Understanding routing processes

Routers route, that's what they do. A packet comes in and a packet goes out. What happens to the packet containing your data inside the router?, how does the router decide which output interface to transmit the packet out of?, what if the router has two routes to the same destination how does it deal with that decision making process?.

For a router to make a routing decision it needs to know how to get to the destination address which is described in the header of the IP Packet. Without this knowledge the router could ultimately drop the packet.

Our first job is to populate the routers routing table with relevant routes, we say relevant since routers will believe and trust any route placed into it's routing table and use it resulting in routing blackholes where traffic is lost in the network where the time to live in the packet headers expire and the traffic is then dropped.

Default Routes

There are various methods for populating the routing table, the most common of routes is called a "default route". A default route is written as follows (I am using Cisco command line)

Ip route 0.0.0.0 0.0.0.0 10.1.1.1

The route is simple to write and straight forward to understand. The first set of zeros after the "ip route" command which we will call the "destination" tells the router which destination this route represents. This route is effectively "matching" every single possible address that can be drawn up with the IPv4 address scheme.

The second row of four zeros which we will call the "prefix "shows the router which bits in the destination the router needs to look at.

Having any octet in the prefix set to all zeros (i.e 0 0 0 0 0 0 0 0 = 0) tells the router that it ought to ignore the corresponding octet in the destination. The order of the octets in the destination field correspond directly to the octets the second row.

The third address "10.1.1.1" is an example of what is referred to as a "next hop", (10.1.1.1 is an example address only for the purposes of this article only, your next hop will be different). The next hop address is the other side of the link on a point to point link or an interface in a multiaccess network like frame relay of Ethernet that can route the data.

The next hop does not have to be the address of the next hop, it can also be the local outbound interface.

All of our home broadband routers will have a default route helping the router to simply pass all traffic up to the ISP. Default routes are also known as "gateways of last resort"



Article Source: http://EzineArticles.com/?expert=Joe_Spoto