JUNOS Cookbook

Chapter 8. IP Routing

Introduction

Routing is the act of forwarding packets toward a given destination from one network segment or interface to the next. Routing tables, also sometimes called routing information bases ( RIBs), are the databases that routers use to route traffic toward their destination. These tables contain the network addresses and prefixes that have been learned from dynamic routing protocols such as RIP, IS-IS, OSPF, and BGP; that have been learned statically from static (configured) routing-table entries; and that have been learned from the router’s network interfaces. Each address and prefix in a routing table has a next hop associated with it that takes the packet one hop closer to its destination.

Each IP packet that a router receives contains two types of information: the packet data itself (the packet’s payload) and information that identifies the packet. In IP packets, the identifying information is at the beginning of the packet, in the header. One of these header fields is the source address, which states the packet’s origin; another, which is key to the routing tables, is the destination address, which tells where the packet is going when the router uses standard destination-based forwarding. (Routing policy can alter the path toward a destination.) When the router is determining the path toward the destination, it checks the routing table for a route that matches the packet’s destination and then sends the packet to the next hop associated with that ...