Chapter 16 - Configuring IPv4 Addresses and Static Routes

Question 1

What is the function of IP Routing?

Answer 1

To carry packets across IP networks from sender to receiver and use routing decisions to decide on the best route for this.

Question 2

What is the process of IP routing with reference to local and non-local addresses specifically for hosts?

Answer 2

If the destination address is in the same local subnet as the host that is sending the packet, it will be sent directly to the destination by:
- Finding the destination host’s MAC address using ARP.
- Encapsulating the packet in a frame with the destination MAC address of the destination host.

If the destination address is not in the same local subnet as the host that is sending the packet, it will be sent to the default gateway by:
- Finding the default gateway’s MAC address using ARP.
- Encapsulating the packet in a frame with the destination MAC address of the default gateway.

Question 3

What is the process of IP routing for routers?

Answer 3

• For each frame received the router decides whether or not to process the frame. If the frame has no errors (as checked in the FCS field) and the frames destination MAC address is the router’s MAC address then it will be processed.
• If the frame is processed then the router will de-encapsulate it to get to the packet.
• Here the router makes a routing decision by comparing the packet’s destination IP to its routing table to find a route that matches the packet’s destination address. This route will identify the outgoing interface and sometimes the net-hop router’s IP address.
• Once the routing decision has been made the router will encapsulate the packet into a frame appropriate for the outgoing interface. When forwarding out of LAN interfaces, the router will use ARP to find the next devices MAC address.
• Finally the frame will be transmitted out of the route’s outgoing interface.

Question 4

What will happen if a router receives a frame that is destined for a unicast MAC other than its own?

Answer 4

The router will ignore said frame.

Question 5

True or false. Routes for connected subnets don’t include a next-hop IP address.

Answer 5

True. Connected subnet routes don’t need to go to another router to reach their destination so they will only include the outgoing interface that they need to follow in order to reach their destination.

Question 6

True or False. Routers can only route packets out of interfaces that are in an up/up state.

Answer 6

True.

Question 7

What are the two methods that a router can add routes to its routing tables? Provide examples

Answer 7

• Static Routes - Added because of the configuration of the ip route command on the local router.
• Dynamic Routes:
  - Routing Protocols - Added via a process that dynamically causes routers to tell each other about
  their networks so they all learn routes.
  - Connected Routes - Added because of the configuration of the ‘interface ip address’ command on
  the local router. (i.e If a subnet is configured on an interface this will be added to the router’s routing
  table).

Question 8

When will a router add a connected route for a connected subnet to its routing table?

Answer 8

• When the interface is in a working state (when it shows up/up) otherwise it will be removed.
• When the interface has an IP address configured to it via the ‘interface ip address’ command.

Question 9

What is a Local Route?

Answer 9

• A local route is a route for a specific address used on the interface of a router instead of an entire subnet. If an interface is configured with 192.168.1.1/24 then a connected route will be added for the entire network (192.168.1.0/24) and a local route will be added for the address of that interface (192.168.1.1/32).
• Local routes are always configured as /32 and therefore have a subnet mask of 255.255.255.255.
• They are used to allow routers to more efficiently forward packets sent to the router itsself.

Question 10

What is the Routing Protocol Code?

Answer 10

These are the letters shown when entering the ‘show ip route’ command that define the characteristics of routes. (e.g. C = Connected L = Local S = Static O = OSPF)

Question 11

How long does it take for an ARP entry to time out on Cisco IOS?

Answer 11

240 minutes after it not being used.

Question 12

How does a router show an ARP entry that will not time out?

Answer 12

It will have a ‘-‘ in the Age collumn.

Question 13

What information will be included when adding a static route?

Answer 13

• A destination address with a subnet ID and mask
• Forwarding instructions which take the form of either the next-hop router’s IP or the outgoing interface that leads to the next-hop router

Question 14

What are the functions of a network route, local route, and default route?

Answer 14

• A network route is a route where the destination is a network/subnet
• A local route is a route that points to a single IP address
• A default route is used if there are no more specific routes in the routing table

Question 15

What does it mean if a static route pointing at an outgoing interface disappears from a routing table?

Answer 15

Either this route has been deleted or the outgoing interface is currently down. If it has not been deleted the route will appear once the interface comes back up.

