4.6 KiB
id, aliases, tags
| id | aliases | tags | ||
|---|---|---|---|---|
| 11. Routing Fundamentals |
|
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)
Show routing table
On cisco router you can show routing table with this command
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
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)