---
id: 11. Routing Fundamentals
aliases: []
tags: 
    - Cisco
    - routers
---

# Routing Fundamentals

## What is routing ?

- **Routing** is the process taht routers use to determine the path that IP packets
should take over a network to reach their destination.
    - Routers store routes to all of their known destinations in a routing table
    - When routers receive packets, they look in the **routing table** to find 
        the best route to forward that packet.

There are two main rounting methods (methods that routers use to learn routes):
- **Dynamic Routing**:
    - Routers use [[dynamic routing protocols]] (ie. [[OSPF]]) to share routing information
    with each other automatically and build their routing tables.
- **Static Routing**:
    - A network engineer/admin manually configures routes on the router.
        
A route tells the router: to send a packet to destination X, you should send the 
[[packet]] to next-hop Y.
            next-hop= the next router in the path to the destination
- or if the destination is directly connected to the router, send the packet directly to the destination.
- or if the destination is the router/s own [[ip address]], receive the packet for yourself (don't forward it)

[[WAN]] Wide Area Network = a network that extends over a large geographical area

## Routing table
### R1 Pre-configuration (IP address)
[[08. IPv4 Addressing (Part 2)]]

### Show routing table
On cisco router you can show routing table with this command

```cisco
R1#show ip route
```

There are two main Output for this command:
- **Codes**
- **Routes**

#### Codes

The Codes legend in the output of show ip route lists the different protocols which 
routers can use to learn routes.
- L - Local
    - A route to the actual IP address configured on the interface. (with a /32 netmask)
- C - connected
    - A route to the network the interface is connected to. (with the actual netmask configured on the interface)

#### Routes

When you configure an IP address on an interface and enable it with no shutdown, 2 routes
(per interface) will automatically be aded to the routing table
    - a connected route
    - a local route

A connected route is a route to the network the interface is connected to
    - R1 G0/2 IP = 192.168.1.1/24
    - network address = 192.168.1.0/24
    - It provides a route to all hosts in that network (192.168.1.2 -> 192.168.1.254)
    - R1 knows "if i need to send a packet to any host in the 192.168.1.0/24 network, i should send it out of G0/2"

A local route is a route to the exact IP address configured on the interface
    - A /32 netmask is used to specify the exact IP address of the interface.
    - /32 means all 32 bits are "fixed", they can't change.
    - Even though R1's G02 is configured as 192.168.1.1/24 the connected route is to 192.168.1.1/32
    - R1 knows "if i receive a packet destined for this IP address, the message is for me"

#### Clarification

// bold part is fixed

**192.168.1**.0/24
**255.255.255**.0

192.168.1.0/24 matches 192.168.1.0 ~ 192.168.1.255
- If R1 receives a packet with a destination in that range, it will send the packet out of G0/2

192.168.1.2 = match
-> Send packet out of G0/2
192.168.1.56 = match
-> Send packet out of G0/2
192.168.2.56 = no match
-> Send the packet using a different route, 
-> or drop the packet there is no matching route

a route matches a packet's destination if the packet's destination IP address is part 
of the network specified in the route.

// bold part is fixed

**192.168.1.1**/32
**255.255.255.255**

192.168.1.1/32 matches Only 192.168.1.1

## Route Selection

A packet destined for 192.168.1.1 is matched by both routes:
- 192.168.1.0/24
- 192.168.1.1/32

it will choose the most **specific matching** route.
- the route to 192.168.1.0/24 includes 256 different IP addresses (192.168.1.0 - 192.168.1.255)
- the route to 192.168.1.1/32 includes only 1 IP addresses (192.168.1.1)
    - This route is **more specific**

*Most specific matching route = the matching route with the longest prefix length*

### Cisco router lines

```cisco
192.168.1.0/24 is variably subnetted, 2 subnets, 2 masks
192.168.1.0/24 is directly connected, GigabitEthernet0.2
192.168.1.1/32 is directly connected, GigabitEthernet0.2
```
these three lines are not routes, they mean the followong:
- 192.168.1.0/24 is variably subnetted, 2 subnets, 2 masks
    - In the routing table, there are two routes to subnets that fit within the 
    192.168.1.0.24 Class C network with two different netmask (24/32).


## Review

- What is routing?
- The routing table on a Cisco [[router]]
    - Connected and local Routes
- Routing fundamentals (route selection)