Question 16

How can you list specific types of routes?

Answer 16

Use the ‘show ip route <parameter>' command. The parameter is where you input one of the definitions from the Routing Protocol Code (e.g. Connected, Static, Local, BGP, etc.).</parameter>

Question 17

True or False. When creating a static route for traffic leaving an Ethernet interface you should always list the next-hop router’s IP rather than the outgoing interface of the current router.

Answer 17

True.

Question 18

True or False. Host routes can use any subnet mask.

Answer 18

False. They will always be 255.255.255.255.

Question 19

True or False. If there are two routes that an IP address can be matched to (e.g. a Network route and a host/local route for an address in the same subnet) the router will choose the most specific route.

Answer 19

True. The most specific route is determined by the longest subnet mask, so in this situation the subnet mask of the local route will be longer (255.255.255.255).

Question 20

What is the Administrative Distance of a route? Provide examples of default Administrative Distances.

Answer 20

This is how a router decides what route to forward traffic over when there are two routes leading to a subnet (in this situation the subnet mask will be the same on both routes since both routes lead to the same subnet). The lower the Administrative Distance, the more suitable the route.

Default OSPF learned rote Administrative Distance - 110
Default static route Administrative Distance - 1

Question 21

What is a Floating Static Route?

Answer 21

A route that moves into and out of a routing table depending on whether better route (lower administrative distance) exists currently. This is used if you want to force a router to use a specific route for a subnet even if a better route is present.

For example if you have an OSPF route (default 110) and a static route (default 1) that lead to the same subnet but you only want to use the OSPF route, instead of disconnecting the static route you would just increase the administrative distance of the static route. This way the static route can be used as a backup if the OSPF route were to fail.

Question 22

How do you create a Floating Static Route?

Answer 22

You would increase the administrative distance of the route you don’t want to be primary (the floating route) by entering the below command:

ip route <ip> <subnet> <next> <administrative></administrative></next></subnet></ip>

Question 23

What command could you use to find the administrative distances of routes to a specific subnet?

Answer 23

show ip route <subnet></subnet>

Question 24

What command would you use to create a static default route?

Answer 24

ip route 0.0.0.0 0.0.0.0 <net-hop/outgoing interface>

Question 25

What character in the Routing Protocol Code shows a candidate for a default route?

Answer 25

An asterisk (i.e. if the default route is static it would be listed as S*)

Question 26

Define Gateway of Last Resort.

Answer 26

This is the chosen default route, out of all default route candidates, over which traffic that doesn’t match any other routes will be forwarded. Synonymous with Default Route.

Question 27

What are the main things to look out for when checking if a static route is correct?

Answer 27

• Will the destination address definitely be included as part of the subnet when using the subnet ID and mask that is listed in the route?
• Is the next-hop IP definitely the IP of the next-hop and not something else (e.g. a PC, another router, unused)?
• Is the outgoing interface correct and listing an interface on the current (local) router (i.e. not a remote router)?

Question 28

True or False. Before IOS adds a static route you have entered to the routing table the outgoing interface must be in an up/up state.

Answer 28

True. This check can be avoided by entering the ‘permanent’ parameter after the ‘ip route…’ command but it will not make the route work if the interface is down, it will only make it remain in the routing table.

Question 29

True or False. For routes that list a next-hop address, the local router must include a route to reach that next-hop address.

Answer 29

True.

Question 30

What command could you use to query the router for what route it would use to reach a specific IP address?

Answer 30

show ip route <address>. The first line lists the route it uses and the next lines list other information such as the next-hop address and the outgoing interface that would be used.

Question 31

What are the different types of static routes?

Answer 31

• Directly Attached - Only the exit interface is specified in the routing information. (In IPv6 these can’t be used if the exit interface is an Ethernet interface. Serial interfaces would work.)
• Recursive - Only the next hop address is specified in the routing information. Recursive means that the router has to check its routing table multiple times. Once to find the address it is attempting to reach, and then a second time to find out how to reach the next hop address.
• Fully Specified - Both the exit interface and next hop address are specified in the routing information.

Question 32

What would happen if multiple static routes to the same destination, with the same administrative distance were added to the routing table?

Answer 32

Traffic to this destination would be load balanced between them. This is known as ECMP (Equal Cost Multi-Path).