Archive for seacug

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

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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

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Service, Service Please! Part II

Posted in Cisco Certification with tags , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , on February 16, 2012 by jjrinehart

TCP and UDP Comparison

TCP (Tansmission Control Protocol) and UDP (Use Datagram Protocol) are a lot like siblings in a family; you can see the resemblance and can tell they are related, but there are striking differences that set them apart.  For example, both use port numbers for accessing applications/services and both operate at Layer 4, but the similarities end there.

Ever met a control freak?  If so, then you have a pretty good idea of how TCP operates, in that it exerts significant control over how traffic is sent and received.  To begin with, TCP won’t send any data without sending up a virtual end-to-end connection between hosts, and it does so with a three-step process to establish that connection, as follows:

1. TCP SYN (synchronization)

2. SYN ACK (SYN acknowledgement)

3. ACK (acknowledgement).

After the connection establishment phase, a TCP connected hosts are free to send data, but does so in a very paranoid and calculated manner.  First, sequence numbers are assigned to the data, in order to reassemble everything in correct order.  Once the data is numbered and sent, the receiving station sends an acknowledgement, and if that acknowledgement is not received, the data is assumed lost and retransmitted.  Since that process can slow data transfer rates, TCP supports a concept called windowing, in which several segments of data can be sent before acknowledgements are required.  Sounds great, right?  Lots of mechanisms for connection-oriented, reliable delivery (which are terms often used to describe TCP).  The problem is, however, that if all of that extensive control is applied to every single piece of data, then everything is likely to take a lot longer to process.

If you are anything like me, you probably don’t follow a 57-point checklist before getting in the car and driving off when you go to work in the morning.  Welcome to the world of UDP, which contains no acknowledgements, no sequencing, no virtual connections, and incredibly low protocol overhead.  UDP is faster than TCP and well-suited to latency-senstitive traffic such as voice (although VoIP requires use of the Real-Time Transport Protocol in order to function).  The terms used for UDP and connectionless and unreliable, due to how it transmits traffic, and it is used often for network transmission.  When you look at both formats side by side (see above), it’s clear how vastly different UDP is from TCP.  The lack of reliability of UDP may seem somewhat random, but when you consider the fact that many applications handle retransmission and data delivery, then the thought of having other protocols do that may seem unnecessary.

Next time, we will dive into the world of IP routing…

– Joe

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Service, Service Please! Part I

Posted in Cisco Certification with tags , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , on January 6, 2012 by jjrinehart

What a Network Dog Says!

Aside from the general principles of IP addressing and subnetting, any CCNA candidate and/or student needs to know about IP Services.  In the design of the Internet Protocol (IPV4 in particular), there are several functions that other parts of the TCP/IP Suite provide to network users, broadly termed services.  The first part of this set of protocols involves Layer 4 of the OSI Model, namely the Transport Layer, with the Transmission Control Protocol (TCP) and the User Datagram Protocol (UDP).

The first aspect of TCP and UDP regarding services are pretty much identical between the two, so let’s consider that first.  Probably the simplest way to think about this is to consider how the television in your home operates; you don’t have to know all of the electronic details to use it, but knowing the concepts is helpful to making full use of the technology.  Using cable TV as an example, a great of content is available, but it would be impractical (not to mention scary) to have all of that sent to your television at once.  Instead, each content provider is assigned a channel, over which they transmit/broadcast programming to subscribers, who tune in on that channel when they want that particular content.  For example, customers wanting sports programming will tune into ESPN (for example purposes, let’s say that’s channel 88), those wanting news might go to CNN (example channel 47), while still others might just want popular movies (example channel 76).  Each subscriber on the cable network would be connecting on a different channel at any one time to get the viewing experience that they wanted.  Sounds simple enough, right?

Now let’s jump from the example to the details of TCP and UDP operation on networks.  Different machines (end users, for example), may want to access different types of information on a server/computer on the network, similar to the content on cable TV networks.  These hosts or workstations “tune in” (connect) to a specific port (similar idea to a channel on a TV network) to access a specific type of information/content on that destination server.  Each type of service uses a different port to access that service, most of which are set by standards groups (e.g., the Internet Assigned Numbers Authority or IANA at www.iana.org).  Common port numbers, for example, are TCP port 80 for http or web access, TCP/UDP port 53 for DNS, and TCP port 23 for telnet.  Any computer wanting to access that type of service requests a connection on that port on the destination device.  Each service coexists on a separate port.

Next time, we will distinguish between TCP and UDP protocols.

– Joe

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Sub(net) Operations! (Part II)

Posted in Cisco Certification with tags , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , on December 6, 2011 by jjrinehart

Subnetting Worksheet

I sincerely hope that the above graphic is readable as I created it myself to assist with subnetting tasks.  There are several ways to perform subnetting tasks, one of which involves rote memorization of tables for each addressing class, and the other involves mathematical calculations.  In my own certification studies I chose the memorization route because the math part scared me to death, although I did the tables the most difficult way imaginable!

The first step to keep in mind is what I mentioned in my first blog, namely that binary is the keys to the kingdom when it comes to networking know-how.  Just for review, remember that the 32-bit (4 octet) address is broken out into eight bits per section, and each bit is part of the powers of two, namely 1, 2, 4, 8, 16, 32, 64, 128 (see the image above).  The first logical step is to break each part of the address back into its binary representation.  For example, 192 is the combination of 128 and 64 (first two bits) which is binary 11000000.  One of the ways to get the most familiar with this process is simply to practice it over and over, you can find an address on a network and break it out or simply make up numbers and do the same thing.  To verify your work you can create a decimal-to-binary converter in Excel, either column by column or using the built-in functions.

One of the typical questions that seemingly appear on all practice exams is the request to find the subnet number, broadcast addresses and range of valid host addresses for a given address and subnet mask.  This is another thing to practice over and over since on the actual exam, time is a very precious resource and not to be wasted.  Here is a breakdown of how to accomplish this task:

1.  Write out the address in the question out in binary (see above for some guidelines).

2. Directly underneath, do a binary conversion of the subnet mask.  Most likely there will be strings of 1’s in the first octets.

3. Perform a Boolean AND Process (not an alien race, but a math operation).  In simple terms, it means to compare the column with the IP address with the column of the subnet mask.  Two 1’s means to write a 1 in the third row, and anything else is a 0.

4. Convert the new row back into decimal.  That is the subnet address.

5. Referring to the row you just converted back, there will be a string of 0’s at the end of that address.  Recopy the 0’s with 1’s and convert it back as well.  This will be the broadcast address.  To get the range of hosts, add 1 to the subnet address and subtract 1 from the broadcast.

More to come…

– Joe

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