Chapter 12. Default Routes and On-Demand Routing

This chapter covers the following subjects:

• Fundamentals of Default Routes

• Fundamentals of On-Demand Routing

• Configuring Default Routes and ODR

Summarization has been examined in several chapters so far. Summarization conserves network resources by reducing the size of route tables and route advertisements. The smaller, simpler route tables can also make management and troubleshooting easier.

A summary address is an address that represents several, sometimes many, more-specific addresses. For example, the following four subnets

     192.168.200.128/27
     192.168.200.160/27
     192.168.200.192/27
     192.168.200.224/27

can be summarized with the single address 192.168.200.128/25.

When examined in binary, the addresses reveal that the summary address is less specific because it consists of fewer network and subnet bits than the addresses being summarized. So put crudely, it might be said that as more zeros are added to the host space and as fewer network bits are used, more addresses are summarized. Taking this concept to its limit, what if so many zeros are added to the host space that no network bits remain? In other words, what if the summary address consists of 32 zeros and has a prefix length of 0 (0.0.0.0/0)? This address summarizes every possible IPv4 address.

0.0.0.0/0 is the IPv4 default address, and a route to 0.0.0.0/0 is a default route.^[1] Similarly, the default IPv6 address ::/0 summarizes every possible IPv6 address. Every other IP address is more specific than the default address, so when a default route exists in a route table, that route will be matched only if a more specific match cannot be made.

^[1] This address is used by all the open IP routing protocols. The Cisco IGRP and EIGRP use an actual network address, advertised as an external route.