Policy routing is the act of routing packets using some intelligence other than normal routing. For example, with policy routing, you can send packets to a different destination than the one determined by the routing protocol running on the router. It does have some limitations, but this feature can get you out of some interesting jams. Figure 14-1 illustrates an example that comes from a real problem I encountered.
Figure 14-1. Policy routing example
Two companies, Company 1 and Company 2, partnered together. To save money, they decided they would build each branch such that it would be a single office that connected directly to both companies’ headquarters. To save more money, they decided they would split the cost of a single router for each branch. One Ethernet interface connected the workers from Company 1, while another Ethernet interface connected the workers from Company 2. The workers from each company, although sitting in the same office, could not interact with workers from the other company using the network. We had to put access lists on the Ethernet interfaces to prevent interaction between the two networks.
This design is an excellent example of politics and money trumping best-practice engineering. Still, our job was not to judge, but rather to make the network function the way the client wanted it to function.
To further complicate the issue, employees ...