Archive for route poisioning

OSPF Part I: The Drill Sergeant of Networking!

Posted in Cisco Certification with tags , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , on July 22, 2012 by jjrinehart
I have never been in any branch of the military, but I have known a number of brave men and women that have served.  For those who may not know how the military induction process works, the first step is known as Basic Training, or more affectionately, hell on earth!  This phase involves heavy physical activity, intense discipline,and hours of a drill sergeant (see graphic above) barking orders in a fierce, shrill tone that makes fire freeze!  By now you might be wondering what on earth any of this has to do with networking, which is a fair question.  Enter the Open Shortest Path First protocol, abbreviated OSPF.
Everything in OSPF is about rules, and ensures that routing takes places in an even and consistent manner across the entire autonomous system (network under a common administration).  There are rules about design, rules about information exchange, rules about the routing hierarchy, and about everything else you can think of, so to speak.  Needless to say, OSPF is also highly structured, and that is part of the appeal of this best-known link-state routing protocol.
In keeping with the military analogy, one of the first things to understand about OSPF is the “chain of command” or hierarchy.  Just as there are platoons, squads, and units of soldiers in any army, there are specific groups of devices in this protocol, which are called areas.  With a small network, you can get away with a single area and keeps things relatively simple.  In larger networks, multiple areas are a given even if just for scalability (a term meaning the ability to grow in a measured fashion), but there are often additional, design-related, reasons as well.  Areas are identified using decimal numbering, such as 0, 1, 2, and so on, although you can also use dotted decimal numbering such as 10.1.4.13; in my own experience I have only seen the digit form of numbering.
Getting back to the “rules”, an important thing to understand is that all areas must connect to a special area, called the Backbone Area, or Area 0.  In other words, any traffic leaving one area and destined for another must cross Area 0.  If you have just one area in the entire network, then you can number it just about any way you like, but in any multi-area OSPF network you simply must have an Area 0, through which traffic passes.  There is an exception process using a special connection called a Virtual-Link (which creates a direct connection to Area 0 by one that is separated by another area), but that is beyond the scope of the CCNA.
Next time we will look at formal neighbor relationships!
– Joe

What a RIP(V1/V2) Off!

Posted in Cisco Certification with tags , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , on July 18, 2012 by jjrinehart
The algorithms that Routing Information Protocol is based on date back to 1957, but the protocol itself was defined in RFC 1058 in 1988, with several revisions since that time.  The first version of RIP was classful, meaning that it only recognized addressing according to the Class A, B and C groups defined for the original Internet.  Subnetting was not practical since subnet masks could not be transmitted in routing updates.  If you ever want to see what this behavior is like, issue the no ip classless command on a Cisco router and specify version 1 of RIP (which is the default unless you explicitly enable version 1).  One of the more interesting things about RIP is the maximum hop-count of 15.  When I first started in networking, that seemed silly and arbitrary, but as I gained a greater grasp of binary, it made perfect sense!  You see, RIP uses a 4-Bit metric, and in binary, that maps out as follows, using the Powers of Two we talked about earlier:
8     4     2     1                Powers of Two
1       1     1     1                Binary Digits
Binary 1111 (8+4+2+1) equals a decimal value of 15!  Simply put, RIP just can’t count any higher!  In addition, because RIP is a Distance Vector Routing Protocol, it uses the loop-prevention mechanisms we talked about previously, and sends out its entire routing table every 30 seconds.  Even the newest engineer can see the glaring limitations of RIP, and due to those shortcomings, Cisco gained huge popularity by introducing its proprietary Interior Gateway Routing Protocol (IGRP).  IGRP could measure more than just hop count and thus represented a quantum leap forward in routing technology.  Unfortunately, both these protocols still used broadcasts, and new protocols such as OSPF created far more attractive update mechanisms by utilizing multicast instead.
Classless routing changed the landscape of routing in general, and in 1993 with RFC 1388, RIP was updated to support classless routing, and with the transmission of subnet masks in routing updates, subnetting was now possible.  In addition, updates using multicast became supported, and some other improvements, although the hop-limit remained at 15.  One critical thing to keep in mind is that a feature called automatic summarization is enabled on RIPV2 by default, which summarizes networks to the nearest classful boundary—in effect, making the classless protocol act classful.  Two command to almost always use when configuring RIP are: 1. Version 2 (V1 is enabled by default) and no auto-summary (disables automatic summarization).
RIP is useful in smaller networks, but in reality is as undesirable as a cheerleader being asked to the prom by the president of the chess club (see graphic above).  As we will discuss next time, there are MUCH better choices available now.
– Joe

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

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