IP Routing in Computer Networks

IP Routing

• IP routing is the process of moving packets from one network to another network using routers.
• Routing protocols are used by routers to dynamically find all networks.
• Examples of routing protocols include RIP, RIPv2, EIGRP, and OSPF.

Routing Basics

• A router must know the following minimum factors to effectively route packets:
  • Destination address
  • Neighbor routers from which it can learn about remote networks
  • Possible routes to all remote networks
  • The best route to each remote network
  • How to maintain and verify routing information
• The router learns about remote networks from neighboring routers or from an administrator.
• The router builds a routing table, which is a map of the internetwork, describing how to find remote networks.

Simple Routing Example

• A simple network example shows a router (Lab_A) with four interfaces.
• The router uses the interface with the best route to forward an IP datagram to a host with a destination IP address of 10.10.10.30.

IP Routing Process

• The IP routing process involves:
  • Host_A sending an IP datagram to Host_B on a different network.
  • The router performing a lookup to find the best route to the destination network.
  • The router forwarding the IP datagram to the next hop on the best route.

Packet-Forwarding Techniques

• Cisco uses three types of packet-forwarding techniques:
  • Process Switching: looks up every destination in the routing table and finds the exit interface for every packet.
  • Fast Switching: uses a cache to store the most recently used destinations.
  • Cisco Express Forwarding (CEF): uses multiple cache tables to improve performance and adapt to network topology changes.

Types of Routing

• Static Routing:
  • Routes are manually configured by the administrator.
  • Requires destination network ID for configuration.
  • Used in small networks.
  • Administrator distance for static route is 1.
• Dynamic Routing:
  • Uses dynamic routing protocols to find networks and update routing tables.
  • Requires directly connected network IDs for configuration.
  • Used in medium and large networks.
  • Administrator work is reduced.

Static Routing Pros and Cons

• Pros:
  • No overhead on the router CPU.
  • No bandwidth usage between routers.
  • Adds security by allowing exclusive routing access to certain networks.
• Cons:
  • Requires administrator to have a thorough knowledge of the internetwork.
  • Tedious to add routes to new networks on all routers.
  • Not feasible for large networks due to high maintenance requirements.

Dynamic Routing Protocols

• Types of dynamic routing protocols:
  • Distance Vector Protocol: RIP (Routing Information Protocol).
  • Link State Protocol: OSPF (Open Shortest Path First).
  • Hybrid Protocol: EIGRP (Enhanced Interior Gateway Routing Protocol).

Administrative Distances

• Administrative Distance (AD) is used to rate the trustworthiness of routing information received on a router from a neighbor router.
• AD is an integer from 0 to 255, where 0 is the most trusted and 255 means no traffic is passed via this route.
• If a router receives two updates listing the same remote network, the route with the lowest AD will be chosen and placed in the routing table.