Of course, security is a broad field, and it is impossible to capture all the details in a short introduction. However, some additional security concepts are worth mentioning briefly:
Confidentiality, integrity, availability? These are the three basic properties of security (some text books add authenticity as a fourth). When defining security, one can be more precise by defining which of the security properties is required to which extent. For example in a military environment, the most important security property is probably confidentiality. In a bank, confidentiality is important, too, but even more important is the integrity of the data: it is imperative that no customer data such as account balances be accidentally or maliciously changed. For an online shopping site, however, availability of the web page is an important factor: every minute the web site is not available leads to direct loss of revenue. Overall, security can usually be defined using these three properties.
In the MPLS context, every VPN customer will have slightly different requirements for these parameters, but generally, customers will expect their data to be private (confidential), such that they are not accessible outside their VPN. They will expect the data not to change in transit, and they will expect the MPLS VPN service overall to be available to them?in other words, that it suffer very few or no outages.
Defense in depth? Because one weak link is sufficient to endanger the security of an overall system, it is common practice to construct several "layers" of security around a solution, such that if one single component breaks, others still defend the assets. The best example of this in enterprise networks is the demilitarized zone (DMZ). In a DMZ, a company's servers are usually highly protected; however, even if this protection fails and a hacker gains access to a server, there is still a firewall to overcome to get into the network.
It is good practice to add several layers of defense around everything that needs to be protected. This design principle is also important in MPLS networks.
Secure failure? The primary mode of operation of any technology is usually well thought through and well secured. However, when the primary method fails, the backup method also needs to be secured appropriately. It is common practice today to use secure shell (SSH) for router configuration; however, a backup method of getting to a router is necessary in case the SSH server fails, for example. This is usually done through out-of-band access, mostly over the telephone network. It is important that this backup mode be as secure as the principal access mode.
There is ample literature available for more detail on security in general and in the networking context. Please refer to Appendix B for recommendations.