WANs II: A New “Lease” on Life!

Private/Leased Line Network

One of the sayings I tend to use when explaining network concepts is a twist on the opening words of the book of Genesis, “In the beginning was the mainframe…” because so many things we take for granted today started with that piece of technology.  In terms of WAN connectivity, the original form of connectivity took place across specialized telephone lines that carried data rather than voice conversations.  Because the end-user/customer paid the IXC for the exclusive use of the line, they were referred to as private lines or point-to-point leased lines.  As mainframes were replaced by personal computers (and networks) these lines connected to devices that convert bits of data to electrical impulses that can be transmitted across these lines for long distances.  The technical term for this device is a Channel Service Unit/Data Service Unit, or CSU/DSU, which used to be an external device but are now integrated on interface modules on routers.  This is actually a good time to introduce two more terms, namely DCE and DTE.  A Data Terminal Equipment (DTE) device is the terminal or end-point sending and receiving information, usually a computer or router.  DCE devices, on the other hand, perform the conversion between raw data and the format needed for transmission, such as a CSU/DSU or modem.  The acronym stands for either Data Communications Equipment or Data Circuit-Terminating Equipment, depending on the publisher.  Cisco prefers the latter term as a general rule.

Private lines utilize electrical circuits to create the pathway between locations, as illustrated in the diagram above; this is in contrast to packet switched networks (which we will deal with later).  Depending on the part of the world you live in, there will be differing names and capacities, such as T1 (1.544 Mbps) or E1 (2.048 mbps), and even higher speeds.  These are typically copper connections, with very high speeds delivered on fiber optic connections (OC-X).  In North America these are the T1/DS1 and T3/DS3 standards, while the rest of the world utilizes the E1/E3 standards.  These lines are charged by mileage and often very expensive as a result, although very secure since a customer has private and full-time use of the connection, although if it ever sits idle that ends up being wasted bandwidth.  The structure of the framing and line coding ca be rather complex, and something you should definitely be familiar with as you pursue your certification studies.

Next time we will look at packet-switched WANs! Don’t you just love this stuff?

– Joe

WANs I: LANs, MAN’s and WAN’s Oh My!

A Wide Area Network

I started my career in the networking industry at a small Internet Service Provider in Seattle, Washington in the United States (this was in the late 1990’s).  At that time, most end-users accessed the Internet using dial-up connectivity, and modem speeds topped out at about 28 Kbps (yes, it sounds like ancient history).  From there I went to work for AT&T, where I spent the next five or so years assisting Fortune 500 businesses connecting multiple locations together; needless to say, I spent a lot less time dealing with Wide Area Networks than Local Area Networks.

Although I have since mastered LAN’s, I still have a great fondness for WANs and enjoy building labs that simulate them. LANs are suspiciously easy to recognize, first because they use Ethernet switches, but also because they occupy a fairly localized geographical area (hence the term LAN).  Wide area networks are also easy to recognize, as they almost universally depend on large telecommunications providers (AT&T, Verizon, British Telecom, etc.) and use an entirely different set of connections to provide services with.  A term that can be confusing, however, is that of a MAN or Metropolitan Area Network, and as such, needs some clarification.

The simplest way to differentiate MANs from WANs is to look at geography once again.  LANs connect computing devices with a floor, building, or campus, but no further than that.  WANs include networks that tie together sites across significant distances, such as nationally or internationally.  MANs are networks that lie within a smaller, more specific region; in a sense all MANs are WANs but not all WANs are MANS.  Think of the concepts like squares and rectangles; all squares are rectangles but not all rectangles are squares (using the classic shape definitions).

The more technical way of explaining Metropolitan Area Networks involves a little bit of US telecommunications history, specifically when AT&T was broken up in the 1980’s.  In the new system following this “divestiture”, new regions were created within states called local Access Transport Areas, or LATAs.  Local phone companies (e.g., Bell Atlantic, Pacific Bell, etc.operated in these LATAs, and Inter-Exchange Carriers (IXC’s) created connectivity between these areas; each had to operate separately.  In this arrangement, a MAN would be between locations within a LATA, while a WAN would be between them.  Usually this would encompass a city and its outlying suburbs and such, hence the term “metropolitan.”  With regulatory changes, these distinctions are not nearly as relevant, which explains why the term MAN is far less frequently used.

Next time we will look at some of the types of WANs that exist today!

– Joe

EIGRP III: Choosing the Winner of the Miss EIGRP Pageant!

The Winner! (Feasible Successor)

Even though EIGRP is a simpler protocol in many ways, there are still important concepts to understand, one of which how it chooses routes.  As we talked about before, the lowest metric (constrained bandwidth + cumulative delay) wins the contest for the best route, without the somewhat confusing exceptions that exist in OSPF.  However, because there are elements of Distance Vector routing in this hybrid protocol, there inevitably have to be some kind of loop prevention mechanisms.

