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A Gentle Introduction to Routing - page 2

Routing Fundamentals

  • October 8, 2008
  • By Charlie Schluting

The idea of a network topology is pretty absurd in the context most people picture it. VLANs (define) turned the world up side down in that regard. But in routing, topology is actually important, if you zoom out a bit.

The whole idea behind routers is that they will �pass it on,� either in the correct direction, or on to their smarter peers. If your network core has a bunch of stubs connected, many of the stub routers will know nothing about each other. But they know �the way to everything� is through the core, and they simply forward packets that way. Hesitantly, we�ll call this a star topology. Of course, I�m insulting your intelligence, because this is the concept of a default route. But pay attention here: this is how many dynamic routing protocols work. It isn�t always the case that you�ll pass a packet onto the all-knowing default router, instead sometimes you�ll be passing the packet to the router that you know handles a certain subnet. The point is that you know nothing about the other routers behind the one that tells you �I am network X.�

The previous paragraph really embodies what routing is. You get packets closer to the destination. Of course, you have to know what�s at each destination, and that�s what routing protocols tell you. It�s really easy to jump back and fourth when talking about routing, so take note that all of the above was with the picture of a single network in mind. This is also known as a routing domain. A routing domain is a set of routers that are all under the same administrative control; presumably all running the same routing protocols.

When routing packets, we have a few paradigms to choose from. The telco world sets up a circuit for your telephone call as soon as you dial. The path is always the same, and it�s very reliable. The IP world does not, and it can handle much more traffic. The tradeoff is that you can get congestion, and sometimes fail to reach certain websites, whereas your telephone call will never drop because of congestion. The IP world can almost do this, through a mechanism called loose source routing. This is how it started: each end node knew what hops it needed to take to reach its destination. Source-based routing doesn�t scale, and introduces security problems. So we use dynamic routing protocols to figure out the paths for us. Take note that each direction can take a different path!

Routing protocols are broken up into a few different categories, in two senses. First, we have IGP, or Interior Gateway Protocols. RIP, OSPF, and ISIS are a few IGP�s you may have heard about. These are routing protocols that deal with intra-domain routing. EGP, Exterior Gateway Protocols, deal with inter-domain routing, between enterprises. Now defunct, EGP was actually a protocol, but BGP is now the standard inter-domain protocol.

Second, routing protocols are said to be of two categories in another sense: link-state, or vector-distance. The vector-distance approach is: �tell your neighbors about the world.� This means that you will broadcast your entire routing table, to all your neighbors. The �vector� is the destination, and the �distance� is really a metric, or hop count. Link-state routing protocols �tell the world about your neighbors.� The idea is to figure out who is �up� and broadcast that information about their link�s state to all other routers. Link-state is very computationally intensive, but it provides an entire view of the network to all routers.

Most people prefer link-state protocols because they converge faster, which means that all of the routers have the same information. Link-state calculations take a long time though, and happen every time we get an update, so they can�t be used Internet-wide. We�ll see why link-state eats CPU when we cover OSPF in the near future. Come back next week for our first routing protocol: RIP.

In a Nutshell
  • Routers send packets toward their destination, normally by shipping it toward a router that knows a bit more about the destination topology.
  • Routing is two one-way problems; it is very common for your packets to take asymmetric routes.
  • Link-state: fast convergence, eats CPU. Vector-distance: slow convergence, easier on the silicon.

Article courtesy of Enterprise Networking Planet, originally published May 18, 2006

When he's not writing for Enterprise Networking Planet or riding his motorcycle, Charlie Schluting is the Associate Director of Computing Infrastructure at Portland State University. Charlie also operates OmniTraining.net, and recently finished Network Ninja, a must-read for every network engineer.

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