Archive for the Cisco Certification Category

Finding the Missing Link…

Posted in Cisco Certification with tags , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , on July 13, 2012 by jjrinehart

One Link to Rule them All?

When you hear the word link these days, it can conjure up all sorts of images, from the main character in Zelda to something you might find on a web page.  At some point in school you were probably taught Darwin’s theory of evolution and the term missing link (meaning some transitional form in the chain of evolution that has yet to be discovered) came up.  In the networking world, however, the term is used of a functional, active connection between devices that allows them to share information.  In addition, the operating condition of those links is referred to as a state, and when you combine the terms, you come up with Link State, a class of routing protocols that take a more global view of the network than the dysfunctional Distance Vector Protocols.  If both of these classes of protocols showed up at a party, DV would be the nerdy, socially inept guy talking to himself in the corner, and LS would be the sharp-dressed, smooth talking fellow with a crowd of people gathered around him.  Why?  Because Link State protocols are infinitely more successful and intelligent about how they operate.

Link State protocols, as the name implies, have accurate and up-to-date information about every operational link throughout the entire network.  They never have to rely on rumor, because they can figure out the entire topology themselves.  This set of information is referred to as the LSDB, or Link-State Database, and does not contain routes but link information.  LS routers unpack the data, process it using an algorithm (for example, the Dijkstra Shortest Path First Algorithm) to calculate potential routes for use by the process.  The most well-known link state protocols are OSPF (Open Shortest Path First) and ISIS (Integrated System to Integrated System), with the latter usually regarded as more popular.

Another distinct difference in the link state world is how routers interact with one another, namely, in a more structured and formal fashion.  In a way, LS routers are like a lot of business people—they only do business with people they know.  Routers actually set up formal relationships, track availability and state of those routers, and send and receive data only with devices they know.  Rumor is no longer a problem, and neither are possible routing loops, because all information is known and available.  Now the focus is not on how far away something is, but rather the cost to reach that network, with cost being the available bandwidth of the links between the source and destination points.

Now that we have talked about the types of protocols, we can discuss specific ones, starting with RIP…

– Joe

Advertisements

Preventing the Epic Fail…Routing by Rumor Part II

Posted in Cisco Certification with tags , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , on July 9, 2012 by jjrinehart

Come, on, SERIOUSLY???

In our last discussion we described Distance Vector protocols as simple, and relying on rumor, namely that a router will just take the information it receives as accurate and reliable.  For any of you that have ever believed a rumor, you probably have discovered that it was anything but accurate, or left out key details? If that it not plainly obvious, watch presidential advertising during elections in the US!  Needless to say, these protocols are not the bright bulbs so to speak, and thus need “extra help” to avoid the creation of routing loops, which are the kiss of death in networking terms.

Enter Loop Prevention Mechanisms for DV routing protocols!  These help avoid routing loops, but also introduce a great deal of delay into the convergence process (convergence is the amount of time it takes the routing process to recover from changes).  Here is a list and brief description of these mechanisms:

1. Triggered Updates: DV protocols, such as RIP, send out their entire routing table at regular intervals.  What if a route fails before the next time interval?  This feature is the essential answer, namely that in the event of a change, the advertising router sends out an update immediately even if the time for a full update has not arrived yet.

2. Poison Reverse: No, this is not an execution by a would-be assassin, but a form of route poisoning, in which the route is declared invalid and marked with an infinite metric so it will be unusable.

3. Hold-Down: While this may sound like a wrestling maneuver, it actually refers to the time during which any changes to the route are essentially ignored.  If, for example, a faulty interface keeps going up and down, the hold down process will prevent the routing table from suffering a nervous breakdown.

4. Counting to Infinity: If all else fails, Distance Vector routing protocols have a numerical value that it considers infinite, and beyond which a route is considered unreachable.  In RIP for example, the maximum hop-count is 15, beyond which a route is unusable.  It’s essentially the “last resort” loop prevention mechanism.

All of these mechanisms together keep loops from forming, and while a little on the paranoid side, they keep the network stable.  The tradeoff is a much longer convergence time, which can cause outages and delays in the networks.  At one time this was the only protocol type available, but now newer, more sophisticated ones can be used in place of that.

Next time we will talk about Link State protocols…

– Joe

Gossip Girl…Routing by RUMOR, Part I

Posted in Cisco Certification with tags , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , on June 26, 2012 by jjrinehart

Distance Vector Routing

If you have ever been in any social setting in your life, then you understand the concept of gossip, where one person essentially reveals something about another person (and often something negative).  You may think you outgrew this in kindergarten or high school, but even if you have never participated in it, you have known about, or even been a victim of gossip.  Sadly enough, the truthfulness of the information being shared is often dubious at best.  Usually the fastest way to break the cycle is to ask something like, “may I quote you on that?”  While certainly a life lesson, the concept of gossip also applies to network routing as well.

