This chapter covers a number of IP configuration topics that won't find their way into simple configurations. However, they are becoming more prevalent in corporate networks: you never know what features you're going to need the next time you redesign your network. In particular, we cover:
So far, we've used routers as routers, which make intelligent decisions about where to send packets based on their IP addresses and information gathered by routing protocols. Cisco routers can also be configured as bridges, which make routing decisions based on MAC addresses (e.g., Ethernet addresses).
Hot standby routing
Cisco's Hot Standby Routing Protocol (HSRP) enables routers to serve as backups for one another.
Network Address Translation (NAT)
Increasingly, the IP addresses visible outside a network are different from the addresses actually in use inside the network. Translating from a small external address space to a much larger internal space conserves addresses (you can have a large network but use a small block of external addresses) and gives you more control over which hosts in your network are visible to the outside world. In these configurations, the router relies on NAT to map your internal addresses to your external addresses.
Tunneling means establishing a TCP/IP connection to another location and then running other protocols through that connection. It can be used as a means of propagating protocols that can't be routed or that don't belong to the TCP/IP family; encrypted tunnels can also be used as part of a security strategy. Dynamic Multipoint VPN (DMVPN) allows easier configuration of secure tunnels.
Multicast is a means of delivering a single packet to multiple hosts. An example would be a video or web broadcast. The sending machine sends only one packet that is delivered to all the hosts that are watching the broadcast (instead of sending the same packet to each host, which wastes time and networking resources).
Multiprotocol Label Switching has become more popular in the last few years. It combines the advantages of switching with the intelligence of routing. In a nutshell, the first router in an MPLS cloud makes the routing decision and tags the packet. After that, all other routers in the cloud need to examine only the MPLS label in order to route the packet.