Chapter 15 - Operating Cisco Routers

Question 1

What is a CSU/DSU?

Answer 1

Channel Service Unit/Data Service Unit is the termination on a Serial WAN Link (basically an ADVA but for serial).

Question 2

What is a serial WAN link?

Answer 2

A serial link is a dedicated WAN connection that is the predecessor to leased lines and Ethernet WAN. They are much slower since, as the name implies, they carry data via serial (one bit at a time).

Question 3

What is an ISR?

Answer 3

Integrated Services Routers are devices that have can function as many other networking devices as well as a router (e.g. Firewall for Security services, APs for Wireless, Switches for Switching).

Question 4

What is a NIM?

Answer 4

A Network Interface Module is a card that allows you to expand the functionality of modular routers. For example you can get NIMs that have serial ports on them so you can connect a router to a serial link.

Question 5

How would you connect a serial WAN link to your router?

Answer 5

This is dependent on whether the router has an internal CSU/DSU or not. If the router doesn’t have an internal CSU/DSU then the serial link that comes into the building will need to be connected to the CSU/DSU and then to the router via a serial port. If the router does have an internal CSU/DSU then the serial link can be connected directly to the router via the CSU/DSU serial port.

Question 6

True or False. Cisco switches and routers both have on/off switches.

Answer 6

False. Typically Cisco routers will have on/off switches but Switches will just power on when connected to a PSU and power off when disconnected.

Question 7

What is the typical definition for an Enterprise router?

Answer 7

A router that resides in a large permanent business location.

Question 8

What is the typical definition for a SOHO router?

Answer 8

A Small Office/Home Office router will typically reside in an employees home office or in a small office with only a few host devices.

Question 9

What are two of the main differences between standard SOHO setups and standard Enterprise setups?

Answer 9

• A SOHO router will almost always use the internet and/or VPNs to connect back to a main office as part of an enterprise.
• A SOHO router will generally be multiple devices bundled into one that have multiple functions (e.g. Switching, Routing, VPNs, Wireless, etc.). Whereas an enterprise would usually have APs for wireless and dedicated switches.

Question 10

What are some of the differences between the CLI of a Cisco switch and the CLI of a Cisco router?

Answer 10

• Configuring IP addresses on a switch must be done per VLAN. However using a router it can be done per interface or VLAN.
• Many Cisco router models have an auxilary port that can be connected to a phone line via a modem which allows remote users to dial into a router and access the CLI by making a phone call. This is not a feature found in Cisco switches.
• By default Cisco routers don’t allow CLI access via Telnet or SSH. Cisco switches do.

Question 11

True or False. The first interface status code relates to the Data-Link status of an interface and the second interface status relates to the Physical status of an interface.

Answer 11

False. The first interface status code relates to the Physical status of an interface and the second interface status code relates to the Data-Link status of an interface.

If the first status code is down then the second one will also always be down. If the first status code is up but the second status code is down then this could indicate a Data-Link misconfiguration (e.g. on a serial link where one interface is configured for HDLC and the other is configured for PPP).

Question 12

True or False. Serial links use HDLC for encapsulation by default.

Answer 12

True, on Cisco routers it is cHDLC, Cisco’s proprietary HDLC. Although they can also use PPP.

Question 13

True or False. If the first status code of an interface is Administratively Down, this shows a layer 1 issue.

Answer 13

Sort of. Itt is the layer 1 status indicator, however, this only means that the shutdown command has been configured on the interface. The status being Down would show a layer 1 issue (e.g. no cable connected to said interface).

Question 14

True or False. Routers will not attempt to send packets or receive packets over an interface that has no IP address or subnet mask configured.

Answer 14

True.

Question 15

What command could you use to confirm the status codes of an interface and the IP address it has been configured with on a router.

Answer 15

show protocols or show ip int brief

show protocols will tell you the subnet mask whereas show ip int brief won’t

Question 16

True or False. The show ip interface brief command lists the IP and subnet mask of an interface.

Answer 16

False. It only lists the IP.

To see the IP and subnet use ‘show IP int’ or ‘show ip protocol’

Question 17

What command would you use to enter the configuration of a Cisco router’s auxilary port?

Answer 17

line aux 0

Question 18

True or False. Not having an IP address configured on an interface would show it as Down or Administratively Down.

Answer 18

False. Not having an IP address configured on an interface does not relate to the status of the interface.

Question 19

What information does the ‘show ip interface brief’ command give you?

Answer 19

• All interfaces on a router
• The IP address assigned to an interface
• The OK collumn which is a legacy feature that determines if the IP address is valid. On modern devices you can’t add an invalid IP address.
• The method by which the IP address was assigned
• The status of the interface (e.g up/up)

Question 20

What is the default state for a Cisco router interface?

Answer 20

Administratively down.

Question 21

What command allows you to map a hostname to an IP address?

Answer 21

ip host <hostname> <IP> (upto 8 addresses). Can be used for purposes such as Telnet so that you can connect to the host instead of having to remember the IP.</IP></hostname>

show hosts allows you to find the hostname mappings configured on a router.

Question 22

True or False. When using the ‘show ip interface brief’ command it is possible to see an interface status of down but a protocol status of up.

Answer 22

False. The status collumn shows the layer 1 status so if this is down there is no connectivity (e.g. nothing connected). The protocol collumn shows the layer 2 status so the reverse could be seen (i.e. You can see a status of up but a protocol of down).

Question 23

True or False. The ‘show version’ command can be used to find IP address information of a router.

Answer 23

False.

Question 24

True or False. Serial data link protocols (e.g. HDLC and PPP) don’t use MAC addresses for transmission.

Answer 24

True.

Question 25

What are the two sides of a serial connection?

Answer 25

• DCE (Data Communications Equipment) - Specifies the clock rate of the link which defines the speed
• DTE (Data Terminal Equipment)

Question 26

How would you find out which side of a serial link is DCE?

Answer 26

Use the ‘show controllers <serial>' command.</serial>

Question 27

What is the Clock Speed when referring to serial interfaces?

Answer 27

The speed of the interface