Distance Vector routing, the first and earliest type of routing protocols, function by gossip, sometimes called routing by rumor.  When a distance vector router receives information from a neighboring device, it simply assumes that the information is accurate, and passes it along to any other devices participating in that process.  Never once does it stop to see if the updates came from a reputable source, or if the information itself is viable in the first place.  This is but the first flaw in distance vector protocols.

A second, but related, issue is the measure by which DV protocols make routing decisions, namely, the defined metric.  As you can guess by the name, these protocols choose routes based on how far away that network is, similar to the way we choose a route of travel (based on mileage, for example).  If one road takes 43 miles to reach a specific city, and another only takes 25 miles, we will usually choose the route with the least mileage.  Routing Information Protocol (versions 1 or 2) use this same basic approach using hop count, or how many Layer 3 networks a route crosses to reach a destination network/subnet.  Simple, right?  Yes indeed, and in reality probably a little bit too simple.  Going back to the example a moment ago, what if the shorter mileage was on surface streets with stop signs, traffic lights, and low speed limits?  The mileage might be shorter, but the travel time would most likely be quite a bit longer.  Simply put, distance vector protocols have no way of knowing or judging the quality of the route, only the distance.  To make this network specific, if one route to a network was using a T1 link (1.544 Mbps) and another was using a 56 Kbps link, RIP would not know any difference, only how many hops away it is.

Distance Vector protocols also require additional mechanisms to prevent routing loops, which we will consider next time.

– Joe

Long Distance Relationships?

Posted in Cisco Certification with tags , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , on June 21, 2012 by jjrinehart

The Concept of DISTANCE

As a sign of the times, may relationships today begin on the Internet, not so much in old-style chat rooms (though I am sure that may still happen), but through online dating and such; this is how my wife and I met in 2005.  In our case, we lived less than 30 miles from one another, but I have heard stories of relationships that span a continent or even the globe!  The good news is that the ability to communicate across the planet can make the world seem much smaller, but that excessive mileage can seem discouraging when far from a loved one.  In this example, distance can create very real barriers that can impact our lives.

In networking, this idea of distance plays out in other ways, first in determining metrics for routing protocols.  In programming, you typically create some sort of value hierarchy to determine the best possible choice, usually using a specific algorithm.  Dynamic routing protocols use a number of these methods and base the choice of the best possible route on the results of the processes used by those algorithms.  The problem is that one protocol may use one set of values, and another a completely different set, with no way to arbitrate between them when multiple protocols are involved.  In multiprotocol environments (more than one set of routing sources involved), there needs to be a way of ranking the best sources to prevent chaos and confusion.  Enter the concept of administrative distance.

In Cisco terms, administrative distance refers to the level of reliability (or believability) of a particular routing source.  Naturally, some are better than others, so a value between 0 and 255 is assigned to a protocol/source, with the lower the number the better the source.  Here is a breakdown:

List of Default Distances

As you can see, the farther up the numbers go, the less desirable the routing source is (iBGP is the highest at 200).  This set of values is local to the device and can be changed with the distance command under the target protocol.  In the CCNA labs I created, I used the distance command to create a failover process (check them out).  More to come…

– Joe

Giving Some STATIC (Routes, that Is!)

Posted in Cisco Certification with tags , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , on June 13, 2012 by jjrinehart

The Original Static Route!

Last time we talked about the IPv4 routing table and how the information populates it in the first place, starting with directly connected routes.  The fact that interfaces with these routes have to be up and functioning to be in the table at all makes them inherently “believable” (the term used to describe routes).  Routes in the IPv4 routing table from this source are marked with a C (connected) and easily recognizable.

That brings us to the topic of static routes, which may evoke images of lightning (see above), or walking across a carpeted room and sending a shock to someone just for fun.  Certainly the word static is used in those contexts, but in this case it simply means fixed or stationary, and refers to the fact that these routes stay permanently in the routing table.  You might think that this sounds like a description of connected routes too, but in reality it means that the routes are manually configured by a network administrator.

You can think of static routes in terms of the large concrete barricades that separate the sides of traffic on the highway.  First, they stay in one place, they do not move, even when an object may strike them with some degree of force.  Static routes remain in the table once configured and do not simply vanish when things are taking place from a routing perspective.  Second, these barriers unforgivingly separate traffic, just as static routes continually direct traffic to their intended destination.  Finally, these concrete dividers are actually put there by highway workers, they do not simply magically appear, just as static routes have to be placed manually in the router’s configuration.

