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.
