Chapter 12. Default Routes and On-Demand Routing

• Fundamentals of Default Routes

• Fundamentals of On-Demand Routing

• Configuring Default Routes and ODR

  Case Study: Static Default Routes

  Case Study: The Default-Network Command

  Case Study: The Default-Information-Originate Command

  Case Study: Configuring On-Demand Routing

Summarization has been examined in several chapters so far. Summarization conserves internetwork resources by reducing the size of routing tables and route advertisements. The smaller, simpler routing 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 (0.0.0.0)? This address summarizes every possible IP address.

Note

The IP default address is 0.0.0.0

0.0.0.0 is the IP default address, and a route to 0.0.0.0 is a default route. ^[1] Every other IP address is more specific than the default address, so when a default route exists in a routing 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. Cisco's IGRP and EIGRP use an actual network address, advertised as an external route.