EIGRP uses two forms of the calculated metric to a network to select the best loop-free route, and at first glance they might sound identical.  The first value is called the Feasible Distance (FD), which is the complete metric from the router to the destination network.  The second value is called the Reported Distance (RD), which is the metric from the point of view of the next-hop router.  If a path is loop-free, the value of the RD will be less than the FD, expressed mathematically as RD < FD.  Paths that could cause loops will have opposite values, which will make EIGRP discard it.  That may sound simple enough, but as you have probably heard on infomercials, “But wait, there’s MORE“!

Routes that meet the RD < FD test (called the Feasible Condition) are held in the EIGRP Topology Table and the best route gets installed in the IP Routing Table.  This best route is called the Successor, or Successor Route in EIGRP, and if other equal-cost routes exist they are also flagged as successors and installed in the routing table.  One of the truly amazing features of EIGRP, however, has to do with backup routes.  In just about every other protocol, if the primary route fails, the entire convergence process for that failed route starts over to select a new one.  In EIGRP, though, another loop-free path is held in reserve for immediate use if the successor fails; this route is called the Feasible Successor.

If you have ever watched any type of beauty pageant (Miss America, Miss Universe, etc.) then you have actually seen this in action!  After all of the contestants have proceeded through the judging process, one of them is crowned the winner (Successor)!  However, the next contest in line, usually called the first runner-up (Feasible Successor) automatically becomes the winner if anything happens to the original victor–no new pageant is required!

That covers the basic principles of IP routing, next time we will start considering Wide Area Networks!

– Joe

EIGRP II: Metric Calculation with a GPS…

GPS via Google Maps

Unlike most of you, I can get lost in my own backyard; imagine how misdirected I can get when I am actually driving!  My beautiful wife Brenda (a feisty little redhead) is a walking, talking, nearly-always-right human GPS, which is wonderful except when I am driving alone and trying to get somewhere.  Fortunately, in our world of GPS devices and smartphones (complete with Google Maps), I have some recourse for not getting lost.  Even so, these devices are not foolproof, as they once told me that a hotel was in the middle of the Potomac River in Washington DC!

These handy little devices are great because they rely on large databases which contain information on mileage, geography, construction, road conditions, etc., when trying to help get you from one place to another.  Just as GPS devices use multiple criteria for recommended a route of travel, so EIGRP relies on several different elements in calculating metric for destination networks.  This is remarkably different from every other interior routing protocol, which typically relies on a single element for its metric.  Here is a breakdown of the five elements of the EIGRP metric:

1. Bandwidth: At first glance you might think that this is identical to the OSPF cost concept, but there are a couple of important differences.  While bandwidth does create a cost-like factor (the higher the better), in EIGRP this cost is not cumulative.  Instead, it is based on the lowest bandwidth along the path to a destination network.  For example, if one path has all 100 Mbps links and another has 100 Mbps links with one 10 Mbps links, the first path will be preferred because the smallest (called constrained) bandwidth is 10 Mbps.  This might sound less ideal than a cumulative cost until you think of how backed up a highway gets when narrowed down to one or two lanes!

2. Delay: Unlike bandwidth, this factor is cumulative along the entire path.  The greater the delay, the less desirable the route is because delay is caused by lower bandwidth and/or congestion.  Why choose a 4 lane superhighway if the traffic is crawling along at a very slow speed?

3. Reliability: As the name implies, this measures how reliable the route is (0-255)

4. Load: How loaded or saturated the route is (0-255)

5. MTU: The IPv4 Maximum Transmission Unit size.

Keep in mind that only bandwidth and delay are enabled by default for metric calculation, and each element is called a K-Value.  Always make sure the K-Values match between neighbors or a relationship will never form.

Next time we will key in on route selection in EIGRP…

– Joe

EIGRP I: The “Borg” of IP Networking

Resistance is Futile!

While not completely universal, there often seems to be a natural affinity between networking geeks and the Star Trek science fiction franchise.  While I grew up watching reruns of the original series, I took a liking to the characters of the “next generation” cast (for you purists, TOS and TNG respectively), and found the Borg to be the most compelling super villains ever!  For those unfamiliar with these mechanical zombies, they are mechanically augmented humanoids all linked by technology to a central mind, being partly biological and partly machine.  Think of Darth Vader as “Borg Lite!”

The complexity of Borg characters is that they are not completely humanoid, though possessing certain biological characteristics, nor completely mechanical, though having aspects of that as well.  In essence, they represent a hybrid of the two, and it is worth noting that the term used for this type of being generally is a cyborg.  In other words, they are not one or the other, but a blend of both.  This is precisely the situation in which EIGRP finds itself, having characteristics of Distance Vector Protocols, as well as features of Link-State Protocols.

When you read white papers, books, and documentation on EIGRP, you will notice this type of duality present in characteristics of EIGRP.  For example, like LS protocols, EIGRP build formal neighbor relationships and tracks the state of those relationships.  Conversely, this protocol also uses the familiar DV loop prevention mechanisms such as split-horizon and hold-down states.  When you peruse the literature on EIGRP you will typically hear the word hybrid to reflect the nature of operations, although I have seem references to balanced-hybrid and advanced distance-vector as well.

