Static Routing | Day 11 (part 2)

• When analysing markets, a range of assumptions are made about the rationality of economic agents involved in the transactions
• The Wealth of Nations was written

  1776
• Rational


  (in classical economic theory) economic agents are able to consider the outcome of their choices and recognise the net benefits of each one
• Rational agents will select the choice which presents the highest benefits
• Producers act rationally by


  Selling goods/services in a way that maximises their profits
• Workers act rationally by


  Balancing welfare at work with consideration of both pay and benefits
• Governments act rationally by


  Placing the interests of the people they serve first in order to maximise their welfare
• Rationality in classical economic theory is a flawed assumption as people usually don't act rationally
• If you add up marginal utility for each unit you get total utility
• Packet forwarding decision


  1. Determine if routing table has matching route

  2. If no matching route, drop packet

  3. If matching route, forward packet to next hop
• Routers need routes to forward packets to destination networks
• Routes provide instructions on where to forward packets
• There are two possible paths for packets from PC1 to PC4: via R3 or via R2
• Using the path via R3, not the path via R2
• Routers need routes to both source and destination networks for two-way reachability
• Routers don't need routes to all networks in the path, just the next hop
• Routes needed by routers

  • R1 route to 192.168.4.0/24
  • R3 route to 192.168.1.0/24
  • R3 route to 192.168.4.0/24
  • R4 route to 192.168.1.0/24
• Next hop IP address


  IP address of the next router interface to forward packets to
• Configuring static routes


  1. Enter global config mode

  2. Use 'ip route' command with destination network, mask, and next hop IP
• Static routes can specify exit interface instead of next hop IP
• Static routes can specify both exit interface and next hop IP
• Ping test successful, indicating two-way reachability between PC1 and PC4
• Packet encapsulation/de-encapsulation


  1. Packet has destination IP, encapsulated in frame with next hop MAC

  2. Frame de-encapsulated, re-encapsulated with new next hop MAC

  3. Final frame has destination IP and MAC of same device
• Configuring static routes


  1. Specify next-hop IP address

  2. Specify exit interface

  3. Specify both next-hop IP and exit interface
• Default route


  Route to 0.0.0.0/0, the least specific route possible that includes every possible destination IP address
• If the router doesn't have any more specific routes that match a packet's destination IP address, the router will forward the packet using the default route
• A default route is often used to direct traffic to the Internet
• Configuring a default route on a Cisco router

  IP ROUTE 0.0.0.0 0.0.0.0 <next-hop IP address>
• The 'Gateway of last resort' is another name for 'default gateway'
• When configuring a default route, the network address and netmask are both 0.0.0.0
• A static route with an asterisk (*) next to it in the routing table means it is a 'candidate default' route
• You cannot specify the prefix length with a slash when configuring a static route on a Cisco router, you must write out the netmask
• When configuring a static route and specifying only the exit interface


  It appears as directly connected in the routing table, using a technique called Proxy ARP
• Proxy ARP is beyond the scope of the CCNA
• The code 'S' in the routing table indicates a static route
• The code 'C' in the routing table indicates a connected route
• You cannot configure a network address on an interface
• To know all destination networks, R3 would need to be configured with 4 static routes