OK so what is this routing thing all about?

Well it seems to me there are two parts to routing, the actual physical routing of data across networks, and the methods in which the network devices keep track of where these route are (the routing protocols). Although there are several different routing protocols in use, they all have the same basic function, to allow routers to share the information of possible paths through the network between each other. Which in turn allows the indivual routers to build up routing tables which they can then use to look up the destination IP address in a packet and determine the next router (hop) to which the packet must be sent.

So what about this routing table? what does it look like and what does it contain?

router# <strong>show ip route</strong>

171.68.0.0/24 is subnetted, 3 subnets
S       171.68.1.0 [1/0] via 171.68.192.201
S       171.68.16.0 [1/0] via 172.16.191.254
C       171.68.192.0 is directly connected, FastEthernet0/0
172.16.0.0/16 is variably subnetted, 2 subnets, 2 masks
S       172.16.88.0/24 [1/0] via 172.16.191.254
C       172.16.191.252/30 is directly connected, Serial2/0
D    192.168.80.0/24 [90/156160] via 171.68.192.201, 00:00:07, FastEthernet0/0
D    192.168.90.0/24 [90/156160] via 171.68.192.201, 00:00:08, FastEthernet0/0
S   0.0.0.0/0 [1/0] via 10.1.1.3

From this output we can see how the routing table functions. Each network/subnet that the router has learnt about there is an entry telling the router where to send a packet that is destined to that network. This destination can be either the IP address of the next router in the path, or the outgoing interface ID. There is also a priority given to each entry for deciding entry to use if a route is added twice due to multiply paths to the same destination network.

The three methods that can add entries to the routing table are , Connected networks (added automatically), statically added routes, and routes learnt through routing protocols. In terms of default priority’s, Connected bet Static which in turn bet those learnt through protocols (where in general from highest to lowest we have BGP, EIGRP, OSPF, IS-IS and RIP). The last entry in the table above is a special case static entry, often know as the “default route” this route of “0.0.0.0 0.0.0.0” will catch any packet that does not match any other entry in to the routing table and forward it to a next hop address. This is commonly used to route packets destined to the internet, so rather than you company router needing to learn the router to every address on the internet, it only knows about internal company address. Anything else is caught with by the default route and passed to the ISP to deal with. This drastically cuts down the size of routing tables and is what allows the internet to function.

However once added the function is the same, as packets enter the router the destination address will be read and checked against the routing table to determine the next step, and then forward the packet out the destination interface. Now although some routers do build up a “map” of the network segment they are part of, once a router has passed on a packet to an upstream router is has no influence on what then happens to that packet. So it is important that all routers in the path have valid routes, and that failers in the network can be notified to downstream routers, so they can route packets around network issues. This is where the routing protocols come in to the picture!

In an ideal world we would not have to add any static routes, we would simple configure IP address on interfaces, enable routing protocols and the routers would teach each other how to reach all the networks. And in fact in many cases this is how it works. Once the interface are set up, a routing protocol is enabled and you simple have to configure what networks you wish to advertise using this protocol and to what neighbouring routers you wish to send the adverts to. This configuration and exact method may change between the different routing protocols, but fundamentals are the same, what do you want to advertise and who do you want to advertise this to. Of course there is far more to it than simply this, and we cover it in more depth later in course. But for now we just want to get a fundamental picture of the why’s and the how’s of routing.

I know once again this is really a bit of revision from CCNA material, but I think it is good to once in a while return to the basics, if you make sure you are clear in your head about them, then later on they can be built on to form the more complex topics. But no matter how complex things get, these fundamentals of what routing is and why we use routing protocols will always hold true.

In the next episode of CCNP ROUTE we shall be looking in more detail at the two main types of routing protocols (link state and Distance vector), and why we may chose one over the other.

Mean while you may want to take a look at CISCO Routing Basic. As well as this one document I would recommend you add a book mark to the handbook as a whole, there is lots of useful info there.

Off to do a bit more study now. 🙂

DevilWAH