of similar interfaces into a larger entity “Port Channel” with the benefits of greater bandwidth and nullifying the blocking issue of multiple ports.  If one link drops, spanning-tree never has to reconverge.
  2. Portfast: Many devices connected to the network (workstations, servers, etc.) pose no loop threat and do not even participate in spanning tree, so the portfast feature puts the port into forwarding mode immediately.
  3. Uplinkfast: Miss America pageants have first runner-up contestants that can immediately be promoted to first place without having to stage an entirely new pageant.  Uplinkfast tracks secondary root (alternate) and designated (backup) ports for this very purpose.
  4. Backbonefast: If indirect links upstream/downstream of the switch fail, the switch can query its neighbors for new path information without waiting for the Max Age timer to expire.

Next time, we will look at 802.1d port states and improvements created in a newer version of spanning-tree, Rapid Spanning Tree (802.1w).

– Joe

The (Necessary) Evils of Spanning Tree, Part I

Posted in Cisco Certification with tags , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , on June 9, 2011 by jjrinehart

The Network Without STP: Chaos

My wife and I had the privilege of getting to visit Europe, specifically Italy, Croatia, Spain, France and Italy several years ago, and got to stand in the middle of St. Mark’s Square (pictured above).  You can clearly see the chaos of clouds of birds descending on the square having no restraint of any kind and the wild disarray as a result.  This very same kind of chaos can result if there are no traffic safeguards in the network, when frames (later 2) run through the network with no protection whatsoever.  These “storms” can literally halt a network in its tracks and prevent any legitimate traffic from getting through.  I personally witnessed this at my workplace, a faulty switch was throwing out bad packets that slowed everything to a standstill.  The global NOC thought there was a virus, but eventually they tracked it down to a cheap switch in one of the conference rooms.

Traditional Spanning Tree (IEEE standard 802.1d) was designed to create a loop free path throughout the network, to prevent situations like the one described above.  Remember that unknown unicast, multicast, and broadcast frames are all flooded out all interfaces (except the originating one) by default, so controlling the paths that frames travel is no trivial matter.  The basic principle is that there is only a single active path allowed end to end; any redundant links are put into a blocking state, meaning that they cannot pass any traffic.  If you only have one set of interfaces active between switches then that precludes the possibility of loops, but then you are susceptible to link failures.

The very center of the Spanning-Tree universe is the root switch, and like human being, it firmly believes that everything rotates around it!  Other switches connected to the network have ports pointing either toward or away from the root switch.  Those pointing back to the root are referred to as root ports, and those leading away (which forward hello messages from the root) are called designated ports.

Now let’s talk just briefly about those hello messages.  Only the root switch sends them out, and they are forwarded by each switch throughout the network.  These messages act as keepalives, help detect link failures, and help elect the root switch when spanning tree starts up.  When switches power up, they all start sending out hello frames claiming to be the best switch for the root, until they hear a better (technical term is superior) hello, usually based on a preconfigured priority and/or the MAC address of the switch.  Eventually the rest of the switches stop sending out hello messages and just wait for the root to send them out.

More to come…

– Joe