Routing is the process of moving information across an internetwork from a source to a destination. Routers maintain routing tables that describe how to get to destinations in the
network. Routers use a routing protocol to exchange routing information with other routers to
update their routing tables.
Routing tables contain information such as:
• Protocol type
• Destination/next hop associations
• Routing metrics
• Outbound interface
Routing versus Switching
Routers and switches both move information from source to destination, but they use different
information to accomplish the task:
• Switching normally occurs at Layer 2 (data link layer) and routing occurs at Layer 3
(network layer)
• Switches act upon the MAC address and routers act upon the IP address
• Switches keep tables of MAC addresses and routers keep tables of IP networks
Static and Dynamic Routing
Static routes are entered into a routing table manually and do not change unless someone
manually changes them. Dynamic routing builds routing tables using routing protocols and
continuously updates the tables as the network topology changes.
Routing Design Goals
Routing algorithms have some of the following goals:
• Optimal Route Selection
• Simplicity
• Robustness and stability
• Rapid convergence
• Flexibility
• Delay
• Load
• Reliability
• Path Length
• Cost – some paths may cost more to use than others. Operators may prefer to use links that
they own because of lower costs.
Dynamic Routing Protocols
Interior Gateway Protocol (IGP)
• Used for exchanging routing information within an autonomous system (AS).
• Uses routing metrics such as hop count
• Commonly used: OSPF, RIP, IS-IS, iBGP
Exterior Gateway Protocol (EGP)
• Used for routing between autonomous systems
• Routing protocols may be different between ASs
• Chosen path may not be optimal since it can’t compare routing metrics from multiple ASs
• Commonly used: BGP-4
Note: An autonomous system (AS) is a network under a single administrative authority.
from Alcatel Lucent Doc.
