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The Illustrated Network- P49

The Illustrated Network- P49:In this chapter, you will learn about the protocol stack used on the global public Internet and how these protocols have been evolving in today’s world. We’ll review some key basic defi nitions and see the network used to illustrate all of the examples in this book, as well as the packet content, the role that hosts and routers play on the network, and how graphic user and command line interfaces (GUI and CLI, respectively) both are used to interact with devices. | CHAPTER 17 MPLS and IP Switching 449 they are stacked in front of the packet. All of the routers shown in practice there will be many more pop and process multiple labels. MPLS domains can be nested for geographical vendor or organizational reasons as well. MPLS and VPNs MPLS forms the basis for many types of VPNs used on IP networks today especially Layer 3 VPNs. LSPs are like the PVCs and SVCs that formed virtually private links across a shared public network such as FR or ATM. LSPs are not really the same as private leased-line links but they appear to be to their users. Of course while the path is constrained the MPLS-based Layer 3 VPN is not actually doing anything special to secure the content of the tunnel or to protect its integrity. So this security value is limited to constraining the path. This reduces the places where snooping or injection can occur but it does not replace other Layer 3 VPN technology for security such as IPSec discussed in Chapter 29 . Nevertheless VPNs are often positioned as a security feature on router networks. This is because like private circuits hackers cannot hack into the middle of an LSP VPN just by spoofing packets. There are labels to be dealt with often nested labels. The ingress and egress routers are more vulnerable but it s not as easy to harm VPNs or the sites they connect as it is to disrupt straight router networks. So VPNs have a lot in common with MPLS and LSPs except that the terms are different For example the transit routers in MPLS are now provider P routers in VPNs. VPNs are discussed further in the security chapters. MPLS Tables The tables used to push pop and swap labels in multiprotocol label switching are different from the tables used to route packets. This makes sense MPLS uses switching and packets are routed. Most MPLS tables are little more than long lists of labels with two key pieces of information attached the output interface to the next-hop router on the LSP and the new value of the label. Other