A few similarities exist between EIGRP and OSPF, beyond neighbor relationships alone.  Although not a Link-State Database, EIGRP does build its own table of subnets called the Topology Database (more on this in another entry).  It also chooses a lowest-cost route to a destination subnet, but the criteria are entirely different from OSPF.

The differences between the protocols are more numerous that the similarities.  First, OSPF is based on an open standard, while EIGRP is Cisco proprietary.  In short, if you have non-Cisco devices in your network, you either have to do some form of redistribution (sharing routes between protocols) or you have to use OSPF or another standardized protocol.  Another significant difference is that EIGRP is not formal—no areas, DR’s/BDR’s, and so forth; no hierarchy exists, which certainly makes it simpler in many respects.

Next time we will dig into the EIGRP metric and route selection process.

– Joe

OSPF V: “You Have Chosen…Wisely” Path Selection Process

You Have Chosen…Wisely

In my opinion, the best movie in the Indiana Jones film franchise was Indiana Jones and the Last Crusade; aside from the pure enjoyment of the action scenes, the film gave great attention between “Indie” and his father.  One of the most-quoted lines from this movie, in pop culture at least, was the statement by the night (pictured above), “You have chosen…wisely.”  The whole concept of making the best possible choice ties in particularly well with OSPF, which puts it head-and-shoulders above Distance Vector routing protocols, since it truly can choose…wisely!  If you recall our earlier discussion about Link-State routing protocols, instead of depending on “mileage” (how far away something is”, the basis for route selection is cost, related to bandwidth.

OSPF uses cost in a cumulative manner–meaning that all of the costs of the links to a destination network are added up together.  If you have ever used a GPS for travel, this makes perfect sense, since the device would recommend the eight-lane Interstate highway (greater “bandwidth”) over the two-lane country road, regardless of the distance.  Ironically, you can even specify the route to avoid toll roads (a different spin on cost), to choose the best way to go.  If you think about it, this makes perfect sense, since you can go faster and have fewer stops on the bigger road, especially if you are tracking the route of travel from end-to-end.  In networking terms, then, OSPF devices will choose a 1.544 T1 link over the vastly inferior 56K link in choosing the best route.  What could be simpler?

Remember how strict and rule-oriented OSPF is?  Well, this applies to route preferences as well, meaning that there are additional selection criteria that will override the cost directive.  This fits into the hierarchy of OSPF areas, and creates the following list of route selection preferences:

1.  Intra-Area: Always choose the path within the area first.

2. Inter-Area: If no routes to the destination exist within the area, choose a path to another area but within the OSPF domain.

3. External Type 1: If no routes to the destination exist within the OSPF domain, choose an E1 route (remember that E1 routes count the cost to the external router in the metric)

4. External Type 2: If no E1 routes exist to the destination, choose an E2 route (remember that E2 routes do not count the cost to the external router in the metric, and redistributed routes are E2 by default)

As you can see, this can substantially change the route selection process.  Next time we will look at EIGRP, which is vastly simpler.

– Joe

OSPF IV: There is No “I” in Team: More about DR’s

DR vs. No DR

One of the unique features of OSPF concerns neighbor relationships across multiaccess networks, such as Ethernet LANs and certain types of WAN’s such as Frame-Relay and ATM (no, not the cash machines at banks).  Remember, neighbor relationships form between connected neighbors across links, so consider the diagrams above to get an idea of what this looks like. Here we have listed six routers, and the math required to calculate the number of relationships is N(N-1)/2, where N is the number of devices.  Solving for 6 in this case creates the values of 6 * 5, yielding 30, and dividing that by 2 results in 15, which is a lot between so few devices, and staggering between many.  To simplify this, OSPF has a single peer on multiaccess networks called the Designated Router, or DR.  Note in the diagram how the 15 relationships is reduced to 5 using a DR, and in reality there are 5 more with a secondary DR, called the BDR.  In keeping with the highly regimented structure of the protocol, all messages, updates, and so forth take place between peers and the DR, and NOT with one another.  The purpose of the BDR is to take over if anything happens to the DR.

As is the case with many other network protocols, there is an election process to determine the DR and BDR roles on the multiaccess networks, and seldom is this optimal.  I have personally had to deal with suboptimal DR selection in networks and labs I have worked on, especially when dealing with redistribution (the process of sharing routes between various routing sources).  Each router on the OSPF multiaccess network has a default priority of 1, which usually results in a tie in the election process, and the highest numerical Router-ID wins if that is the case.  To set the priority manually, use the ip ospf priority <0-255>  command on the interface, understanding that the higher the priority, the better (I routinely use 200 for the DR and 190 for the BDR).  To remove a router from the election process, just specify zero (0) as the priority, which is useful in hub-and-spoke topologies such as Frame-Relay (having a spoke router as the DR or BDR is not helpful at all).

Let me share a quick word on WAN topologies in OSPF because they can drive you crazy at times.  Frame-Relay and ATM do not forward broadcasts naturally, and special configuration is required is you want that functionality.  If you have to work with non-broadcast links, use the ospf neighbor command under the OSPF process.

Next time, we will wrap up our OSPF discussion by looking at route selection.

– Joe