Routing Fundamentals | Day 11 (part 1)

• Welcome to Jeremy's IT Lab. This is a complete course for the CCNA, Cisco Certified Network Associate, exam.
• In this video we will cover routing fundamentals, a very important topic to understand for the CCNA exam.
• In the Ethernet LAN Switching videos we looked at how switches forward frames to their proper destination.
• The process routers use to forward packets to their proper destination is different, and that's what we'll look at in this video.
• What we'll cover in this video


  • What is routing?
  • Routing table on a Cisco router
  • Connected and Local routes
  • Routing fundamentals, specifically route selection
• Routing


  The process that routers use to determine the path that IP packets should take over a network to reach their destination
• Routing table


  Where routers store routes to all of their known destinations
• Two main routing methods


  • Dynamic routing
  • Static routing
• A route tells the router: to send a packet to destination X, you should send the packet to next-hop Y
• Next-hop means the next router in the path to the destination
• If the destination is directly connected to the router, send the packet directly to the destination
• If the destination is the router's own IP address, receive the packet for yourself, don't forward it
• This example network has four routers connected together, representing a WAN
• R1 and R4 have LANs connected to them
• IP addresses used in the example network


  • 192.168.1.0/24
  • 12.0/24
  • 13.0/24
  • 24.0/24
  • 34.0/24
  • 192.168.4.0/24
• In the next video, we will configure static routes on the routers to allow PC1 and PC4 to communicate with each other over the network
• This video will focus on two types of routes automatically added to a router's routing table: Connected and Local routes
• Configuring IP addresses on R1


  1. Configure G0/0 interface

  2. Configure G0/1 interface

  3. Configure G0/2 interface
• When an interface is configured and enabled, two routes per interface are automatically added to the routing table: a connected route and a local route
• Connected route


  A route to the network the interface is connected to
• Local route


  A route to the exact IP address configured on the interface
• A /32 netmask is used for a local route to specify the exact IP address of the interface
• A route 'matches' a packet's destination if the packet's destination IP address is part of the network specified in the route
• When a packet's destination matches multiple routes, the router will choose the 'most specific' matching route
• The 'most specific' route is the one with the longest prefix length (smaller subnet mask)
• Route selection


  The process of a router determining the best path to forward a packet to its destination
• R1 has a connected route to 192.168.1.0/24 and a local route to 1.1/32
• R1 receives a packet with destination IP 192.168.1.1


  The packet matches both the 192.168.1.0/24 and 1.1/32 routes
• Most specific matching route


  The matching route with the longest prefix length
• R1 will select the 1.1/32 route for the packet destined for 192.168.1.1
• Local route


  A route that tells the router to keep the packet, don't forward it
• R1 will receive the packet destined for 192.168.1.1 and look at the contents, because it is addressed to R1 itself
• 192.168.1.0/24 is variably subnetted, 2 subnets, 2 masks
• These lines about subnetting are not actual routes in the routing table
• R1 receives a packet destined for 192.168.1.1


  R1 will select the 1.1/32 local route
• R1 receives a packet destined for 192.168.13.3


  R1 will select the 13.0/24 connected route and forward the packet
• R1 receives a packet destined for 192.168.1.244


  R1 will select the 1.0/24 connected route and forward the packet
• R1 receives a packet destined for 192.168.12.1


  R1 will select the 12.1/32 local route and receive the packet
• R1 receives a packet destined for 192.168.4.10


  R1 will drop the packet since there is no matching route
• Routers store information about destinations they know in their routing table