Archive for asassin

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