Differences between routers and switches:

- One minor difference between Cisco switches and routers is that routers support a much wider variety of interfaces.
- Today, LAN switches support Ethernet LAN interfaces of various speeds.
- Routers support a variety of other types of interfaces, including serial interfaces, cable TV, DSL, 3G/4G wireless, and others not mentioned in this book.
- Some Cisco routers have serial interfaces.
- As you might recall from Chapter 3, Cisco routers use serial interfaces to connect to a serial link.
- Each point-to-point serial link can then use High-Level Data Link Control (HDLC, the default) or Point-to-Point Protocol (PPP).
- Types of router interfaces:
- interface ethernet 0
- interface fastEthernet 0/1
- interface gigabitethernet 0/0
- interface serial 1/0/1
- Two of the most common commands to display the interfaces, and their status, are the show ip interface brief and show interfaces commands.

- Each interface has two interface status codes.
- To be usable, the two interface status codes must be in an “up” state.
- The first status code refers essentially to whether Layer 1 is working, and the second status code mainly (but not always) refers to whether the data link layer protocol is working.






Bandwidth and clock rate on serial interfaces:
- WAN serial links can run at a wide variety of speeds.
- To deal with the wide range of speeds, routers physically slave themselves to the speed as dictated by the CSU/DSU through a process called clocking.
- As a result, routers can use serial links without the need for additional configuration or auto-negotiation to sense the serial link’s speed.
- The CSU/DSU knows the speed, the CSU/DSU sends clock pulses over the cable to the router, and the router reacts to the clocking signal.

- Using the correct cabling works, as long as you add one command: the clock rate interface subcommand.
- This command tells that router the speed at which to transmit bits on a serial link like the one shown in Figure 17-7.
- The clock rate command is not needed on real serial links, because the CSU/DSU provides the clocking.
- When you create a serial link in the lab using cables, without any real CSU/DSUs on the link, the router with the DCE cable must supply that clocking function, and the clock rate command tells the router to provide it.

- The clock rate command does not allow just any speed to be configured.
- However, the list of speeds does vary from router to router.
- The clock rate command sets the actual Layer 1 speed used on the link, if no CSU/DSU is used, as just described.
- clock rate 128000 command sets the clock rate to 128 kbps

Router auxiliary port
- Both routers and switches have a console port to allow administrative access, but most Cisco routers have an extra physical port called an auxiliary (Aux) port.
- The Aux port typically serves as a means to make a phone call to connect into the router to issue commands from the CLI.