The configuration of a static is remarkably straightforward and uncomplicated, using the following syntax:

ip route <destination-prefix> <subnet-mask> <next-hop-address | exit-interface> [<distance>]

The destination prefix is a subnet and/or network using standard decimal notation (e.g., 192.168.2.0), followed by a subnet mask, just as you probably have seen and used numerous times in your networking job and/or studies.  The next part is critical to understand, as you have options you need to consider.  You can choose simply to specify the outgoing interface, or you can specify the IPv4 address of the device you want to send the traffic to (typically directly connected).  In either case, if the interface is down and/or the next hop is unreachable, the route will not appear in the table.  The optional distance command allows you to specify the administrative distance, which we will cover next time!

– Joe

Road Trip, the Sequel! (aka, Introduction to IP Routing II)

Posted in Cisco Certification with tags , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , on June 4, 2012 by jjrinehart

Cross Country RV Trip

Continuing the theme of road trips, my wife Brenda, our son Josh and daughter-in law Liz, and two dogs got into our thirty-one foot RV and headed across the country, not metaphorically, but literally. The real part of the adventure actually happened before the trip when I decided to do the brakes myself. We had been given a $3-6000 estimate on doing that at a professional garage, and after I came to, I decided that was far too spendy and would do the job myself. Have you ever launched into something and then halfway through decided it was a stupid idea? It took almost forever, and even though I have done brakes before, it’s a much greater challenge on a vehicle that enormous. I would probably do it all over again, but with a lot more trepidation.

Our “navigator map guy” was a team of my son and wife, who plotted out the route of travel, keeping our two-week timeframe squarely in the middle of the calculations. To their credit (they did a lot of the driving as well), we stayed fairly close to our plan all along the way. There were a variety of factors that they had to keep in mind:

All of those added together formed the basis of the various routes we needed to take to get all the way across the country step by step. Our plan was written out, with the details needed to get to the next step of the journey.

 

In the world of IPV4 routing, we call this the ip routing table. Just like the vacation driving plan, there are several critical pieces of the table:

The “means by which the route is chosen” needs further expansion, since it describes how the route was learned and ended up in the table to begin with.  There are three primary means why which routes are installed in the ip routing table:

Directly Connected Routes are those that “live” on the device itself, usually a router.  These routes come from functioning interfaces with IPV4 addresses learned by DHCP or manually configured.  This makes them the most reliable and believable routes on the device.

Next time, we will dig into static routes

–          Joe

Leave a comment »

Road Trip! (aka, Introduction to IP Routing I)

Posted in Cisco Certification with tags , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , on May 31, 2012 by jjrinehart

Route Sign

As you may recall from the very first entries on this blog, networking is all about communication—getting information from one location to another.  IPV4 packets carry that data from a source address to a destination address and all, but how does the process know how to get there in the first place?  The simple version of the answer is just IP routing!  Routing is the process of figuring out the best path to travel between any two given points in the journey.

To explain this a little bit, let me describe how I remember family vacations when I was a kid.  My Dad would load up the station wagon (yes, the nerdiest possible vehicle in history, I know), load us all inside, and then head out on the road.  My father was not the typical “Red Foreman” dad (in case you don’t remember, he was the colorful character from That 70’s Show).  My Dad is an engineer, and approaches everything with a carefully calculated plan by which to succeed, without much tolerance for variance.  Out would come the map, and he would move through and figure out the safest, most efficient path of travel, all before we ever actually got out on the road.

In a nutshell, that’s almost the same exact process that network devices (routers) use to figure out how to send data from the source to the destination.  When there are multiple possible ways to get there, a process is used to determine the most efficient manner to get from end to end.  Each part of the journey involves various networks, addresses, interfaces and media, not unlike the various highways and access roads used on a long-distance road trip.  And no, none of the small packets in the back seat keep asking “Are we there yet” over and over!

Each segment of the “trip” of an IP packet is managed by a Layer 3 routing device, typically a router or Layer 3 switch.  These devices maintain a list of networks it can reach, and by which exit point from the device (interface) it uses to reach those.  Once the packet leaves that device, it becomes the responsibility of the next one in the chain, which uses a similar process (route lookup) to send traffic on to the next part of the journey.  Once it reaches the destination, the process is repeated for return traffic, just as a road trip usually involves heading back to the point of origin.

Next time, we will take a look at the various methods used for specifically accomplishing the forwarding process.

–          Joe

1 Comment »