Routing

This page provides introduction to Routing.

Overview

Routing is the process that routers uses to determine the path that IP packets should take over a network to reach their destination. Routers stores routes to all of their known destinations in a routing table.

There are two main routing methods:

• Dynamic Routing: Routers uses dynamic rounting protocol(OSPF, BGP, etc.) to exchange routing information with other routers.
• Static Routing: A Network Administrator manually configures the routing table.

Route Selection

When a router receives a packet, it selects the most specific route to use to send the packet. The most specific route is the route that matches the destination IP address the most.

For example, a packet destined for 192.168.1.1 is matched by both routes as below:

• 192.168.1.0/24 (192.168.1.0 is the network address and 24 is the subnet mask which means all addresses from 192.168.1.0 to 192.168.1.255 are included)
• 192.168.1.1/32 (192.168.1.1 is the destination IP address and 32 is the subnet mask which means only the address 192.168.1.1 is included)

The router will select the most specific route to use to send the packet. In this case, the router will select the route with the longest prefix match.

Default Gateway

The default gateway is a route that is used when the router doesn't have a more specific route to use. Router uses the default gateway to send a packet to destination outside of the router's network. A default route is route to $0.0.0.0/0$ which means all destinations. Default gateway is also known as the gateway of last resort.

Routing Configuration

When you configure an IP address on a router interface, a local route is automatically created. For example, if you configure the interface e$0$/$0$ with the IP address $192.168.1.1/24$, the local route $192.168.1.0/24$ will be automatically added to the routing table.

Let's say we have the following network and we want to connect PC$1$ to PC$4$.

Routing configuration will look like below. We are selecting a route through R$2$ to reach PC$4$.

Router	Destination	Next Hop
R1	192.168.1.0/24	Connected
	192.168.4.0/24	192.168.12.1
R2	192.168.1.0/24	192.168.12.1
	192.168.4.0/24	192.168.24.4
R4	192.168.1.0/24	192.168.24.2
	192.168.4.0/24	Connected

Life of a Packet

Let's see the life of a packet when PC$1$($192.168.1.10$) wants to send a data to PC$4$($192.168.4.10$).

Step 1: Since they are in different networks, PC$1$ will send an ARP request to its default gateway which is R$1$.

ARP Request:
Src IP: 192.168.1.10
Dst IP: 192.168.1.1
Src MAC: 11:11:11:11:11:11
Dst MAC: FF:FF:FF:FF:FF:FF

Step 2: R$1$ will receive the ARP request and send an ARP reply to PC$1$.

ARP Reply:
Src IP: 192.168.1.1
Dst IP: 192.168.1.10
Src MAC: 22:22:22:22:22:22
Dst MAC: 11:11:11:11:11:11

PC$1$ will receive the ARP reply and update its ARP cache.

ARP Cache:
192.168.1.1: 22:22:22:22:22:22

Now, PC$1$ knows the mac address of it's default gateway R$1$, so PC$1$ will send a frame to R$1$.

Frame:
Src IP: 192.168.1.10
Dst IP: 192.168.4.10
Src MAC: 11:11:11:11:11:11
Dst MAC: 22:22:22:22:22:22

info

Note: The destination IP address remains as PC4's IP address (192.168.4.10), only destination mac address changes to R$1$'s mac address.

R$1$ will receive the frame and looks up the destination in its routing table. Most specific route is $192.168.4.10/24$.