Chapter 17 – Operating Cisco Routers

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.