1. Introduction to Networking
Routers

2. Routing
Routing is the process of moving packets of data from one network to another
Packets will bounce from router to router along a path to the destination host until it hits a router directly connected to the network the destination host is in
Routers make forwarding decisions based on its routing table. The routing table lists all known destination networks along with the interface connected, the next router in the path and a value that identifies the cost to reach the destination network

3. Routing Table
For directly connected networks the routing table will just store the network address and the interface connected. That is all we need to know to send a packet to an address on that network
For remote networks we also need to know the next hop router. This allows us to send the packet we receive to the next router so it can handle it
If we connect to the internet we’ll also want to know the default route

4. Frame Structure and Routing
When a host is trying to send a message outside of its network, it will create a packet with its IP address, and the IP address of its destination. It will then wrap the packet in a frame that contains its mac address and the mac address of its default gateway as the destination
The router will see its own MAC address and strip the frame header to analyze the packet. It will then compare the destination IP address to its routing table to identify the destination network
If it is not a local network, the router will wrap the packet in a new frame with the MAC address of the port it is using as the source MAC, and the next hop router’s MAC address as the destination
Eventually the local router will get the frame, and then the destination MAC address of the host will be used, and the packet will be delivered

5. Building the Routing Table
The router will be able to poll any network it is local to to build its routing table, but how do we get our routing table remote entries? There are two ways
Static
The information is manually entered
The network entries will remain in the routing table until manually removed, which means that these entries must be modified, added or removed in response to network changes
Dynamic
We can use a protocol to allow routers to share information with other routers to help each other build their routing table
The protocols will define the information contained in the table, how messages are routed from one network to another
How topology changes are communicated between routers. This can take time as the change information spreads through the network. When all routers have a consistent network view it is called “convergence”

6. Static Routing vs Dynamic Routing
You can have both dynamic and static entries in the same routing table based on your needs
Dynamic entries are usually used for larger private networks to minimize work for the network administrators
Static entries must be used for a private network connected to the internet. You may want to configure your private network with dynamic entries, but you won’t want to share your private network information over the network, and you don’t want to try to create a routing table for the entirety of the internet
Instead you would make a special static entry called the default route

7. Default Route
The default route is the destination to send any packet addressed to a network that the router doesn’t know
If there is no default route the packet would be dropped
The default route entry has a network address of and a mask of
For a router connected to the internet, this default entry will typically be a router belonging to an ISP. The ISP router would then use its routing table to send the message through the internet

8. Dynamic Entries
When configuring a dynamic routing protocol you must first enable the protocol on the router
You then identify which interfaces will use the protocol to dynamically learn routing information
For instance, if I was configuring the router on the boundary between my network and the internet, and wanted to dynamically learn my private network but not the internet, I would enable only the interfaces connecting to my network